# 内置类型 ¶

## 真值测试 ¶

• 定义为 False 的常量： ``` None ``` and ``` False ``` .

• 任何数值类型的零： ``` 0 ``` , ``` 0.0 ``` , ``` 0j ``` , ``` Decimal(0) ``` , ``` Fraction(0, 1) ```

• 空序列和集合： ``` '' ``` , ``` () ``` , ``` [] ``` , ``` {} ``` , ``` set() ``` , ``` range(0) ```

## 布尔运算 — ``` and ``` , ``` or ``` , ``` not ``` ¶

``` x or y ```

if x 为 False，那么 y ，否则 x

(1)

``` x and y ```

if x 为 False，那么 x ，否则 y

(2)

``` not x ```

if x 为 False，那么 ``` True ``` ，否则 ``` False ```

(3)

1. This is a short-circuit operator, so it only evaluates the second argument if the first one is false.

2. This is a short-circuit operator, so it only evaluates the second argument if the first one is true.

3. ``` not ``` has a lower priority than non-Boolean operators, so ``` not a == b ``` is interpreted as ``` not (a == b) ``` ，和 ``` a == not b ``` 是句法错误。

## 比较 ¶

There are eight comparison operations in Python. They all have the same priority (which is higher than that of the Boolean operations). Comparisons can be chained arbitrarily; for example, ``` x < y <= z ``` 相当于 ``` x < y and y <= z ``` ，除了 y is evaluated only once (but in both cases z is not evaluated at all when ``` x < y ``` is found to be false).

``` < ```

``` <= ```

``` > ```

``` >= ```

``` == ```

equal

``` != ```

``` is ```

``` is not ```

Objects of different types, except different numeric types, never compare equal. ``` == ``` operator is always defined but for some object types (for example, class objects) is equivalent to ``` is ``` ``` < ``` , ``` <= ``` , ``` > ``` and ``` >= ``` operators are only defined where they make sense; for example, they raise a ``` TypeError ``` exception when one of the arguments is a complex number.

Non-identical instances of a class normally compare as non-equal unless the class defines the ``` __eq__() ``` 方法。

Instances of a class cannot be ordered with respect to other instances of the same class, or other types of object, unless the class defines enough of the methods ``` __lt__() ``` , ``` __le__() ``` , ``` __gt__() ``` ，和 ``` __ge__() ``` (in general, ``` __lt__() ``` and ``` __eq__() ``` are sufficient, if you want the conventional meanings of the comparison operators).

The behavior of the ``` is ``` and ``` is not ``` operators cannot be customized; also they can be applied to any two objects and never raise an exception.

Two more operations with the same syntactic priority, ``` in ``` and ``` not in ``` , are supported by types that are iterable 或实现 ``` __contains__() ``` 方法。

## 数值类型 — ``` int ``` , ``` float ``` , ``` complex ``` ¶

Numbers are created by numeric literals or as the result of built-in functions and operators. Unadorned integer literals (including hex, octal and binary numbers) yield integers. Numeric literals containing a decimal point or an exponent sign yield floating point numbers. Appending ``` 'j' ``` or ``` 'J' ``` to a numeric literal yields an imaginary number (a complex number with a zero real part) which you can add to an integer or float to get a complex number with real and imaginary parts.

Python fully supports mixed arithmetic: when a binary arithmetic operator has operands of different numeric types, the operand with the “narrower” type is widened to that of the other, where integer is narrower than floating point, which is narrower than complex. A comparison between numbers of different types behaves as though the exact values of those numbers were being compared. 2

All numeric types (except complex) support the following operations (for priorities of the operations, see 运算符优先级 ):

``` x + y ```

sum of x and y

``` x - y ```

difference of x and y

``` x * y ```

product of x and y

``` x / y ```

quotient of x and y

``` x // y ```

floored quotient of x and y

(1)

``` x % y ```

remainder of ``` x / y ```

(2)

``` -x ```

x negated

``` +x ```

x unchanged

``` abs(x) ```

absolute value or magnitude of x

``` int(x) ```

x converted to integer

(3)(6)

``` float(x) ```

x converted to floating point

(4)(6)

``` complex(re, im) ```

a complex number with real part re , imaginary part im . im defaults to zero.

(6)

``` c.conjugate() ```

conjugate of the complex number c

``` divmod(x, y) ```

the pair ``` (x // y, x % y) ```

(2)

``` pow(x, y) ```

x to the power y

(5)

``` x ** y ```

x to the power y

(5)

1. Also referred to as integer division. The resultant value is a whole integer, though the result’s type is not necessarily int. The result is always rounded towards minus infinity: ``` 1//2 ``` is ``` 0 ``` , ``` (-1)//2 ``` is ``` -1 ``` , ``` 1//(-2) ``` is ``` -1 ``` ，和 ``` (-1)//(-2) ``` is ``` 0 ``` .

2. 不适用于复数。相反，转换为浮点数使用 ``` abs() ``` 若合适。

3. Conversion from floating point to integer may round or truncate as in C; see functions ``` math.floor() ``` and ``` math.ceil() ``` for well-defined conversions.

4. float also accepts the strings “nan” and “inf” with an optional prefix “+” or “-” for Not a Number (NaN) and positive or negative infinity.

5. Python 定义 ``` pow(0, 0) ``` and ``` 0 ** 0 ``` to be ``` 1 ``` , as is common for programming languages.

6. The numeric literals accepted include the digits ``` 0 ``` to ``` 9 ``` or any Unicode equivalent (code points with the ``` Nd ``` 特性)。

https://www.unicode.org/Public/13.0.0/ucd/extracted/DerivedNumericType.txt for a complete list of code points with the ``` Nd ``` 特性。

x truncated to ``` Integral ```

x 四舍五入到 n digits, rounding half to even. If n is omitted, it defaults to 0.

the greatest ``` Integral ``` <= x

the least ``` Integral ``` >= x

### 整数类型的按位运算 ¶

Bitwise operations only make sense for integers. The result of bitwise operations is calculated as though carried out in two’s complement with an infinite number of sign bits.

The priorities of the binary bitwise operations are all lower than the numeric operations and higher than the comparisons; the unary operation ``` ~ ``` 具有相同优先级，如同其它一元数值运算 ( ``` + ``` and ``` - ``` ).

``` x | y ```

bitwise or of x and y

(4)

``` x ^ y ```

bitwise exclusive or of x and y

(4)

``` x & y ```

bitwise and of x and y

(4)

``` x << n ```

x shifted left by n bits

(1)(2)

``` x >> n ```

x shifted right by n bits

(1)(3)

``` ~x ```

the bits of x inverted

1. 负移位计数是非法的，并会导致 ``` ValueError ``` 被引发。

2. 向左移位 n 位，相当于乘以 ``` pow(2, n) ``` .

3. 向右移位 n bits is equivalent to floor division by ``` pow(2, n) ``` .

4. 履行这些计算，采用至少一额外符号扩展位表示有限的 2 的补码 (工作位宽 ``` 1 + max(x.bit_length(), y.bit_length()) ``` 或更多) 足以获取与无穷多符号位相同的结果。

### 额外整数类型方法 ¶

int 类型实现 ``` numbers.Integral ``` 抽象基类 。此外，它还提供一些其它方法：

``` int. ``` ``` bit_length ``` ( )

Return the number of bits necessary to represent an integer in binary, excluding the sign and leading zeros:

```>>> n = -37
>>> bin(n)
'-0b100101'
>>> n.bit_length()
6
```

```def bit_length(self):
s = bin(self)       # binary representation:  bin(-37) --> '-0b100101'
s = s.lstrip('-0b') # remove leading zeros and minus sign
return len(s)       # len('100101') --> 6
```

3.1 版新增。

``` int. ``` ``` to_bytes ``` ( length , byteorder , * , signed=False )

Return an array of bytes representing an integer.

```>>> (1024).to_bytes(2, byteorder='big')
b'\x04\x00'
>>> (1024).to_bytes(10, byteorder='big')
b'\x00\x00\x00\x00\x00\x00\x00\x00\x04\x00'
>>> (-1024).to_bytes(10, byteorder='big', signed=True)
b'\xff\xff\xff\xff\xff\xff\xff\xff\xfc\x00'
>>> x = 1000
>>> x.to_bytes((x.bit_length() + 7) // 8, byteorder='little')
b'\xe8\x03'
```

The integer is represented using length bytes. An ``` OverflowError ``` is raised if the integer is not representable with the given number of bytes.

byteorder argument determines the byte order used to represent the integer. If byteorder is ``` "big" ``` , the most significant byte is at the beginning of the byte array. If byteorder is ``` "little" ``` , the most significant byte is at the end of the byte array. To request the native byte order of the host system, use ``` sys.byteorder ``` as the byte order value.

signed argument determines whether two’s complement is used to represent the integer. If signed is ``` False ``` and a negative integer is given, an ``` OverflowError ``` is raised. The default value for signed is ``` False ``` .

3.2 版新增。

classmethod ``` int. ``` ``` from_bytes ``` ( bytes , byteorder , * , signed=False )

Return the integer represented by the given array of bytes.

```>>> int.from_bytes(b'\x00\x10', byteorder='big')
16
>>> int.from_bytes(b'\x00\x10', byteorder='little')
4096
>>> int.from_bytes(b'\xfc\x00', byteorder='big', signed=True)
-1024
>>> int.from_bytes(b'\xfc\x00', byteorder='big', signed=False)
64512
>>> int.from_bytes([255, 0, 0], byteorder='big')
16711680
```

byteorder argument determines the byte order used to represent the integer. If byteorder is ``` "big" ``` , the most significant byte is at the beginning of the byte array. If byteorder is ``` "little" ``` , the most significant byte is at the end of the byte array. To request the native byte order of the host system, use ``` sys.byteorder ``` as the byte order value.

signed argument indicates whether two’s complement is used to represent the integer.

3.2 版新增。

``` int. ``` ``` as_integer_ratio ``` ( )

Return a pair of integers whose ratio is exactly equal to the original integer and with a positive denominator. The integer ratio of integers (whole numbers) is always the integer as the numerator and ``` 1 ``` as the denominator.

3.8 版新增。

### 额外浮点方法 ¶

``` float. ``` ``` as_integer_ratio ``` ( )

Return a pair of integers whose ratio is exactly equal to the original float and with a positive denominator. Raises ``` OverflowError ``` on infinities and a ``` ValueError ``` on NaNs.

``` float. ``` ``` is_integer ``` ( )

```>>> (-2.0).is_integer()
True
>>> (3.2).is_integer()
False
```

Two methods support conversion to and from hexadecimal strings. Since Python’s floats are stored internally as binary numbers, converting a float to or from a decimal string usually involves a small rounding error. In contrast, hexadecimal strings allow exact representation and specification of floating-point numbers. This can be useful when debugging, and in numerical work.

``` float. ``` ``` hex ``` ( )

Return a representation of a floating-point number as a hexadecimal string. For finite floating-point numbers, this representation will always include a leading ``` 0x ``` and a trailing ``` p ``` and exponent.

classmethod ``` float. ``` ``` fromhex ``` ( s )

Class method to return the float represented by a hexadecimal string s . The string s may have leading and trailing whitespace.

A hexadecimal string takes the form:

```[sign] ['0x'] integer ['.' fraction] ['p' exponent]
```

where the optional ``` sign ``` may by either ``` + ``` or ``` - ``` , ``` integer ``` and ``` fraction ``` are strings of hexadecimal digits, and ``` exponent ``` is a decimal integer with an optional leading sign. Case is not significant, and there must be at least one hexadecimal digit in either the integer or the fraction. This syntax is similar to the syntax specified in section 6.4.4.2 of the C99 standard, and also to the syntax used in Java 1.5 onwards. In particular, the output of ``` float.hex() ``` is usable as a hexadecimal floating-point literal in C or Java code, and hexadecimal strings produced by C’s ``` %a ``` format character or Java’s ``` Double.toHexString ``` are accepted by ``` float.fromhex() ``` .

Note that the exponent is written in decimal rather than hexadecimal, and that it gives the power of 2 by which to multiply the coefficient. For example, the hexadecimal string ``` 0x3.a7p10 ``` represents the floating-point number ``` (3 + 10./16 + 7./16**2) * 2.0**10 ``` ，或 ``` 3740.0 ``` :

```>>> float.fromhex('0x3.a7p10')
3740.0
```

Applying the reverse conversion to ``` 3740.0 ``` gives a different hexadecimal string representing the same number:

```>>> float.hex(3740.0)
'0x1.d380000000000p+11'
```

### 数值类型的哈希 ¶

For numbers ``` x ``` and ``` y ``` , possibly of different types, it’s a requirement that ``` hash(x) == hash(y) ``` whenever ``` x == y ``` (见 ``` __hash__() ``` method documentation for more details). For ease of implementation and efficiency across a variety of numeric types (including ``` int ``` , ``` float ``` , ``` decimal.Decimal ``` and ``` fractions.Fraction ``` ) Python’s hash for numeric types is based on a single mathematical function that’s defined for any rational number, and hence applies to all instances of ``` int ``` and ``` fractions.Fraction ``` , and all finite instances of ``` float ``` and ``` decimal.Decimal ``` . Essentially, this function is given by reduction modulo ``` P ``` for a fixed prime ``` P ``` . The value of ``` P ``` is made available to Python as the ``` modulus ``` attribute of ``` sys.hash_info ``` .

CPython 实现细节： 目前，使用的素数是 ``` P = 2**31 - 1 ``` on machines with 32-bit C longs and ``` P = 2**61 - 1 ``` on machines with 64-bit C longs.

• ``` x = m / n ``` is a nonnegative rational number and ``` n ``` is not divisible by ``` P ``` , define ``` hash(x) ``` as ``` m * invmod(n, P) % P ``` ，其中 ``` invmod(n, P) ``` gives the inverse of ``` n ``` modulo ``` P ``` .

• ``` x = m / n ``` is a nonnegative rational number and ``` n ``` is divisible by ``` P ``` (但 ``` m ``` is not) then ``` n ``` has no inverse modulo ``` P ``` and the rule above doesn’t apply; in this case define ``` hash(x) ``` to be the constant value ``` sys.hash_info.inf ``` .

• ``` x = m / n ``` is a negative rational number define ``` hash(x) ``` as ``` -hash(-x) ``` 。若结果哈希为 ``` -1 ``` ，替换它采用 ``` -2 ``` .

• The particular values ``` sys.hash_info.inf ``` , ``` -sys.hash_info.inf ``` and ``` sys.hash_info.nan ``` are used as hash values for positive infinity, negative infinity, or nans (respectively). (All hashable nans have the same hash value.)

• For a ``` complex ``` number ``` z ``` , the hash values of the real and imaginary parts are combined by computing ``` hash(z.real) + sys.hash_info.imag * hash(z.imag) ``` , reduced modulo ``` 2**sys.hash_info.width ``` so that it lies in ``` range(-2**(sys.hash_info.width - 1), 2**(sys.hash_info.width - 1)) ``` . Again, if the result is ``` -1 ``` , it’s replaced with ``` -2 ``` .

To clarify the above rules, here’s some example Python code, equivalent to the built-in hash, for computing the hash of a rational number, ``` float ``` ，或 ``` complex ``` :

```import sys, math
def hash_fraction(m, n):
"""Compute the hash of a rational number m / n.
Assumes m and n are integers, with n positive.
Equivalent to hash(fractions.Fraction(m, n)).
"""
P = sys.hash_info.modulus
# Remove common factors of P.  (Unnecessary if m and n already coprime.)
while m % P == n % P == 0:
m, n = m // P, n // P
if n % P == 0:
hash_value = sys.hash_info.inf
else:
# Fermat's Little Theorem: pow(n, P-1, P) is 1, so
# pow(n, P-2, P) gives the inverse of n modulo P.
hash_value = (abs(m) % P) * pow(n, P - 2, P) % P
if m < 0:
hash_value = -hash_value
if hash_value == -1:
hash_value = -2
return hash_value
def hash_float(x):
"""Compute the hash of a float x."""
if math.isnan(x):
return sys.hash_info.nan
elif math.isinf(x):
return sys.hash_info.inf if x > 0 else -sys.hash_info.inf
else:
return hash_fraction(*x.as_integer_ratio())
def hash_complex(z):
"""Compute the hash of a complex number z."""
hash_value = hash_float(z.real) + sys.hash_info.imag * hash_float(z.imag)
# do a signed reduction modulo 2**sys.hash_info.width
M = 2**(sys.hash_info.width - 1)
hash_value = (hash_value & (M - 1)) - (hash_value & M)
if hash_value == -1:
hash_value = -2
return hash_value
```

## 迭代器类型 ¶

Python supports a concept of iteration over containers. This is implemented using two distinct methods; these are used to allow user-defined classes to support iteration. Sequences, described below in more detail, always support the iteration methods.

``` container. ``` ``` __iter__ ``` ( )

Return an iterator object. The object is required to support the iterator protocol described below. If a container supports different types of iteration, additional methods can be provided to specifically request iterators for those iteration types. (An example of an object supporting multiple forms of iteration would be a tree structure which supports both breadth-first and depth-first traversal.) This method corresponds to the ``` tp_iter ``` slot of the type structure for Python objects in the Python/C API.

The iterator objects themselves are required to support the following two methods, which together form the 迭代器协议 :

``` iterator. ``` ``` __iter__ ``` ( )

Return the iterator object itself. This is required to allow both containers and iterators to be used with the ``` for ``` and ``` in ``` statements. This method corresponds to the ``` tp_iter ``` slot of the type structure for Python objects in the Python/C API.

``` iterator. ``` ``` __next__ ``` ( )

Return the next item from the container. If there are no further items, raise the ``` StopIteration ``` exception. This method corresponds to the ``` tp_iternext ``` slot of the type structure for Python objects in the Python/C API.

Python defines several iterator objects to support iteration over general and specific sequence types, dictionaries, and other more specialized forms. The specific types are not important beyond their implementation of the iterator protocol.

### 生成器类型 ¶

Python 的 generator s provide a convenient way to implement the iterator protocol. If a container object’s ``` __iter__() ``` method is implemented as a generator, it will automatically return an iterator object (technically, a generator object) supplying the ``` __iter__() ``` and ``` __next__() ``` methods. More information about generators can be found in the documentation for the yield expression .

## 序列类型 — ``` list ``` , ``` tuple ``` , ``` range ``` ¶

There are three basic sequence types: lists, tuples, and range objects. Additional sequence types tailored for processing of 二进制数据 and 文本字符串 are described in dedicated sections.

### 常见序列操作 ¶

The operations in the following table are supported by most sequence types, both mutable and immutable. The ``` collections.abc.Sequence ``` ABC is provided to make it easier to correctly implement these operations on custom sequence types.

This table lists the sequence operations sorted in ascending priority. In the table, s and t are sequences of the same type, n , i , j and k are integers and x is an arbitrary object that meets any type and value restrictions imposed by s .

``` in ``` and ``` not in ``` operations have the same priorities as the comparison operations. The ``` + ``` (concatenation) and ``` * ``` (repetition) operations have the same priority as the corresponding numeric operations. 3

``` x in s ```

``` True ``` if an item of s 等于 x ，否则 ``` False ```

(1)

``` x not in s ```

``` False ``` if an item of s 等于 x ，否则 ``` True ```

(1)

``` s + t ```

the concatenation of s and t

(6)(7)

``` s * n ``` or ``` n * s ```

equivalent to adding s to itself n times

(2)(7)

``` s[i] ```

i th item of s , origin 0

(3)

``` s[i:j] ```

slice of s from i to j

(3)(4)

``` s[i:j:k] ```

slice of s from i to j with step k

(3)(5)

``` len(s) ```

length of s

``` min(s) ```

smallest item of s

``` max(s) ```

largest item of s

``` s.index(x[, i[, j]]) ```

index of the first occurrence of x in s (at or after index i and before index j )

(8)

``` s.count(x) ```

total number of occurrences of x in s

Sequences of the same type also support comparisons. In particular, tuples and lists are compared lexicographically by comparing corresponding elements. This means that to compare equal, every element must compare equal and the two sequences must be of the same type and have the same length. (For full details see 比较 in the language reference.)

1. While the ``` in ``` and ``` not in ``` operations are used only for simple containment testing in the general case, some specialised sequences (such as ``` str ``` , ``` bytes ``` and ``` bytearray ``` ) also use them for subsequence testing:

```>>> "gg" in "eggs"
True
```
2. Values of n less than ``` 0 ``` are treated as ``` 0 ``` (which yields an empty sequence of the same type as s ). Note that items in the sequence s are not copied; they are referenced multiple times. This often haunts new Python programmers; consider:

```>>> lists = [[]] * 3
>>> lists
[[], [], []]
>>> lists.append(3)
>>> lists
[, , ]
```

What has happened is that ``` [[]] ``` is a one-element list containing an empty list, so all three elements of ``` [[]] * 3 ``` are references to this single empty list. Modifying any of the elements of ``` lists ``` modifies this single list. You can create a list of different lists this way:

```>>> lists = [[] for i in range(3)]
>>> lists.append(3)
>>> lists.append(5)
>>> lists.append(7)
>>> lists
[, , ]
```

Further explanation is available in the FAQ entry How do I create a multidimensional list? .

3. i or j is negative, the index is relative to the end of sequence s : ``` len(s) + i ``` or ``` len(s) + j ``` is substituted. But note that ``` -0 ``` is still ``` 0 ``` .

4. The slice of s from i to j is defined as the sequence of items with index k 这样 ``` i <= k < j ``` 。若 i or j 大于 ``` len(s) ``` ，使用 ``` len(s) ``` 。若 i 被省略或 ``` None ``` ，使用 ``` 0 ``` 。若 j 被省略或 ``` None ``` ，使用 ``` len(s) ``` 。若 i is greater than or equal to j , the slice is empty.

5. The slice of s from i to j with step k is defined as the sequence of items with index ``` x = i + n*k ``` 这样 ``` 0 <= n < (j-i)/k ``` . In other words, the indices are ``` i ``` , ``` i+k ``` , ``` i+2*k ``` , ``` i+3*k ``` and so on, stopping when j is reached (but never including j ). When k is positive, i and j are reduced to ``` len(s) ``` if they are greater. When k is negative, i and j are reduced to ``` len(s) - 1 ``` if they are greater. If i or j are omitted or ``` None ``` , they become “end” values (which end depends on the sign of k ). Note, k cannot be zero. If k is ``` None ``` , it is treated like ``` 1 ``` .

6. Concatenating immutable sequences always results in a new object. This means that building up a sequence by repeated concatenation will have a quadratic runtime cost in the total sequence length. To get a linear runtime cost, you must switch to one of the alternatives below:

7. Some sequence types (such as ``` range ``` ) only support item sequences that follow specific patterns, and hence don’t support sequence concatenation or repetition.

8. ``` index ``` 引发 ``` ValueError ``` when x is not found in s . Not all implementations support passing the additional arguments i and j . These arguments allow efficient searching of subsections of the sequence. Passing the extra arguments is roughly equivalent to using ``` s[i:j].index(x) ``` , only without copying any data and with the returned index being relative to the start of the sequence rather than the start of the slice.

### 不可变序列类型 ¶

The only operation that immutable sequence types generally implement that is not also implemented by mutable sequence types is support for the ``` hash() ``` 内置。

This support allows immutable sequences, such as ``` tuple ``` instances, to be used as ``` dict ``` keys and stored in ``` set ``` and ``` frozenset ``` 实例。

Attempting to hash an immutable sequence that contains unhashable values will result in ``` TypeError ``` .

### 可变序列类型 ¶

The operations in the following table are defined on mutable sequence types. ``` collections.abc.MutableSequence ``` ABC is provided to make it easier to correctly implement these operations on custom sequence types.

In the table s is an instance of a mutable sequence type, t is any iterable object and x is an arbitrary object that meets any type and value restrictions imposed by s (例如， ``` bytearray ``` only accepts integers that meet the value restriction ``` 0 <= x <= 255 ``` ).

``` s[i] = x ```

item i of s is replaced by x

``` s[i:j] = t ```

slice of s from i to j is replaced by the contents of the iterable t

``` del s[i:j] ```

``` s[i:j:k] = t ```

the elements of ``` s[i:j:k] ``` are replaced by those of t

(1)

``` del s[i:j:k] ```

removes the elements of ``` s[i:j:k] ``` from the list

``` s.append(x) ```

appends x to the end of the sequence (same as ``` s[len(s):len(s)] = [x] ``` )

``` s.clear() ```

removes all items from s (same as ``` del s[:] ``` )

(5)

``` s.copy() ```

creates a shallow copy of s (same as ``` s[:] ``` )

(5)

``` s.extend(t) ``` or ``` s += t ```

extends s with the contents of t (for the most part the same as ``` s[len(s):len(s)] = t ``` )

``` s *= n ```

(6)

``` s.insert(i, x) ```

inserts x into s at the index given by i (same as ``` s[i:i] = [x] ``` )

``` s.pop() ``` or ``` s.pop(i) ```

retrieves the item at i and also removes it from s

(2)

``` s.remove(x) ```

remove the first item from s where ``` s[i] ``` 等于 x

(3)

``` s.reverse() ```

reverses the items of s in place

(4)

1. t 必须与要替换切片具有相同的长度。

2. 可选自变量 i 默认为 ``` -1 ``` ，因此默认情况下，最后项会被移除并返回。

3. ``` remove() ``` 引发 ``` ValueError ``` when x is not found in s .

4. ``` reverse() ``` method modifies the sequence in place for economy of space when reversing a large sequence. To remind users that it operates by side effect, it does not return the reversed sequence.

5. ``` clear() ``` and ``` copy() ``` are included for consistency with the interfaces of mutable containers that don’t support slicing operations (such as ``` dict ``` and ``` set ``` ). ``` copy() ``` is not part of the ``` collections.abc.MutableSequence ``` ABC, but most concrete mutable sequence classes provide it.

3.3 版新增： ``` clear() ``` and ``` copy() ``` 方法。

6. n is an integer, or an object implementing ``` __index__() ``` . Zero and negative values of n clear the sequence. Items in the sequence are not copied; they are referenced multiple times, as explained for ``` s * n ``` under 常见序列操作 .

### 列表 ¶

class ``` list ``` ( [ iterable ] )

• 使用一对方括号表示空列表： ``` [] ```

• 使用方括号，采用逗号分隔项： ``` [a] ``` , ``` [a, b, c] ```

• 使用列表推导： ``` [x for x in iterable] ```

• 使用类型构造函数： ``` list() ``` or ``` list(iterable) ```

The constructor builds a list whose items are the same and in the same order as iterable ’s items. iterable may be either a sequence, a container that supports iteration, or an iterator object. If iterable is already a list, a copy is made and returned, similar to ``` iterable[:] ``` 。例如， ``` list('abc') ``` 返回 ``` ['a', 'b', 'c'] ``` and ``` list( (1, 2, 3) ) ``` 返回 ``` [1, 2, 3] ``` . If no argument is given, the constructor creates a new empty list, ``` [] ``` .

``` sort ``` ( * , key=None , reverse=False )

``` sort() ``` 接受仅通过关键字传递的 2 自变量 ( 仅关键词自变量 ):

key specifies a function of one argument that is used to extract a comparison key from each list element (for example, ``` key=str.lower ``` ). The key corresponding to each item in the list is calculated once and then used for the entire sorting process. The default value of ``` None ``` means that list items are sorted directly without calculating a separate key value.

``` functools.cmp_to_key() ``` utility is available to convert a 2.x style cmp 函数到 key 函数。

reverse 是布尔值。若设为 ``` True ``` ，则对列表元素排序，就好像反转每一比较。

This method modifies the sequence in place for economy of space when sorting a large sequence. To remind users that it operates by side effect, it does not return the sorted sequence (use ``` sorted() ``` to explicitly request a new sorted list instance).

``` sort() ``` method is guaranteed to be stable. A sort is stable if it guarantees not to change the relative order of elements that compare equal — this is helpful for sorting in multiple passes (for example, sort by department, then by salary grade).

CPython 实现细节： While a list is being sorted, the effect of attempting to mutate, or even inspect, the list is undefined. The C implementation of Python makes the list appear empty for the duration, and raises ``` ValueError ``` if it can detect that the list has been mutated during a sort.

### 元组 ¶

Tuples are immutable sequences, typically used to store collections of heterogeneous data (such as the 2-tuples produced by the ``` enumerate() ``` built-in). Tuples are also used for cases where an immutable sequence of homogeneous data is needed (such as allowing storage in a ``` set ``` or ``` dict ``` 实例)。

class ``` tuple ``` ( [ iterable ] )

• Using a pair of parentheses to denote the empty tuple: ``` () ```

• Using a trailing comma for a singleton tuple: ``` a, ``` or ``` (a,) ```

• 采用逗号分隔项： ``` a, b, c ``` or ``` (a, b, c) ```

• 使用 ``` tuple() ``` 内置： ``` tuple() ``` or ``` tuple(iterable) ```

The constructor builds a tuple whose items are the same and in the same order as iterable ’s items. iterable may be either a sequence, a container that supports iteration, or an iterator object. If iterable is already a tuple, it is returned unchanged. For example, ``` tuple('abc') ``` 返回 ``` ('a', 'b', 'c') ``` and ``` tuple( [1, 2, 3] ) ``` 返回 ``` (1, 2, 3) ``` . If no argument is given, the constructor creates a new empty tuple, ``` () ``` .

Note that it is actually the comma which makes a tuple, not the parentheses. The parentheses are optional, except in the empty tuple case, or when they are needed to avoid syntactic ambiguity. For example, ``` f(a, b, c) ``` is a function call with three arguments, while ``` f((a, b, c)) ``` is a function call with a 3-tuple as the sole argument.

For heterogeneous collections of data where access by name is clearer than access by index, ``` collections.namedtuple() ``` may be a more appropriate choice than a simple tuple object.

### 范围 ¶

``` range ``` type represents an immutable sequence of numbers and is commonly used for looping a specific number of times in ``` for ``` 循环。

class ``` range ``` ( stop )
class ``` range ``` ( start , stop [ , step ] )

The arguments to the range constructor must be integers (either built-in ``` int ``` or any object that implements the ``` __index__ ``` special method). If the step argument is omitted, it defaults to ``` 1 ``` 。若 start argument is omitted, it defaults to ``` 0 ``` 。若 step is zero, ``` ValueError ``` 被引发。

A range object will be empty if ``` r ``` does not meet the value constraint. Ranges do support negative indices, but these are interpreted as indexing from the end of the sequence determined by the positive indices.

Ranges containing absolute values larger than ``` sys.maxsize ``` are permitted but some features (such as ``` len() ``` ) 可能引发 ``` OverflowError ``` .

```>>> list(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> list(range(1, 11))
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> list(range(0, 30, 5))
[0, 5, 10, 15, 20, 25]
>>> list(range(0, 10, 3))
[0, 3, 6, 9]
>>> list(range(0, -10, -1))
[0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
>>> list(range(0))
[]
>>> list(range(1, 0))
[]
```

Ranges implement all of the common sequence operations except concatenation and repetition (due to the fact that range objects can only represent sequences that follow a strict pattern and repetition and concatenation will usually violate that pattern).

``` start ```

The value of the start parameter (or ``` 0 ``` if the parameter was not supplied)

``` stop ```

The value of the stop parameter

``` step ```

The value of the step parameter (or ``` 1 ``` if the parameter was not supplied)

The advantage of the ``` range ``` type over a regular ``` list ``` or ``` tuple ``` is that a ``` range ``` object will always take the same (small) amount of memory, no matter the size of the range it represents (as it only stores the ``` start ``` , ``` stop ``` and ``` step ``` values, calculating individual items and subranges as needed).

Range objects implement the ``` collections.abc.Sequence ``` ABC, and provide features such as containment tests, element index lookup, slicing and support for negative indices (see 序列类型 — list tuple range ):

```>>> r = range(0, 20, 2)
>>> r
range(0, 20, 2)
>>> 11 in r
False
>>> 10 in r
True
>>> r.index(10)
5
>>> r
10
>>> r[:5]
range(0, 10, 2)
>>> r[-1]
18
```

Testing range objects for equality with ``` == ``` and ``` != ``` compares them as sequences. That is, two range objects are considered equal if they represent the same sequence of values. (Note that two range objects that compare equal might have different ``` start ``` , ``` stop ``` and ``` step ``` attributes, for example ``` range(0) == range(2, 1, 3) ``` or ``` range(0, 3, 2) == range(0, 4, 2) ``` .)

3.2 版改变： Implement the Sequence ABC. Support slicing and negative indices. Test ``` int ``` objects for membership in constant time instead of iterating through all items.

3.3 版改变： Define ‘==’ and ‘!=’ to compare range objects based on the sequence of values they define (instead of comparing based on object identity).

• linspace recipe shows how to implement a lazy version of range suitable for floating point applications.

## 文本序列类型 — ``` str ``` ¶

Textual data in Python is handled with ``` str ``` objects, or strings . Strings are immutable sequences of Unicode code points. String literals are written in a variety of ways:

• 单引号： ``` 'allows embedded "double" quotes' ```

• 双引号： ``` "allows embedded 'single' quotes" ``` .

• 三引号： ``` '''Three single quotes''' ``` , ``` """Three double quotes""" ```

Triple quoted strings may span multiple lines - all associated whitespace will be included in the string literal.

String literals that are part of a single expression and have only whitespace between them will be implicitly converted to a single string literal. That is, ``` ("spam " "eggs") == "spam eggs" ``` .

Strings may also be created from other objects using the ``` str ``` 构造函数。

Since there is no separate “character” type, indexing a string produces strings of length 1. That is, for a non-empty string s , ``` s == s[0:1] ``` .

There is also no mutable string type, but ``` str.join() ``` or ``` io.StringIO ``` can be used to efficiently construct strings from multiple fragments.

3.3 版改变： For backwards compatibility with the Python 2 series, the ``` u ``` prefix is once again permitted on string literals. It has no effect on the meaning of string literals and cannot be combined with the ``` r ``` 前缀。

class ``` str ``` ( object='' )
class ``` str ``` ( object=b'' , encoding='utf-8' , errors='strict' )

encoding nor errors 有给定， ``` str(object) ``` 返回 ``` object.__str__() ``` ，这是非正式或很好可打印字符串表示的 object 。对于字符串对象，这是字符串自身。若 object 没有 ``` __str__() ``` 方法，那么 ``` str() ``` 回退以返回 ``` repr(object) ``` .

```>>> str(b'Zoot!')
"b'Zoot!'"
```

### 字符串方法 ¶

Strings also support two styles of string formatting, one providing a large degree of flexibility and customization (see ``` str.format() ``` , 格式字符串语法 and 自定义字符串格式化 ) and the other based on C ``` printf ``` style formatting that handles a narrower range of types and is slightly harder to use correctly, but is often faster for the cases it can handle ( printf 样式字符串格式化 ).

``` str. ``` ``` capitalize ``` ( )

Return a copy of the string with its first character capitalized and the rest lowercased.

3.8 版改变： The first character is now put into titlecase rather than uppercase. This means that characters like digraphs will only have their first letter capitalized, instead of the full character.

``` str. ``` ``` casefold ``` ( )

Return a casefolded copy of the string. Casefolded strings may be used for caseless matching.

Casefolding is similar to lowercasing but more aggressive because it is intended to remove all case distinctions in a string. For example, the German lowercase letter ``` 'ß' ``` 相当于 ``` "ss" ``` . Since it is already lowercase, ``` lower() ``` would do nothing to ``` 'ß' ``` ; ``` casefold() ``` converts it to ``` "ss" ``` .

The casefolding algorithm is described in section 3.13 of the Unicode Standard.

3.3 版新增。

``` str. ``` ``` center ``` ( width [ , fillchar ] )

Return centered in a string of length width 。填充使用指定 fillchar (默认为 ASCII 空格)。返回原始字符串若 width 小于等于 ``` len(s) ``` .

``` str. ``` ``` count ``` ( sub [ , start [ , end ] ] )

Return the number of non-overlapping occurrences of substring sub in the range [ start , end ]。可选自变量 start and end are interpreted as in slice notation.

``` str. ``` ``` encode ``` ( encoding="utf-8" , errors="strict" )

Return an encoded version of the string as a bytes object. Default encoding is ``` 'utf-8' ``` . errors may be given to set a different error handling scheme. The default for errors is ``` 'strict' ``` , meaning that encoding errors raise a ``` UnicodeError ``` 。其它可能值包括 ``` 'ignore' ``` , ``` 'replace' ``` , ``` 'xmlcharrefreplace' ``` , ``` 'backslashreplace' ``` and any other name registered via ``` codecs.register_error() ``` ，见章节 错误处理程序 。对于可能的编码列表，见章节 标准编码 .

3.1 版改变： 添加支持关键词自变量。

3.9 版改变： errors 现在在开发模式和调试模式下有校验。

``` str. ``` ``` endswith ``` ( suffix [ , start [ , end ] ] )

``` str. ``` ``` expandtabs ``` ( tabsize=8 )

Return a copy of the string where all tab characters are replaced by one or more spaces, depending on the current column and the given tab size. Tab positions occur every tabsize characters (default is 8, giving tab positions at columns 0, 8, 16 and so on). To expand the string, the current column is set to zero and the string is examined character by character. If the character is a tab ( ``` \t ``` ), one or more space characters are inserted in the result until the current column is equal to the next tab position. (The tab character itself is not copied.) If the character is a newline ( ``` \n ``` ) 或返回 ( ``` \r ``` ), it is copied and the current column is reset to zero. Any other character is copied unchanged and the current column is incremented by one regardless of how the character is represented when printed.

```>>> '01\t012\t0123\t01234'.expandtabs()
'01      012     0123    01234'
>>> '01\t012\t0123\t01234'.expandtabs(4)
'01  012 0123    01234'
```
``` str. ``` ``` find ``` ( sub [ , start [ , end ] ] )

``` find() ``` 方法才应被使用，若需要知道位置为 sub 。要校验若 sub 是子字符串或不是，使用 ``` in ``` 运算符：

```>>> 'Py' in 'Python'
True
```
``` str. ``` ``` format ``` ( *args , **kwargs )

Perform a string formatting operation. The string on which this method is called can contain literal text or replacement fields delimited by braces ``` {} ``` . Each replacement field contains either the numeric index of a positional argument, or the name of a keyword argument. Returns a copy of the string where each replacement field is replaced with the string value of the corresponding argument.

```>>> "The sum of 1 + 2 is {0}".format(1+2)
'The sum of 1 + 2 is 3'
```

3.7 版改变： 当格式化数字采用 ``` n ``` 类型，函数设置临时 ``` LC_CTYPE ``` 区域设置到 ``` LC_NUMERIC ``` 区域设置在某些情况下。

``` str. ``` ``` format_map ``` ( mapping )

```>>> class Default(dict):
...     def __missing__(self, key):
...         return key
...
>>> '{name} was born in {country}'.format_map(Default(name='Guido'))
'Guido was born in country'
```

3.2 版新增。

``` str. ``` ``` index ``` ( sub [ , start [ , end ] ] )
``` str. ``` ``` isalnum ``` ( )

``` str. ``` ``` isalpha ``` ( )

``` str. ``` ``` isascii ``` ( )

3.7 版新增。

``` str. ``` ``` isdecimal ``` ( )

``` str. ``` ``` isdigit ``` ( )

``` str. ``` ``` isidentifier ``` ( )

```>>> from keyword import iskeyword
>>> 'hello'.isidentifier(), iskeyword('hello')
True, False
>>> 'def'.isidentifier(), iskeyword('def')
True, True
```
``` str. ``` ``` islower ``` ( )

``` str. ``` ``` isnumeric ``` ( )

``` str. ``` ``` isprintable ``` ( )

``` str. ``` ``` isspace ``` ( )

A character is whitespace if in the Unicode character database (see ``` unicodedata ``` ), either its general category is ``` Zs ``` (“Separator, space”), or its bidirectional class is one of ``` WS ``` , ``` B ``` ，或 ``` S ``` .

``` str. ``` ``` istitle ``` ( )

``` str. ``` ``` isupper ``` ( )

```>>> 'BANANA'.isupper()
True
>>> 'banana'.isupper()
False
>>> 'baNana'.isupper()
False
>>> ' '.isupper()
False
```
``` str. ``` ``` join ``` ( iterable )

Return a string which is the concatenation of the strings in iterable ``` TypeError ``` will be raised if there are any non-string values in iterable ，包括 ``` bytes ``` objects. The separator between elements is the string providing this method.

``` str. ``` ``` ljust ``` ( width [ , fillchar ] )

Return the string left justified in a string of length width 。填充使用指定 fillchar (默认为 ASCII 空格)。返回原始字符串若 width 小于等于 ``` len(s) ``` .

``` str. ``` ``` lower ``` ( )

Return a copy of the string with all the cased characters 4 converted to lowercase.

The lowercasing algorithm used is described in section 3.13 of the Unicode Standard.

``` str. ``` ``` lstrip ``` ( [ chars ] )

```>>> '   spacious   '.lstrip()
'spacious   '
>>> 'www.example.com'.lstrip('cmowz.')
'example.com'
```

``` str.removeprefix() ``` for a method that will remove a single prefix string rather than all of a set of characters. For example:

```>>> 'Arthur: three!'.lstrip('Arthur: ')
'ee!'
>>> 'Arthur: three!'.removeprefix('Arthur: ')
'three!'
```
static ``` str. ``` ``` maketrans ``` ( x [ , y [ , z ] ] )

This static method returns a translation table usable for ``` str.translate() ``` .

If there is only one argument, it must be a dictionary mapping Unicode ordinals (integers) or characters (strings of length 1) to Unicode ordinals, strings (of arbitrary lengths) or ``` None ``` . Character keys will then be converted to ordinals.

If there are two arguments, they must be strings of equal length, and in the resulting dictionary, each character in x will be mapped to the character at the same position in y. If there is a third argument, it must be a string, whose characters will be mapped to ``` None ``` in the result.

``` str. ``` ``` partition ``` ( sep )

Split the string at the first occurrence of sep , and return a 3-tuple containing the part before the separator, the separator itself, and the part after the separator. If the separator is not found, return a 3-tuple containing the string itself, followed by two empty strings.

``` str. ``` ``` removeprefix ``` ( prefix , / )

```>>> 'TestHook'.removeprefix('Test')
'Hook'
>>> 'BaseTestCase'.removeprefix('Test')
'BaseTestCase'
```

3.9 版新增。

``` str. ``` ``` removesuffix ``` ( suffix , / )

```>>> 'MiscTests'.removesuffix('Tests')
'Misc'
>>> 'TmpDirMixin'.removesuffix('Tests')
'TmpDirMixin'
```

3.9 版新增。

``` str. ``` ``` replace ``` ( old , new [ , count ] )

Return a copy of the string with all occurrences of substring old replaced by new . If the optional argument count is given, only the first count occurrences are replaced.

``` str. ``` ``` rfind ``` ( sub [ , start [ , end ] ] )

Return the highest index in the string where substring sub 被发现，这种 sub is contained within ``` s[start:end] ``` 。可选自变量 start and end 按切片表示法解释。返回 ``` -1 ``` 当故障时。

``` str. ``` ``` rindex ``` ( sub [ , start [ , end ] ] )
``` str. ``` ``` rjust ``` ( width [ , fillchar ] )

``` str. ``` ``` rpartition ``` ( sep )

Split the string at the last occurrence of sep , and return a 3-tuple containing the part before the separator, the separator itself, and the part after the separator. If the separator is not found, return a 3-tuple containing two empty strings, followed by the string itself.

``` str. ``` ``` rsplit ``` ( sep=None , maxsplit=-1 )

Return a list of the words in the string, using sep as the delimiter string. If maxsplit is given, at most maxsplit splits are done, the rightmost ones. If sep 未指定或 ``` None ``` , any whitespace string is a separator. Except for splitting from the right, ``` rsplit() ``` behaves like ``` split() ``` which is described in detail below.

``` str. ``` ``` rstrip ``` ( [ chars ] )

```>>> '   spacious   '.rstrip()
'   spacious'
>>> 'mississippi'.rstrip('ipz')
'mississ'
```

``` str.removesuffix() ``` for a method that will remove a single suffix string rather than all of a set of characters. For example:

```>>> 'Monty Python'.rstrip(' Python')
'M'
>>> 'Monty Python'.removesuffix(' Python')
'Monty'
```
``` str. ``` ``` split ``` ( sep=None , maxsplit=-1 )

Return a list of the words in the string, using sep as the delimiter string. If maxsplit is given, at most maxsplit splits are done (thus, the list will have at most ``` maxsplit+1 ``` elements). If maxsplit 未指定或 ``` -1 ``` , then there is no limit on the number of splits (all possible splits are made).

sep is given, consecutive delimiters are not grouped together and are deemed to delimit empty strings (for example, ``` '1,,2'.split(',') ``` 返回 ``` ['1', '', '2'] ``` )。 sep argument may consist of multiple characters (for example, ``` '1<>2<>3'.split('<>') ``` 返回 ``` ['1', '2', '3'] ``` ). Splitting an empty string with a specified separator returns ``` [''] ``` .

```>>> '1,2,3'.split(',')
['1', '2', '3']
>>> '1,2,3'.split(',', maxsplit=1)
['1', '2,3']
>>> '1,2,,3,'.split(',')
['1', '2', '', '3', '']
```

sep 未指定或是 ``` None ``` , a different splitting algorithm is applied: runs of consecutive whitespace are regarded as a single separator, and the result will contain no empty strings at the start or end if the string has leading or trailing whitespace. Consequently, splitting an empty string or a string consisting of just whitespace with a ``` None ``` separator returns ``` [] ``` .

```>>> '1 2 3'.split()
['1', '2', '3']
>>> '1 2 3'.split(maxsplit=1)
['1', '2 3']
>>> '   1   2   3   '.split()
['1', '2', '3']
```
``` str. ``` ``` splitlines ``` ( [ keepends ] )

Return a list of the lines in the string, breaking at line boundaries. Line breaks are not included in the resulting list unless keepends is given and true.

This method splits on the following line boundaries. In particular, the boundaries are a superset of 通用换行符 .

``` \n ```

``` \r ```

CR (回车)

``` \r\n ```

Carriage Return + Line Feed

``` \v ``` or ``` \x0b ```

Line Tabulation

``` \f ``` or ``` \x0c ```

Form Feed

``` \x1c ```

``` \x1d ```

``` \x1e ```

``` \x85 ```

Next Line (C1 Control Code)

``` \u2028 ```

``` \u2029 ```

3.2 版改变： ``` \v ``` and ``` \f ``` added to list of line boundaries.

```>>> 'ab c\n\nde fg\rkl\r\n'.splitlines()
['ab c', '', 'de fg', 'kl']
>>> 'ab c\n\nde fg\rkl\r\n'.splitlines(keepends=True)
['ab c\n', '\n', 'de fg\r', 'kl\r\n']
```

```>>> "".splitlines()
[]
>>> "One line\n".splitlines()
['One line']
```

```>>> ''.split('\n')
['']
>>> 'Two lines\n'.split('\n')
['Two lines', '']
```
``` str. ``` ``` startswith ``` ( prefix [ , start [ , end ] ] )

``` str. ``` ``` strip ``` ( [ chars ] )

```>>> '   spacious   '.strip()
'spacious'
>>> 'www.example.com'.strip('cmowz.')
'example'
```

```>>> comment_string = '#....... Section 3.2.1 Issue #32 .......'
>>> comment_string.strip('.#! ')
'Section 3.2.1 Issue #32'
```
``` str. ``` ``` swapcase ``` ( )

Return a copy of the string with uppercase characters converted to lowercase and vice versa. Note that it is not necessarily true that ``` s.swapcase().swapcase() == s ``` .

``` str. ``` ``` title ``` ( )

Return a titlecased version of the string where words start with an uppercase character and the remaining characters are lowercase.

```>>> 'Hello world'.title()
'Hello World'
```

The algorithm uses a simple language-independent definition of a word as groups of consecutive letters. The definition works in many contexts but it means that apostrophes in contractions and possessives form word boundaries, which may not be the desired result:

```>>> "they're bill's friends from the UK".title()
"They'Re Bill'S Friends From The Uk"
```

A workaround for apostrophes can be constructed using regular expressions:

```>>> import re
>>> def titlecase(s):
...     return re.sub(r"[A-Za-z]+('[A-Za-z]+)?",
...                   lambda mo: mo.group(0).capitalize(),
...                   s)
...
>>> titlecase("they're bill's friends.")
"They're Bill's Friends."
```
``` str. ``` ``` translate ``` ( table )

Return a copy of the string in which each character has been mapped through the given translation table. The table must be an object that implements indexing via ``` __getitem__() ``` , typically a 映射 or sequence . When indexed by a Unicode ordinal (an integer), the table object can do any of the following: return a Unicode ordinal or a string, to map the character to one or more other characters; return ``` None ``` , to delete the character from the return string; or raise a ``` LookupError ``` exception, to map the character to itself.

``` str. ``` ``` upper ``` ( )

Return a copy of the string with all the cased characters 4 converted to uppercase. Note that ``` s.upper().isupper() ``` might be ``` False ``` if ``` s ``` contains uncased characters or if the Unicode category of the resulting character(s) is not “Lu” (Letter, uppercase), but e.g. “Lt” (Letter, titlecase).

The uppercasing algorithm used is described in section 3.13 of the Unicode Standard.

``` str. ``` ``` zfill ``` ( width )

Return a copy of the string left filled with ASCII ``` '0' ``` digits to make a string of length width . A leading sign prefix ( ``` '+' ``` / ``` '-' ``` ) is handled by inserting the padding after the sign character rather than before. The original string is returned if width 小于等于 ``` len(s) ``` .

```>>> "42".zfill(5)
'00042'
>>> "-42".zfill(5)
'-0042'
```

### ``` printf ``` 样式字符串格式化 ¶

The formatting operations described here exhibit a variety of quirks that lead to a number of common errors (such as failing to display tuples and dictionaries correctly). Using the newer 格式化字符串文字 ``` str.format() ``` interface, or 模板字符串 may help avoid these errors. Each of these alternatives provides their own trade-offs and benefits of simplicity, flexibility, and/or extensibility.

String objects have one unique built-in operation: the ``` % ``` operator (modulo). This is also known as the string formatting or interpolation operator. Given ``` format % values ``` (在哪里 format 是字符串)， ``` % ``` conversion specifications in format are replaced with zero or more elements of values . The effect is similar to using the ``` sprintf() ``` 在 C 语言中。

format requires a single argument, values may be a single non-tuple object. 5 否则， values must be a tuple with exactly the number of items specified by the format string, or a single mapping object (for example, a dictionary).

A conversion specifier contains two or more characters and has the following components, which must occur in this order:

1. ``` '%' ``` character, which marks the start of the specifier.

2. Mapping key (optional), consisting of a parenthesised sequence of characters (for example, ``` (somename) ``` ).

3. Conversion flags (optional), which affect the result of some conversion types.

4. Minimum field width (optional). If specified as an ``` '*' ``` (asterisk), the actual width is read from the next element of the tuple in values , and the object to convert comes after the minimum field width and optional precision.

5. Precision (optional), given as a ``` '.' ``` (dot) followed by the precision. If specified as ``` '*' ``` (an asterisk), the actual precision is read from the next element of the tuple in values , and the value to convert comes after the precision.

6. Length modifier (optional).

7. 转换类型。

When the right argument is a dictionary (or other mapping type), then the formats in the string must include a parenthesised mapping key into that dictionary inserted immediately after the ``` '%' ``` character. The mapping key selects the value to be formatted from the mapping. For example:

```>>> print('%(language)s has %(number)03d quote types.' %
...       {'language': "Python", "number": 2})
Python has 002 quote types.
```

In this case no ``` * ``` specifiers may occur in a format (since they require a sequential parameter list).

Flag

``` '#' ```

The value conversion will use the “alternate form” (where defined below).

``` '0' ```

The conversion will be zero padded for numeric values.

``` '-' ```

The converted value is left adjusted (overrides the ``` '0' ``` conversion if both are given).

``` ' ' ```

(a space) A blank should be left before a positive number (or empty string) produced by a signed conversion.

``` '+' ```

A sign character ( ``` '+' ``` or ``` '-' ``` ) will precede the conversion (overrides a “space” flag).

A length modifier ( ``` h ``` , ``` l ``` ，或 ``` L ``` ) may be present, but is ignored as it is not necessary for Python – so e.g. ``` %ld ``` is identical to ``` %d ``` .

Conversion

``` 'd' ```

``` 'i' ```

``` 'o' ```

(1)

``` 'u' ```

Obsolete type – it is identical to ``` 'd' ``` .

(6)

``` 'x' ```

(2)

``` 'X' ```

(2)

``` 'e' ```

Floating point exponential format (lowercase).

(3)

``` 'E' ```

Floating point exponential format (uppercase).

(3)

``` 'f' ```

(3)

``` 'F' ```

(3)

``` 'g' ```

Floating point format. Uses lowercase exponential format if exponent is less than -4 or not less than precision, decimal format otherwise.

(4)

``` 'G' ```

Floating point format. Uses uppercase exponential format if exponent is less than -4 or not less than precision, decimal format otherwise.

(4)

``` 'c' ```

Single character (accepts integer or single character string).

``` 'r' ```

String (converts any Python object using ``` repr() ``` ).

(5)

``` 's' ```

String (converts any Python object using ``` str() ``` ).

(5)

``` 'a' ```

String (converts any Python object using ``` ascii() ``` ).

(5)

``` '%' ```

No argument is converted, results in a ``` '%' ``` character in the result.

1. The alternate form causes a leading octal specifier ( ``` '0o' ``` ) to be inserted before the first digit.

2. The alternate form causes a leading ``` '0x' ``` or ``` '0X' ``` (depending on whether the ``` 'x' ``` or ``` 'X' ``` format was used) to be inserted before the first digit.

3. The alternate form causes the result to always contain a decimal point, even if no digits follow it.

The precision determines the number of digits after the decimal point and defaults to 6.

4. The alternate form causes the result to always contain a decimal point, and trailing zeroes are not removed as they would otherwise be.

The precision determines the number of significant digits before and after the decimal point and defaults to 6.

5. If precision is ``` N ``` , the output is truncated to ``` N ``` 字符。

6. PEP 237 .

Since Python strings have an explicit length, ``` %s ``` conversions do not assume that ``` '\0' ``` is the end of the string.

3.1 版改变： ``` %f ``` conversions for numbers whose absolute value is over 1e50 are no longer replaced by ``` %g ``` conversions.

## 二进制序列类型 — ``` bytes ``` , ``` bytearray ``` , ``` memoryview ``` ¶

The core built-in types for manipulating binary data are ``` bytes ``` and ``` bytearray ``` . They are supported by ``` memoryview ``` 其使用 缓冲协议 to access the memory of other binary objects without needing to make a copy.

``` array ``` module supports efficient storage of basic data types like 32-bit integers and IEEE754 double-precision floating values.

### 字节对象 ¶

Bytes objects are immutable sequences of single bytes. Since many major binary protocols are based on the ASCII text encoding, bytes objects offer several methods that are only valid when working with ASCII compatible data and are closely related to string objects in a variety of other ways.

class ``` bytes ``` ( [ source [ , encoding [ , errors ] ] ] )

• 单引号： ``` b'still allows embedded "double" quotes' ```

• 双引号： ``` b"still allows embedded 'single' quotes" ``` .

• 三引号： ``` b'''3 single quotes''' ``` , ``` b"""3 double quotes""" ```

Only ASCII characters are permitted in bytes literals (regardless of the declared source code encoding). Any binary values over 127 must be entered into bytes literals using the appropriate escape sequence.

As with string literals, bytes literals may also use a ``` r ``` prefix to disable processing of escape sequences. See 字符串和 bytes 文字 for more about the various forms of bytes literal, including supported escape sequences.

While bytes literals and representations are based on ASCII text, bytes objects actually behave like immutable sequences of integers, with each value in the sequence restricted such that ``` 0 <= x < 256 ``` (attempts to violate this restriction will trigger ``` ValueError ``` ). This is done deliberately to emphasise that while many binary formats include ASCII based elements and can be usefully manipulated with some text-oriented algorithms, this is not generally the case for arbitrary binary data (blindly applying text processing algorithms to binary data formats that are not ASCII compatible will usually lead to data corruption).

In addition to the literal forms, bytes objects can be created in a number of other ways:

• A zero-filled bytes object of a specified length: ``` bytes(10) ```

• 来自整数迭代： ``` bytes(range(20)) ```

• Copying existing binary data via the buffer protocol: ``` bytes(obj) ```

Since 2 hexadecimal digits correspond precisely to a single byte, hexadecimal numbers are a commonly used format for describing binary data. Accordingly, the bytes type has an additional class method to read data in that format:

classmethod ``` fromhex ``` ( string )

This ``` bytes ``` class method returns a bytes object, decoding the given string object. The string must contain two hexadecimal digits per byte, with ASCII whitespace being ignored.

```>>> bytes.fromhex('2Ef0 F1f2  ')
b'.\xf0\xf1\xf2'
```

3.7 版改变： ``` bytes.fromhex() ``` 现在跳过字符串中的所有 ASCII 空白，不仅仅空格。

A reverse conversion function exists to transform a bytes object into its hexadecimal representation.

``` hex ``` ( [ sep [ , bytes_per_sep ] ] )

Return a string object containing two hexadecimal digits for each byte in the instance.

```>>> b'\xf0\xf1\xf2'.hex()
'f0f1f2'
```

If you want to make the hex string easier to read, you can specify a single character separator sep parameter to include in the output. By default between each byte. A second optional bytes_per_sep parameter controls the spacing. Positive values calculate the separator position from the right, negative values from the left.

```>>> value = b'\xf0\xf1\xf2'
>>> value.hex('-')
'f0-f1-f2'
>>> value.hex('_', 2)
'f0_f1f2'
>>> b'UUDDLRLRAB'.hex(' ', -4)
'55554444 4c524c52 4142'
```

3.5 版新增。

3.8 版改变： ``` bytes.hex() ``` 现在支持可选 sep and bytes_per_sep parameters to insert separators between bytes in the hex output.

Since bytes objects are sequences of integers (akin to a tuple), for a bytes object b , ``` b ``` will be an integer, while ``` b[0:1] ``` will be a bytes object of length 1. (This contrasts with text strings, where both indexing and slicing will produce a string of length 1)

The representation of bytes objects uses the literal format ( ``` b'...' ``` ) since it is often more useful than e.g. ``` bytes([46, 46, 46]) ``` . You can always convert a bytes object into a list of integers using ``` list(b) ``` .

For Python 2.x users: In the Python 2.x series, a variety of implicit conversions between 8-bit strings (the closest thing 2.x offers to a built-in binary data type) and Unicode strings were permitted. This was a backwards compatibility workaround to account for the fact that Python originally only supported 8-bit text, and Unicode text was a later addition. In Python 3.x, those implicit conversions are gone - conversions between 8-bit binary data and Unicode text must be explicit, and bytes and string objects will always compare unequal.

### bytearray 对象 ¶

class ``` bytearray ``` ( [ source [ , encoding [ , errors ] ] ] )

There is no dedicated literal syntax for bytearray objects, instead they are always created by calling the constructor:

• 创建空实例： ``` bytearray() ```

• Creating a zero-filled instance with a given length: ``` bytearray(10) ```

• 来自整数迭代： ``` bytearray(range(20)) ```

• Copying existing binary data via the buffer protocol: ``` bytearray(b'Hi!') ```

As bytearray objects are mutable, they support the 可变 sequence operations in addition to the common bytes and bytearray operations described in bytes 和 bytearray 操作 .

Since 2 hexadecimal digits correspond precisely to a single byte, hexadecimal numbers are a commonly used format for describing binary data. Accordingly, the bytearray type has an additional class method to read data in that format:

classmethod ``` fromhex ``` ( string )

This ``` bytearray ``` class method returns bytearray object, decoding the given string object. The string must contain two hexadecimal digits per byte, with ASCII whitespace being ignored.

```>>> bytearray.fromhex('2Ef0 F1f2  ')
bytearray(b'.\xf0\xf1\xf2')
```

3.7 版改变： ``` bytearray.fromhex() ``` 现在跳过字符串中的所有 ASCII 空白，不仅仅空格。

A reverse conversion function exists to transform a bytearray object into its hexadecimal representation.

``` hex ``` ( [ sep [ , bytes_per_sep ] ] )

Return a string object containing two hexadecimal digits for each byte in the instance.

```>>> bytearray(b'\xf0\xf1\xf2').hex()
'f0f1f2'
```

3.5 版新增。

3.8 版改变： 类似 ``` bytes.hex() ``` , ``` bytearray.hex() ``` 现在支持可选 sep and bytes_per_sep parameters to insert separators between bytes in the hex output.

Since bytearray objects are sequences of integers (akin to a list), for a bytearray object b , ``` b ``` will be an integer, while ``` b[0:1] ``` will be a bytearray object of length 1. (This contrasts with text strings, where both indexing and slicing will produce a string of length 1)

The representation of bytearray objects uses the bytes literal format ( ``` bytearray(b'...') ``` ) since it is often more useful than e.g. ``` bytearray([46, 46, 46]) ``` . You can always convert a bytearray object into a list of integers using ``` list(b) ``` .

### bytes 和 bytearray 操作 ¶

Both bytes and bytearray objects support the common sequence operations. They interoperate not just with operands of the same type, but with any 像字节对象 . Due to this flexibility, they can be freely mixed in operations without causing errors. However, the return type of the result may depend on the order of operands.

The methods on bytes and bytearray objects don’t accept strings as their arguments, just as the methods on strings don’t accept bytes as their arguments. For example, you have to write:

```a = "abc"
b = a.replace("a", "f")
```

and:

```a = b"abc"
b = a.replace(b"a", b"f")
```

Some bytes and bytearray operations assume the use of ASCII compatible binary formats, and hence should be avoided when working with arbitrary binary data. These restrictions are covered below.

Using these ASCII based operations to manipulate binary data that is not stored in an ASCII based format may lead to data corruption.

The following methods on bytes and bytearray objects can be used with arbitrary binary data.

`bytes.` ``` count ``` ( sub [ , start [ , end ] ] )
``` bytearray. ``` ``` count ``` ( sub [ , start [ , end ] ] )

Return the number of non-overlapping occurrences of subsequence sub in the range [ start , end ]。可选自变量 start and end are interpreted as in slice notation.

The subsequence to search for may be any 像字节对象 or an integer in the range 0 to 255.

3.3 版改变： Also accept an integer in the range 0 to 255 as the subsequence.

`bytes.` ``` removeprefix ``` ( prefix , / )
``` bytearray. ``` ``` removeprefix ``` ( prefix , / )

```>>> b'TestHook'.removeprefix(b'Test')
b'Hook'
>>> b'BaseTestCase'.removeprefix(b'Test')
b'BaseTestCase'
```

prefix 可以是任何 像字节对象 .

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

3.9 版新增。

`bytes.` ``` removesuffix ``` ( suffix , / )
``` bytearray. ``` ``` removesuffix ``` ( suffix , / )

If the binary data ends with the suffix 字符串和 suffix 不为空，返回 ``` bytes[:-len(suffix)] ``` . Otherwise, return a copy of the original binary data:

```>>> b'MiscTests'.removesuffix(b'Tests')
b'Misc'
>>> b'TmpDirMixin'.removesuffix(b'Tests')
b'TmpDirMixin'
```

suffix 可以是任何 像字节对象 .

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

3.9 版新增。

`bytes.` ``` decode ``` ( encoding="utf-8" , errors="strict" )
``` bytearray. ``` ``` decode ``` ( encoding="utf-8" , errors="strict" )

Return a string decoded from the given bytes. Default encoding is ``` 'utf-8' ``` . errors may be given to set a different error handling scheme. The default for errors is ``` 'strict' ``` , meaning that encoding errors raise a ``` UnicodeError ``` 。其它可能值包括 ``` 'ignore' ``` , ``` 'replace' ``` and any other name registered via ``` codecs.register_error() ``` ，见章节 错误处理程序 。对于可能的编码列表，见章节 标准编码 .

Passing the encoding 自变量为 ``` str ``` 允许解码任何 像字节对象 directly, without needing to make a temporary bytes or bytearray object.

3.1 版改变： 添加支持关键词自变量。

3.9 版改变： errors 现在在开发模式和调试模式下有校验。

`bytes.` ``` endswith ``` ( suffix [ , start [ , end ] ] )
``` bytearray. ``` ``` endswith ``` ( suffix [ , start [ , end ] ] )

The suffix(es) to search for may be any 像字节对象 .

`bytes.` ``` find ``` ( sub [ , start [ , end ] ] )
``` bytearray. ``` ``` find ``` ( sub [ , start [ , end ] ] )

Return the lowest index in the data where the subsequence sub 被发现，这种 sub is contained in the slice ``` s[start:end] ``` 。可选自变量 start and end 按切片表示法解释。返回 ``` -1 ``` if sub 找不到。

The subsequence to search for may be any 像字节对象 or an integer in the range 0 to 255.

``` find() ``` 方法才应被使用，若需要知道位置为 sub 。要校验若 sub 是子字符串或不是，使用 ``` in ``` 运算符：

```>>> b'Py' in b'Python'
True
```

3.3 版改变： Also accept an integer in the range 0 to 255 as the subsequence.

`bytes.` ``` index ``` ( sub [ , start [ , end ] ] )
``` bytearray. ``` ``` index ``` ( sub [ , start [ , end ] ] )

``` find() ``` ，但会引发 ``` ValueError ``` when the subsequence is not found.

The subsequence to search for may be any 像字节对象 or an integer in the range 0 to 255.

3.3 版改变： Also accept an integer in the range 0 to 255 as the subsequence.

`bytes.` ``` join ``` ( iterable )
``` bytearray. ``` ``` join ``` ( iterable )

Return a bytes or bytearray object which is the concatenation of the binary data sequences in iterable ``` TypeError ``` will be raised if there are any values in iterable that are not 像字节对象 ，包括 ``` str ``` objects. The separator between elements is the contents of the bytes or bytearray object providing this method.

static `bytes.` ``` maketrans ``` ( from , to )
static ``` bytearray. ``` ``` maketrans ``` ( from , to )

This static method returns a translation table usable for ``` bytes.translate() ``` that will map each character in from into the character at the same position in to ; from and to must both be 像字节对象 and have the same length.

3.1 版新增。

`bytes.` ``` partition ``` ( sep )
``` bytearray. ``` ``` partition ``` ( sep )

Split the sequence at the first occurrence of sep , and return a 3-tuple containing the part before the separator, the separator itself or its bytearray copy, and the part after the separator. If the separator is not found, return a 3-tuple containing a copy of the original sequence, followed by two empty bytes or bytearray objects.

The separator to search for may be any 像字节对象 .

`bytes.` ``` replace ``` ( old , new [ , count ] )
``` bytearray. ``` ``` replace ``` ( old , new [ , count ] )

Return a copy of the sequence with all occurrences of subsequence old replaced by new . If the optional argument count is given, only the first count occurrences are replaced.

The subsequence to search for and its replacement may be any 像字节对象 .

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

`bytes.` ``` rfind ``` ( sub [ , start [ , end ] ] )
``` bytearray. ``` ``` rfind ``` ( sub [ , start [ , end ] ] )

Return the highest index in the sequence where the subsequence sub 被发现，这种 sub is contained within ``` s[start:end] ``` 。可选自变量 start and end 按切片表示法解释。返回 ``` -1 ``` 当故障时。

The subsequence to search for may be any 像字节对象 or an integer in the range 0 to 255.

3.3 版改变： Also accept an integer in the range 0 to 255 as the subsequence.

`bytes.` ``` rindex ``` ( sub [ , start [ , end ] ] )
``` bytearray. ``` ``` rindex ``` ( sub [ , start [ , end ] ] )

``` rfind() ``` 但引发 ``` ValueError ``` when the subsequence sub 找不到。

The subsequence to search for may be any 像字节对象 or an integer in the range 0 to 255.

3.3 版改变： Also accept an integer in the range 0 to 255 as the subsequence.

`bytes.` ``` rpartition ``` ( sep )
``` bytearray. ``` ``` rpartition ``` ( sep )

Split the sequence at the last occurrence of sep , and return a 3-tuple containing the part before the separator, the separator itself or its bytearray copy, and the part after the separator. If the separator is not found, return a 3-tuple containing two empty bytes or bytearray objects, followed by a copy of the original sequence.

The separator to search for may be any 像字节对象 .

`bytes.` ``` startswith ``` ( prefix [ , start [ , end ] ] )
``` bytearray. ``` ``` startswith ``` ( prefix [ , start [ , end ] ] )

The prefix(es) to search for may be any 像字节对象 .

`bytes.` ``` translate ``` ( table , / , delete=b'' )
``` bytearray. ``` ``` translate ``` ( table , / , delete=b'' )

Return a copy of the bytes or bytearray object where all bytes occurring in the optional argument delete are removed, and the remaining bytes have been mapped through the given translation table, which must be a bytes object of length 256.

```>>> b'read this short text'.translate(None, b'aeiou')
b'rd ths shrt txt'
```

3.6 版改变： delete is now supported as a keyword argument.

The following methods on bytes and bytearray objects have default behaviours that assume the use of ASCII compatible binary formats, but can still be used with arbitrary binary data by passing appropriate arguments. Note that all of the bytearray methods in this section do not operate in place, and instead produce new objects.

`bytes.` ``` center ``` ( width [ , fillbyte ] )
``` bytearray. ``` ``` center ``` ( width [ , fillbyte ] )

Return a copy of the object centered in a sequence of length width 。填充使用指定 fillbyte (default is an ASCII space). For ``` bytes ``` objects, the original sequence is returned if width 小于等于 ``` len(s) ``` .

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

`bytes.` ``` ljust ``` ( width [ , fillbyte ] )
``` bytearray. ``` ``` ljust ``` ( width [ , fillbyte ] )

Return a copy of the object left justified in a sequence of length width 。填充使用指定 fillbyte (default is an ASCII space). For ``` bytes ``` objects, the original sequence is returned if width 小于等于 ``` len(s) ``` .

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

`bytes.` ``` lstrip ``` ( [ chars ] )
``` bytearray. ``` ``` lstrip ``` ( [ chars ] )

Return a copy of the sequence with specified leading bytes removed. The chars argument is a binary sequence specifying the set of byte values to be removed - the name refers to the fact this method is usually used with ASCII characters. If omitted or ``` None ``` chars argument defaults to removing ASCII whitespace. The chars 自变量不是前缀；在一定程度上，会剥离其值的所有组合：

```>>> b'   spacious   '.lstrip()
b'spacious   '
>>> b'www.example.com'.lstrip(b'cmowz.')
b'example.com'
```

The binary sequence of byte values to remove may be any 像字节对象 。见 ``` removeprefix() ``` for a method that will remove a single prefix string rather than all of a set of characters. For example:

```>>> b'Arthur: three!'.lstrip(b'Arthur: ')
b'ee!'
>>> b'Arthur: three!'.removeprefix(b'Arthur: ')
b'three!'
```

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

`bytes.` ``` rjust ``` ( width [ , fillbyte ] )
``` bytearray. ``` ``` rjust ``` ( width [ , fillbyte ] )

Return a copy of the object right justified in a sequence of length width 。填充使用指定 fillbyte (default is an ASCII space). For ``` bytes ``` objects, the original sequence is returned if width 小于等于 ``` len(s) ``` .

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

`bytes.` ``` rsplit ``` ( sep=None , maxsplit=-1 )
``` bytearray. ``` ``` rsplit ``` ( sep=None , maxsplit=-1 )

Split the binary sequence into subsequences of the same type, using sep as the delimiter string. If maxsplit is given, at most maxsplit splits are done, the rightmost ones. If sep 未指定或 ``` None ``` , any subsequence consisting solely of ASCII whitespace is a separator. Except for splitting from the right, ``` rsplit() ``` behaves like ``` split() ``` which is described in detail below.

`bytes.` ``` rstrip ``` ( [ chars ] )
``` bytearray. ``` ``` rstrip ``` ( [ chars ] )

Return a copy of the sequence with specified trailing bytes removed. The chars argument is a binary sequence specifying the set of byte values to be removed - the name refers to the fact this method is usually used with ASCII characters. If omitted or ``` None ``` chars argument defaults to removing ASCII whitespace. The chars 自变量不是后缀；在一定程度上，会剥离其值的所有组合：

```>>> b'   spacious   '.rstrip()
b'   spacious'
>>> b'mississippi'.rstrip(b'ipz')
b'mississ'
```

The binary sequence of byte values to remove may be any 像字节对象 。见 ``` removesuffix() ``` for a method that will remove a single suffix string rather than all of a set of characters. For example:

```>>> b'Monty Python'.rstrip(b' Python')
b'M'
>>> b'Monty Python'.removesuffix(b' Python')
b'Monty'
```

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

`bytes.` ``` split ``` ( sep=None , maxsplit=-1 )
``` bytearray. ``` ``` split ``` ( sep=None , maxsplit=-1 )

Split the binary sequence into subsequences of the same type, using sep as the delimiter string. If maxsplit is given and non-negative, at most maxsplit splits are done (thus, the list will have at most ``` maxsplit+1 ``` elements). If maxsplit 未指定或是 ``` -1 ``` , then there is no limit on the number of splits (all possible splits are made).

sep is given, consecutive delimiters are not grouped together and are deemed to delimit empty subsequences (for example, ``` b'1,,2'.split(b',') ``` 返回 ``` [b'1', b'', b'2'] ``` )。 sep argument may consist of a multibyte sequence (for example, ``` b'1<>2<>3'.split(b'<>') ``` 返回 ``` [b'1', b'2', b'3'] ``` ). Splitting an empty sequence with a specified separator returns ``` [b''] ``` or ``` [bytearray(b'')] ``` depending on the type of object being split. The sep argument may be any 像字节对象 .

```>>> b'1,2,3'.split(b',')
[b'1', b'2', b'3']
>>> b'1,2,3'.split(b',', maxsplit=1)
[b'1', b'2,3']
>>> b'1,2,,3,'.split(b',')
[b'1', b'2', b'', b'3', b'']
```

sep 未指定或是 ``` None ``` , a different splitting algorithm is applied: runs of consecutive ASCII whitespace are regarded as a single separator, and the result will contain no empty strings at the start or end if the sequence has leading or trailing whitespace. Consequently, splitting an empty sequence or a sequence consisting solely of ASCII whitespace without a specified separator returns ``` [] ``` .

```>>> b'1 2 3'.split()
[b'1', b'2', b'3']
>>> b'1 2 3'.split(maxsplit=1)
[b'1', b'2 3']
>>> b'   1   2   3   '.split()
[b'1', b'2', b'3']
```
`bytes.` ``` strip ``` ( [ chars ] )
``` bytearray. ``` ``` strip ``` ( [ chars ] )

Return a copy of the sequence with specified leading and trailing bytes removed. The chars argument is a binary sequence specifying the set of byte values to be removed - the name refers to the fact this method is usually used with ASCII characters. If omitted or ``` None ``` chars argument defaults to removing ASCII whitespace. The chars 自变量不是前缀 (或后缀)；在一定程度上，会剥离其值的所有组合：

```>>> b'   spacious   '.strip()
b'spacious'
>>> b'www.example.com'.strip(b'cmowz.')
b'example'
```

The binary sequence of byte values to remove may be any 像字节对象 .

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

The following methods on bytes and bytearray objects assume the use of ASCII compatible binary formats and should not be applied to arbitrary binary data. Note that all of the bytearray methods in this section do not operate in place, and instead produce new objects.

`bytes.` ``` capitalize ``` ( )
``` bytearray. ``` ``` capitalize ``` ( )

Return a copy of the sequence with each byte interpreted as an ASCII character, and the first byte capitalized and the rest lowercased. Non-ASCII byte values are passed through unchanged.

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

`bytes.` ``` expandtabs ``` ( tabsize=8 )
``` bytearray. ``` ``` expandtabs ``` ( tabsize=8 )

Return a copy of the sequence where all ASCII tab characters are replaced by one or more ASCII spaces, depending on the current column and the given tab size. Tab positions occur every tabsize bytes (default is 8, giving tab positions at columns 0, 8, 16 and so on). To expand the sequence, the current column is set to zero and the sequence is examined byte by byte. If the byte is an ASCII tab character ( ``` b'\t' ``` ), one or more space characters are inserted in the result until the current column is equal to the next tab position. (The tab character itself is not copied.) If the current byte is an ASCII newline ( ``` b'\n' ``` ) or carriage return ( ``` b'\r' ``` ), it is copied and the current column is reset to zero. Any other byte value is copied unchanged and the current column is incremented by one regardless of how the byte value is represented when printed:

```>>> b'01\t012\t0123\t01234'.expandtabs()
b'01      012     0123    01234'
>>> b'01\t012\t0123\t01234'.expandtabs(4)
b'01  012 0123    01234'
```

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

`bytes.` ``` isalnum ``` ( )
``` bytearray. ``` ``` isalnum ``` ( )

```>>> b'ABCabc1'.isalnum()
True
>>> b'ABC abc1'.isalnum()
False
```
`bytes.` ``` isalpha ``` ( )
``` bytearray. ``` ``` isalpha ``` ( )

```>>> b'ABCabc'.isalpha()
True
>>> b'ABCabc1'.isalpha()
False
```
`bytes.` ``` isascii ``` ( )
``` bytearray. ``` ``` isascii ``` ( )

3.7 版新增。

`bytes.` ``` isdigit ``` ( )
``` bytearray. ``` ``` isdigit ``` ( )

```>>> b'1234'.isdigit()
True
>>> b'1.23'.isdigit()
False
```
`bytes.` ``` islower ``` ( )
``` bytearray. ``` ``` islower ``` ( )

```>>> b'hello world'.islower()
True
>>> b'Hello world'.islower()
False
```

Lowercase ASCII characters are those byte values in the sequence ``` b'abcdefghijklmnopqrstuvwxyz' ``` . Uppercase ASCII characters are those byte values in the sequence ``` b'ABCDEFGHIJKLMNOPQRSTUVWXYZ' ``` .

`bytes.` ``` isspace ``` ( )
``` bytearray. ``` ``` isspace ``` ( )

`bytes.` ``` istitle ``` ( )
``` bytearray. ``` ``` istitle ``` ( )

```>>> b'Hello World'.istitle()
True
>>> b'Hello world'.istitle()
False
```
`bytes.` ``` isupper ``` ( )
``` bytearray. ``` ``` isupper ``` ( )

```>>> b'HELLO WORLD'.isupper()
True
>>> b'Hello world'.isupper()
False
```

Lowercase ASCII characters are those byte values in the sequence ``` b'abcdefghijklmnopqrstuvwxyz' ``` . Uppercase ASCII characters are those byte values in the sequence ``` b'ABCDEFGHIJKLMNOPQRSTUVWXYZ' ``` .

`bytes.` ``` lower ``` ( )
``` bytearray. ``` ``` lower ``` ( )

Return a copy of the sequence with all the uppercase ASCII characters converted to their corresponding lowercase counterpart.

```>>> b'Hello World'.lower()
b'hello world'
```

Lowercase ASCII characters are those byte values in the sequence ``` b'abcdefghijklmnopqrstuvwxyz' ``` . Uppercase ASCII characters are those byte values in the sequence ``` b'ABCDEFGHIJKLMNOPQRSTUVWXYZ' ``` .

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

`bytes.` ``` splitlines ``` ( keepends=False )
``` bytearray. ``` ``` splitlines ``` ( keepends=False )

Return a list of the lines in the binary sequence, breaking at ASCII line boundaries. This method uses the 通用换行符 approach to splitting lines. Line breaks are not included in the resulting list unless keepends is given and true.

```>>> b'ab c\n\nde fg\rkl\r\n'.splitlines()
[b'ab c', b'', b'de fg', b'kl']
>>> b'ab c\n\nde fg\rkl\r\n'.splitlines(keepends=True)
[b'ab c\n', b'\n', b'de fg\r', b'kl\r\n']
```

```>>> b"".split(b'\n'), b"Two lines\n".split(b'\n')
([b''], [b'Two lines', b''])
>>> b"".splitlines(), b"One line\n".splitlines()
([], [b'One line'])
```
`bytes.` ``` swapcase ``` ( )
``` bytearray. ``` ``` swapcase ``` ( )

Return a copy of the sequence with all the lowercase ASCII characters converted to their corresponding uppercase counterpart and vice-versa.

```>>> b'Hello World'.swapcase()
b'hELLO wORLD'
```

Lowercase ASCII characters are those byte values in the sequence ``` b'abcdefghijklmnopqrstuvwxyz' ``` . Uppercase ASCII characters are those byte values in the sequence ``` b'ABCDEFGHIJKLMNOPQRSTUVWXYZ' ``` .

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

`bytes.` ``` title ``` ( )
``` bytearray. ``` ``` title ``` ( )

Return a titlecased version of the binary sequence where words start with an uppercase ASCII character and the remaining characters are lowercase. Uncased byte values are left unmodified.

```>>> b'Hello world'.title()
b'Hello World'
```

Lowercase ASCII characters are those byte values in the sequence ``` b'abcdefghijklmnopqrstuvwxyz' ``` . Uppercase ASCII characters are those byte values in the sequence ``` b'ABCDEFGHIJKLMNOPQRSTUVWXYZ' ``` . All other byte values are uncased.

The algorithm uses a simple language-independent definition of a word as groups of consecutive letters. The definition works in many contexts but it means that apostrophes in contractions and possessives form word boundaries, which may not be the desired result:

```>>> b"they're bill's friends from the UK".title()
b"They'Re Bill'S Friends From The Uk"
```

A workaround for apostrophes can be constructed using regular expressions:

```>>> import re
>>> def titlecase(s):
...     return re.sub(rb"[A-Za-z]+('[A-Za-z]+)?",
...                   lambda mo: mo.group(0)[0:1].upper() +
...                              mo.group(0)[1:].lower(),
...                   s)
...
>>> titlecase(b"they're bill's friends.")
b"They're Bill's Friends."
```

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

`bytes.` ``` upper ``` ( )
``` bytearray. ``` ``` upper ``` ( )

Return a copy of the sequence with all the lowercase ASCII characters converted to their corresponding uppercase counterpart.

```>>> b'Hello World'.upper()
b'HELLO WORLD'
```

Lowercase ASCII characters are those byte values in the sequence ``` b'abcdefghijklmnopqrstuvwxyz' ``` . Uppercase ASCII characters are those byte values in the sequence ``` b'ABCDEFGHIJKLMNOPQRSTUVWXYZ' ``` .

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

`bytes.` ``` zfill ``` ( width )
``` bytearray. ``` ``` zfill ``` ( width )

Return a copy of the sequence left filled with ASCII ``` b'0' ``` digits to make a sequence of length width . A leading sign prefix ( ``` b'+' ``` / ``` b'-' ``` ) is handled by inserting the padding after the sign character rather than before. For ``` bytes ``` objects, the original sequence is returned if width 小于等于 ``` len(seq) ``` .

```>>> b"42".zfill(5)
b'00042'
>>> b"-42".zfill(5)
b'-0042'
```

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

### ``` printf ``` 样式字节格式化 ¶

The formatting operations described here exhibit a variety of quirks that lead to a number of common errors (such as failing to display tuples and dictionaries correctly). If the value being printed may be a tuple or dictionary, wrap it in a tuple.

Bytes objects ( ``` bytes ``` / ``` bytearray ``` ) have one unique built-in operation: the ``` % ``` operator (modulo). This is also known as the bytes formatting or interpolation operator. Given ``` format % values ``` (在哪里 format is a bytes object), ``` % ``` conversion specifications in format are replaced with zero or more elements of values . The effect is similar to using the ``` sprintf() ``` 在 C 语言中。

format requires a single argument, values may be a single non-tuple object. 5 否则， values must be a tuple with exactly the number of items specified by the format bytes object, or a single mapping object (for example, a dictionary).

A conversion specifier contains two or more characters and has the following components, which must occur in this order:

1. ``` '%' ``` character, which marks the start of the specifier.

2. Mapping key (optional), consisting of a parenthesised sequence of characters (for example, ``` (somename) ``` ).

3. Conversion flags (optional), which affect the result of some conversion types.

4. Minimum field width (optional). If specified as an ``` '*' ``` (asterisk), the actual width is read from the next element of the tuple in values , and the object to convert comes after the minimum field width and optional precision.

5. Precision (optional), given as a ``` '.' ``` (dot) followed by the precision. If specified as ``` '*' ``` (an asterisk), the actual precision is read from the next element of the tuple in values , and the value to convert comes after the precision.

6. Length modifier (optional).

7. 转换类型。

When the right argument is a dictionary (or other mapping type), then the formats in the bytes object must include a parenthesised mapping key into that dictionary inserted immediately after the ``` '%' ``` character. The mapping key selects the value to be formatted from the mapping. For example:

```>>> print(b'%(language)s has %(number)03d quote types.' %
...       {b'language': b"Python", b"number": 2})
b'Python has 002 quote types.'
```

In this case no ``` * ``` specifiers may occur in a format (since they require a sequential parameter list).

Flag

``` '#' ```

The value conversion will use the “alternate form” (where defined below).

``` '0' ```

The conversion will be zero padded for numeric values.

``` '-' ```

The converted value is left adjusted (overrides the ``` '0' ``` conversion if both are given).

``` ' ' ```

(a space) A blank should be left before a positive number (or empty string) produced by a signed conversion.

``` '+' ```

A sign character ( ``` '+' ``` or ``` '-' ``` ) will precede the conversion (overrides a “space” flag).

A length modifier ( ``` h ``` , ``` l ``` ，或 ``` L ``` ) may be present, but is ignored as it is not necessary for Python – so e.g. ``` %ld ``` is identical to ``` %d ``` .

Conversion

``` 'd' ```

``` 'i' ```

``` 'o' ```

(1)

``` 'u' ```

Obsolete type – it is identical to ``` 'd' ``` .

(8)

``` 'x' ```

(2)

``` 'X' ```

(2)

``` 'e' ```

Floating point exponential format (lowercase).

(3)

``` 'E' ```

Floating point exponential format (uppercase).

(3)

``` 'f' ```

(3)

``` 'F' ```

(3)

``` 'g' ```

Floating point format. Uses lowercase exponential format if exponent is less than -4 or not less than precision, decimal format otherwise.

(4)

``` 'G' ```

Floating point format. Uses uppercase exponential format if exponent is less than -4 or not less than precision, decimal format otherwise.

(4)

``` 'c' ```

Single byte (accepts integer or single byte objects).

``` 'b' ```

Bytes (any object that follows the 缓冲协议 or has ``` __bytes__() ``` ).

(5)

``` 's' ```

``` 's' ``` 是别名化的 ``` 'b' ``` and should only be used for Python2/3 code bases.

(6)

``` 'a' ```

Bytes (converts any Python object using ``` repr(obj).encode('ascii','backslashreplace) ``` ).

(5)

``` 'r' ```

``` 'r' ``` 是别名化的 ``` 'a' ``` and should only be used for Python2/3 code bases.

(7)

``` '%' ```

No argument is converted, results in a ``` '%' ``` character in the result.

1. The alternate form causes a leading octal specifier ( ``` '0o' ``` ) to be inserted before the first digit.

2. The alternate form causes a leading ``` '0x' ``` or ``` '0X' ``` (depending on whether the ``` 'x' ``` or ``` 'X' ``` format was used) to be inserted before the first digit.

3. The alternate form causes the result to always contain a decimal point, even if no digits follow it.

The precision determines the number of digits after the decimal point and defaults to 6.

4. The alternate form causes the result to always contain a decimal point, and trailing zeroes are not removed as they would otherwise be.

The precision determines the number of significant digits before and after the decimal point and defaults to 6.

5. If precision is ``` N ``` , the output is truncated to ``` N ``` 字符。

6. ``` b'%s' ``` is deprecated, but will not be removed during the 3.x series.

7. ``` b'%r' ``` is deprecated, but will not be removed during the 3.x series.

8. PEP 237 .

The bytearray version of this method does not operate in place - it always produces a new object, even if no changes were made.

PEP 461 - Adding % formatting to bytes and bytearray

3.5 版新增。

### 内存视图 ¶

``` memoryview ``` objects allow Python code to access the internal data of an object that supports the 缓冲协议 without copying.

class ``` memoryview ``` ( object )

A ``` memoryview ``` has the notion of an element , which is the atomic memory unit handled by the originating object . For many simple types such as ``` bytes ``` and ``` bytearray ``` , an element is a single byte, but other types such as ``` array.array ``` may have bigger elements.

``` len(view) ``` is equal to the length of ``` tolist ``` 。若 ``` view.ndim = 0 ``` , the length is 1. If ``` view.ndim = 1 ``` , the length is equal to the number of elements in the view. For higher dimensions, the length is equal to the length of the nested list representation of the view. The ``` itemsize ``` attribute will give you the number of bytes in a single element.

A ``` memoryview ``` supports slicing and indexing to expose its data. One-dimensional slicing will result in a subview:

```>>> v = memoryview(b'abcefg')
>>> v
98
>>> v[-1]
103
>>> v[1:4]
<memory at 0x7f3ddc9f4350>
>>> bytes(v[1:4])
b'bce'
```

``` format ``` is one of the native format specifiers from the ``` struct ``` module, indexing with an integer or a tuple of integers is also supported and returns a single element with the correct type. One-dimensional memoryviews can be indexed with an integer or a one-integer tuple. Multi-dimensional memoryviews can be indexed with tuples of exactly ndim integers where ndim is the number of dimensions. Zero-dimensional memoryviews can be indexed with the empty tuple.

Here is an example with a non-byte format:

```>>> import array
>>> a = array.array('l', [-11111111, 22222222, -33333333, 44444444])
>>> m = memoryview(a)
>>> m
-11111111
>>> m[-1]
44444444
>>> m[::2].tolist()
[-11111111, -33333333]
```

If the underlying object is writable, the memoryview supports one-dimensional slice assignment. Resizing is not allowed:

```>>> data = bytearray(b'abcefg')
>>> v = memoryview(data)
False
>>> v = ord(b'z')
>>> data
bytearray(b'zbcefg')
>>> v[1:4] = b'123'
>>> data
bytearray(b'z123fg')
>>> v[2:3] = b'spam'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: memoryview assignment: lvalue and rvalue have different structures
>>> v[2:6] = b'spam'
>>> data
bytearray(b'z1spam')
```

One-dimensional memoryviews of hashable (read-only) types with formats ‘B’, ‘b’ or ‘c’ are also hashable. The hash is defined as ``` hash(m) == hash(m.tobytes()) ``` :

```>>> v = memoryview(b'abcefg')
>>> hash(v) == hash(b'abcefg')
True
>>> hash(v[2:4]) == hash(b'ce')
True
>>> hash(v[::-2]) == hash(b'abcefg'[::-2])
True
```

3.3 版改变： One-dimensional memoryviews can now be sliced. One-dimensional memoryviews with formats ‘B’, ‘b’ or ‘c’ are now hashable.

3.4 版改变： memoryview is now registered automatically with ``` collections.abc.Sequence ```

3.5 版改变： memoryviews can now be indexed with tuple of integers.

``` __eq__ ``` ( exporter )

A memoryview and a PEP 3118 exporter are equal if their shapes are equivalent and if all corresponding values are equal when the operands’ respective format codes are interpreted using ``` struct ``` 句法。

For the subset of ``` struct ``` format strings currently supported by ``` tolist() ``` , ``` v ``` and ``` w ``` are equal if ``` v.tolist() == w.tolist() ``` :

```>>> import array
>>> a = array.array('I', [1, 2, 3, 4, 5])
>>> b = array.array('d', [1.0, 2.0, 3.0, 4.0, 5.0])
>>> c = array.array('b', [5, 3, 1])
>>> x = memoryview(a)
>>> y = memoryview(b)
>>> x == a == y == b
True
>>> x.tolist() == a.tolist() == y.tolist() == b.tolist()
True
>>> z = y[::-2]
>>> z == c
True
>>> z.tolist() == c.tolist()
True
```

If either format string is not supported by the ``` struct ``` module, then the objects will always compare as unequal (even if the format strings and buffer contents are identical):

```>>> from ctypes import BigEndianStructure, c_long
>>> class BEPoint(BigEndianStructure):
...     _fields_ = [("x", c_long), ("y", c_long)]
...
>>> point = BEPoint(100, 200)
>>> a = memoryview(point)
>>> b = memoryview(point)
>>> a == point
False
>>> a == b
False
```

Note that, as with floating point numbers, ``` v is w ``` does not imply ``` v == w ``` 对于 memoryview 对象。

3.3 版改变： Previous versions compared the raw memory disregarding the item format and the logical array structure.

``` tobytes ``` ( order=None )

Return the data in the buffer as a bytestring. This is equivalent to calling the ``` bytes ``` constructor on the memoryview.

```>>> m = memoryview(b"abc")
>>> m.tobytes()
b'abc'
>>> bytes(m)
b'abc'
```

For non-contiguous arrays the result is equal to the flattened list representation with all elements converted to bytes. ``` tobytes() ``` supports all format strings, including those that are not in ``` struct ``` module syntax.

3.8 版新增： order can be {‘C’, ‘F’, ‘A’}. When order is ‘C’ or ‘F’, the data of the original array is converted to C or Fortran order. For contiguous views, ‘A’ returns an exact copy of the physical memory. In particular, in-memory Fortran order is preserved. For non-contiguous views, the data is converted to C first. order=None 如同 order=’C’ .

``` hex ``` ( [ sep [ , bytes_per_sep ] ] )

Return a string object containing two hexadecimal digits for each byte in the buffer.

```>>> m = memoryview(b"abc")
>>> m.hex()
'616263'
```

3.5 版新增。

3.8 版改变： 类似 ``` bytes.hex() ``` , ``` memoryview.hex() ``` 现在支持可选 sep and bytes_per_sep parameters to insert separators between bytes in the hex output.

``` tolist ``` ( )

Return the data in the buffer as a list of elements.

```>>> memoryview(b'abc').tolist()
[97, 98, 99]
>>> import array
>>> a = array.array('d', [1.1, 2.2, 3.3])
>>> m = memoryview(a)
>>> m.tolist()
[1.1, 2.2, 3.3]
```

3.3 版改变： ``` tolist() ``` now supports all single character native formats in ``` struct ``` module syntax as well as multi-dimensional representations.

``` toreadonly ``` ( )

Return a readonly version of the memoryview object. The original memoryview object is unchanged.

```>>> m = memoryview(bytearray(b'abc'))
>>> mm.tolist()
[89, 98, 99]
>>> mm = 42
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
>>> m = 43
>>> mm.tolist()
[43, 98, 99]
```

3.8 版新增。

``` release ``` ( )

Release the underlying buffer exposed by the memoryview object. Many objects take special actions when a view is held on them (for example, a ``` bytearray ``` would temporarily forbid resizing); therefore, calling release() is handy to remove these restrictions (and free any dangling resources) as soon as possible.

After this method has been called, any further operation on the view raises a ``` ValueError ``` (except ``` release() ``` itself which can be called multiple times):

```>>> m = memoryview(b'abc')
>>> m.release()
>>> m
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: operation forbidden on released memoryview object
```

The context management protocol can be used for a similar effect, using the ``` with ``` 语句：

```>>> with memoryview(b'abc') as m:
...     m
...
97
>>> m
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: operation forbidden on released memoryview object
```

3.2 版新增。

``` cast ``` ( format [ , shape ] )

Cast a memoryview to a new format or shape. shape 默认为 ``` [byte_length//new_itemsize] ``` , which means that the result view will be one-dimensional. The return value is a new memoryview, but the buffer itself is not copied. Supported casts are 1D -> C- contiguous and C-contiguous -> 1D.

The destination format is restricted to a single element native format in ``` struct ``` syntax. One of the formats must be a byte format (‘B’, ‘b’ or ‘c’). The byte length of the result must be the same as the original length.

Cast 1D/long to 1D/unsigned bytes:

```>>> import array
>>> a = array.array('l', [1,2,3])
>>> x = memoryview(a)
>>> x.format
'l'
>>> x.itemsize
8
>>> len(x)
3
>>> x.nbytes
24
>>> y = x.cast('B')
>>> y.format
'B'
>>> y.itemsize
1
>>> len(y)
24
>>> y.nbytes
24
```

Cast 1D/unsigned bytes to 1D/char:

```>>> b = bytearray(b'zyz')
>>> x = memoryview(b)
>>> x = b'a'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: memoryview: invalid value for format "B"
>>> y = x.cast('c')
>>> y = b'a'
>>> b
bytearray(b'ayz')
```

Cast 1D/bytes to 3D/ints to 1D/signed char:

```>>> import struct
>>> buf = struct.pack("i"*12, *list(range(12)))
>>> x = memoryview(buf)
>>> y = x.cast('i', shape=[2,2,3])
>>> y.tolist()
[[[0, 1, 2], [3, 4, 5]], [[6, 7, 8], [9, 10, 11]]]
>>> y.format
'i'
>>> y.itemsize
4
>>> len(y)
2
>>> y.nbytes
48
>>> z = y.cast('b')
>>> z.format
'b'
>>> z.itemsize
1
>>> len(z)
48
>>> z.nbytes
48
```

Cast 1D/unsigned long to 2D/unsigned long:

```>>> buf = struct.pack("L"*6, *list(range(6)))
>>> x = memoryview(buf)
>>> y = x.cast('L', shape=[2,3])
>>> len(y)
2
>>> y.nbytes
48
>>> y.tolist()
[[0, 1, 2], [3, 4, 5]]
```

3.3 版新增。

3.5 版改变： 不再限定源格式当铸造成字节视图时。

``` obj ```

```>>> b  = bytearray(b'xyz')
>>> m = memoryview(b)
>>> m.obj is b
True
```

3.3 版新增。

``` nbytes ```

``` nbytes == product(shape) * itemsize == len(m.tobytes()) ``` . This is the amount of space in bytes that the array would use in a contiguous representation. It is not necessarily equal to ``` len(m) ``` :

```>>> import array
>>> a = array.array('i', [1,2,3,4,5])
>>> m = memoryview(a)
>>> len(m)
5
>>> m.nbytes
20
>>> y = m[::2]
>>> len(y)
3
>>> y.nbytes
12
>>> len(y.tobytes())
12
```

```>>> import struct
>>> buf = struct.pack("d"*12, *[1.5*x for x in range(12)])
>>> x = memoryview(buf)
>>> y = x.cast('d', shape=[3,4])
>>> y.tolist()
[[0.0, 1.5, 3.0, 4.5], [6.0, 7.5, 9.0, 10.5], [12.0, 13.5, 15.0, 16.5]]
>>> len(y)
3
>>> y.nbytes
96
```

3.3 版新增。

``` readonly ```

A bool indicating whether the memory is read only.

``` format ```

A string containing the format (in ``` struct ``` module style) for each element in the view. A memoryview can be created from exporters with arbitrary format strings, but some methods (e.g. ``` tolist() ``` ) are restricted to native single element formats.

3.3 版改变： format ``` 'B' ``` is now handled according to the struct module syntax. This means that ``` memoryview(b'abc') == b'abc' == 97 ``` .

``` itemsize ```

The size in bytes of each element of the memoryview:

```>>> import array, struct
>>> m = memoryview(array.array('H', [32000, 32001, 32002]))
>>> m.itemsize
2
>>> m
32000
>>> struct.calcsize('H') == m.itemsize
True
```
``` ndim ```

An integer indicating how many dimensions of a multi-dimensional array the memory represents.

``` shape ```

A tuple of integers the length of ``` ndim ``` giving the shape of the memory as an N-dimensional array.

3.3 版改变： An empty tuple instead of ``` None ``` when ndim = 0.

``` strides ```

A tuple of integers the length of ``` ndim ``` giving the size in bytes to access each element for each dimension of the array.

3.3 版改变： An empty tuple instead of ``` None ``` when ndim = 0.

``` suboffsets ```

Used internally for PIL-style arrays. The value is informational only.

``` c_contiguous ```

A bool indicating whether the memory is C- contiguous .

3.3 版新增。

``` f_contiguous ```

A bool indicating whether the memory is Fortran contiguous .

3.3 版新增。

``` contiguous ```

A bool indicating whether the memory is contiguous .

3.3 版新增。

## 集类型 — ``` set ``` , ``` frozenset ``` ¶

A set object is an unordered collection of distinct hashable objects. Common uses include membership testing, removing duplicates from a sequence, and computing mathematical operations such as intersection, union, difference, and symmetric difference. (For other containers see the built-in ``` dict ``` , ``` list ``` ，和 ``` tuple ``` 类，和 ``` collections ``` 模块。)

Like other collections, sets support ``` x in set ``` , ``` len(set) ``` ，和 ``` for x in set ``` . Being an unordered collection, sets do not record element position or order of insertion. Accordingly, sets do not support indexing, slicing, or other sequence-like behavior.

There are currently two built-in set types, ``` set ``` and ``` frozenset ``` . ``` set ``` type is mutable — the contents can be changed using methods like ``` add() ``` and ``` remove() ``` . Since it is mutable, it has no hash value and cannot be used as either a dictionary key or as an element of another set. The ``` frozenset ``` type is immutable and hashable — its contents cannot be altered after it is created; it can therefore be used as a dictionary key or as an element of another set.

Non-empty sets (not frozensets) can be created by placing a comma-separated list of elements within braces, for example: ``` {'jack', 'sjoerd'} ``` , in addition to the ``` set ``` 构造函数。

class ``` set ``` ( [ iterable ] )
class ``` frozenset ``` ( [ iterable ] )

Return a new set or frozenset object whose elements are taken from iterable . The elements of a set must be hashable . To represent sets of sets, the inner sets must be ``` frozenset ``` 对象。若 iterable 未指定，返回新的空集。

• Use a comma-separated list of elements within braces: ``` {'jack', 'sjoerd'} ```

• Use a set comprehension: ``` {c for c in 'abracadabra' if c not in 'abc'} ```

• 使用类型构造函数： ``` set() ``` , ``` set('foobar') ``` , ``` set(['a', 'b', 'foo']) ```

``` len(s) ```

Return the number of elements in set s (cardinality of s ).

``` x in s ```

``` x not in s ```

``` isdisjoint ``` ( other )

``` issubset ``` ( other )
``` set <= other ```

``` set < other ```

Test whether the set is a proper subset of other , that is, ``` set <= other and set != other ``` .

``` issuperset ``` ( other )
``` set >= other ```

Test whether every element in other is in the set.

``` set > other ```

Test whether the set is a proper superset of other , that is, ``` set >= other and set != other ``` .

``` union ``` ( *others )
``` set | other | ... ```

``` intersection ``` ( *others )
``` set & other & ... ```

``` difference ``` ( *others )
``` set - other - ... ```

``` symmetric_difference ``` ( other )
``` set ^ other ```

Return a new set with elements in either the set or other but not both.

``` copy ``` ( )

Note, the non-operator versions of ``` union() ``` , ``` intersection() ``` , ``` difference() ``` ，和 ``` symmetric_difference() ``` , ``` issubset() ``` ，和 ``` issuperset() ``` methods will accept any iterable as an argument. In contrast, their operator based counterparts require their arguments to be sets. This precludes error-prone constructions like ``` set('abc') & 'cbs' ``` in favor of the more readable ``` set('abc').intersection('cbs') ``` .

Both ``` set ``` and ``` frozenset ``` support set to set comparisons. Two sets are equal if and only if every element of each set is contained in the other (each is a subset of the other). A set is less than another set if and only if the first set is a proper subset of the second set (is a subset, but is not equal). A set is greater than another set if and only if the first set is a proper superset of the second set (is a superset, but is not equal).

The subset and equality comparisons do not generalize to a total ordering function. For example, any two nonempty disjoint sets are not equal and are not subsets of each other, so all of the following return ``` False ``` : ``` a<b ``` , ``` a==b ``` ，或 ``` a>b ``` .

Since sets only define partial ordering (subset relationships), the output of the ``` list.sort() ``` method is undefined for lists of sets.

Binary operations that mix ``` set ``` instances with ``` frozenset ``` return the type of the first operand. For example: ``` frozenset('ab') | set('bc') ``` 返回实例化的 ``` frozenset ``` .

The following table lists operations available for ``` set ``` that do not apply to immutable instances of ``` frozenset ``` :

``` update ``` ( *others )
``` set |= other | ... ```

``` intersection_update ``` ( *others )
``` set &= other & ... ```

``` difference_update ``` ( *others )
``` set -= other | ... ```

``` symmetric_difference_update ``` ( other )
``` set ^= other ```

Update the set, keeping only elements found in either set, but not in both.

``` add ``` ( elem )

``` remove ``` ( elem )

``` discard ``` ( elem )

``` pop ``` ( )

``` clear ``` ( )

## 映射类型 — ``` dict ``` ¶

class ``` dict ``` ( **kwarg )
class ``` dict ``` ( mapping , **kwarg )
class ``` dict ``` ( iterable , **kwarg )

• 使用逗号分隔的列表 ``` key: value ``` 对在花括号内： ``` {'jack': 4098, 'sjoerd': 4127} ``` or ``` {4098: 'jack', 4127: 'sjoerd'} ```

• 使用字典推导： ``` {} ``` , ``` {x: x ** 2 for x in range(10)} ```

• 使用类型构造函数： ``` dict() ``` , ``` dict([('foo', 100), ('bar', 200)]) ``` , ``` dict(foo=100, bar=200) ```

If no positional argument is given, an empty dictionary is created. If a positional argument is given and it is a mapping object, a dictionary is created with the same key-value pairs as the mapping object. Otherwise, the positional argument must be an iterable object. Each item in the iterable must itself be an iterable with exactly two objects. The first object of each item becomes a key in the new dictionary, and the second object the corresponding value. If a key occurs more than once, the last value for that key becomes the corresponding value in the new dictionary.

If keyword arguments are given, the keyword arguments and their values are added to the dictionary created from the positional argument. If a key being added is already present, the value from the keyword argument replaces the value from the positional argument.

```>>> a = dict(one=1, two=2, three=3)
>>> b = {'one': 1, 'two': 2, 'three': 3}
>>> c = dict(zip(['one', 'two', 'three'], [1, 2, 3]))
>>> d = dict([('two', 2), ('one', 1), ('three', 3)])
>>> e = dict({'three': 3, 'one': 1, 'two': 2})
>>> f = dict({'one': 1, 'three': 3}, two=2)
>>> a == b == c == d == e == f
True
```

Providing keyword arguments as in the first example only works for keys that are valid Python identifiers. Otherwise, any valid keys can be used.

These are the operations that dictionaries support (and therefore, custom mapping types should support too):

``` list(d) ```

Return a list of all the keys used in the dictionary d .

``` len(d) ```

``` d[key] ```

```>>> class Counter(dict):
...     def __missing__(self, key):
...         return 0
>>> c = Counter()
>>> c['red']
0
>>> c['red'] += 1
>>> c['red']
1
```

The example above shows part of the implementation of ``` collections.Counter ``` . A different ``` __missing__ ``` method is used by ``` collections.defaultdict ``` .

``` d[key] = value ```

Set ``` d[key] ``` to value .

``` del d[key] ```

``` key in d ```

``` key not in d ```

``` iter(d) ```

``` clear ``` ( )

``` copy ``` ( )

classmethod ``` fromkeys ``` ( iterable [ , value ] )

``` fromkeys() ``` 是返回新字典的类方法。 value 默认为 ``` None ``` . All of the values refer to just a single instance, so it generally doesn’t make sense for value to be a mutable object such as an empty list. To get distinct values, use a 字典推导 代替。

``` get ``` ( key [ , default ] )

``` items ``` ( )

``` keys ``` ( )

``` pop ``` ( key [ , default ] )

key 在字典中，移除它并返回其值，否则返回 default 。若 default 未给定且 key 不在字典中， ``` KeyError ``` 被引发。

``` popitem ``` ( )

``` popitem() ``` 对破坏性迭代字典很有用，这常用于集合算法。若字典为空，调用 ``` popitem() ``` 引发 ``` KeyError ``` .

3.7 版改变： LIFO (后进先出) 次序现在是保证的。在之前版本中， ``` popitem() ``` 将返回任意键/值对。

``` reversed(d) ```

3.8 版新增。

``` setdefault ``` ( key [ , default ] )

key 在字典中，返回其值。若不在，插入 key 采用值 default 并返回 default . default 默认为 ``` None ``` .

``` update ``` ( [ other ] )

``` update() ``` 接受另一字典对象或键/值对可迭代 (作为 2 长元组或其它可迭代)。若指定关键词自变量，则采用这些键/值对更新字典： ``` d.update(red=1, blue=2) ``` .

``` values ``` ( )

```>>> d = {'a': 1}
>>> d.values() == d.values()
False
```
``` d | other ```

Create a new dictionary with the merged keys and values of d and other , which must both be dictionaries. The values of other take priority when d and other share keys.

3.9 版新增。

``` d |= other ```

Update the dictionary d with keys and values from other , which may be either a 映射 or an iterable of key/value pairs. The values of other take priority when d and other share keys.

3.9 版新增。

Dictionaries compare equal if and only if they have the same ``` (key, value) ``` pairs (regardless of ordering). Order comparisons (‘<’, ‘<=’, ‘>=’, ‘>’) raise ``` TypeError ``` .

Dictionaries preserve insertion order. Note that updating a key does not affect the order. Keys added after deletion are inserted at the end.

```>>> d = {"one": 1, "two": 2, "three": 3, "four": 4}
>>> d
{'one': 1, 'two': 2, 'three': 3, 'four': 4}
>>> list(d)
['one', 'two', 'three', 'four']
>>> list(d.values())
[1, 2, 3, 4]
>>> d["one"] = 42
>>> d
{'one': 42, 'two': 2, 'three': 3, 'four': 4}
>>> del d["two"]
>>> d["two"] = None
>>> d
{'one': 42, 'three': 3, 'four': 4, 'two': None}
```

3.7 版改变： Dictionary order is guaranteed to be insertion order. This behavior was an implementation detail of CPython from 3.6.

```>>> d = {"one": 1, "two": 2, "three": 3, "four": 4}
>>> d
{'one': 1, 'two': 2, 'three': 3, 'four': 4}
>>> list(reversed(d))
['four', 'three', 'two', 'one']
>>> list(reversed(d.values()))
[4, 3, 2, 1]
>>> list(reversed(d.items()))
[('four', 4), ('three', 3), ('two', 2), ('one', 1)]
```

3.8 版改变： 字典现在是可逆的。

### 字典视图对象 ¶

Dictionary views can be iterated over to yield their respective data, and support membership tests:

``` len(dictview) ```

``` iter(dictview) ```

Return an iterator over the keys, values or items (represented as tuples of ``` (key, value) ``` ) 在字典中。

Keys and values are iterated over in insertion order. This allows the creation of ``` (value, key) ``` pairs using ``` zip() ``` : ``` pairs = zip(d.values(), d.keys()) ``` . Another way to create the same list is ``` pairs = [(v, k) for (k, v) in d.items()] ``` .

Iterating views while adding or deleting entries in the dictionary may raise a ``` RuntimeError ``` or fail to iterate over all entries.

3.7 版改变： Dictionary order is guaranteed to be insertion order.

``` x in dictview ```

``` reversed(dictview) ```

Return a reverse iterator over the keys, values or items of the dictionary. The view will be iterated in reverse order of the insertion.

3.8 版改变： Dictionary views are now reversible.

Keys views are set-like since their entries are unique and hashable. If all values are hashable, so that ``` (key, value) ``` pairs are unique and hashable, then the items view is also set-like. (Values views are not treated as set-like since the entries are generally not unique.) For set-like views, all of the operations defined for the abstract base class ``` collections.abc.Set ``` are available (for example, ``` == ``` , ``` < ``` ，或 ``` ^ ``` ).

An example of dictionary view usage:

```>>> dishes = {'eggs': 2, 'sausage': 1, 'bacon': 1, 'spam': 500}
>>> keys = dishes.keys()
>>> values = dishes.values()
>>> # iteration
>>> n = 0
>>> for val in values:
...     n += val
>>> print(n)
504
>>> # keys and values are iterated over in the same order (insertion order)
>>> list(keys)
['eggs', 'sausage', 'bacon', 'spam']
>>> list(values)
[2, 1, 1, 500]
>>> # view objects are dynamic and reflect dict changes
>>> del dishes['eggs']
>>> del dishes['sausage']
>>> list(keys)
['bacon', 'spam']
>>> # set operations
>>> keys & {'eggs', 'bacon', 'salad'}
{'bacon'}
>>> keys ^ {'sausage', 'juice'}
{'juice', 'sausage', 'bacon', 'spam'}
```

## 上下文管理器类型 ¶

Python 的 ``` with ``` statement supports the concept of a runtime context defined by a context manager. This is implemented using a pair of methods that allow user-defined classes to define a runtime context that is entered before the statement body is executed and exited when the statement ends:

``` contextmanager. ``` ``` __enter__ ``` ( )

Enter the runtime context and return either this object or another object related to the runtime context. The value returned by this method is bound to the identifier in the ``` as ``` clause of ``` with ``` statements using this context manager.

An example of a context manager that returns itself is a 文件对象 . File objects return themselves from __enter__() to allow ``` open() ``` to be used as the context expression in a ``` with ``` 语句。

An example of a context manager that returns a related object is the one returned by ``` decimal.localcontext() ``` . These managers set the active decimal context to a copy of the original decimal context and then return the copy. This allows changes to be made to the current decimal context in the body of the ``` with ``` statement without affecting code outside the ``` with ``` 语句。

``` contextmanager. ``` ``` __exit__ ``` ( exc_type , exc_val , exc_tb )

Exit the runtime context and return a Boolean flag indicating if any exception that occurred should be suppressed. If an exception occurred while executing the body of the ``` with ``` statement, the arguments contain the exception type, value and traceback information. Otherwise, all three arguments are ``` None ``` .

Returning a true value from this method will cause the ``` with ``` statement to suppress the exception and continue execution with the statement immediately following the ``` with ``` statement. Otherwise the exception continues propagating after this method has finished executing. Exceptions that occur during execution of this method will replace any exception that occurred in the body of the ``` with ``` 语句。

The exception passed in should never be reraised explicitly - instead, this method should return a false value to indicate that the method completed successfully and does not want to suppress the raised exception. This allows context management code to easily detect whether or not an ``` __exit__() ``` method has actually failed.

Python defines several context managers to support easy thread synchronisation, prompt closure of files or other objects, and simpler manipulation of the active decimal arithmetic context. The specific types are not treated specially beyond their implementation of the context management protocol. See the ``` contextlib ``` module for some examples.

Python 的 generator s and the ``` contextlib.contextmanager ``` decorator provide a convenient way to implement these protocols. If a generator function is decorated with the ``` contextlib.contextmanager ``` decorator, it will return a context manager implementing the necessary ``` __enter__() ``` and ``` __exit__() ``` methods, rather than the iterator produced by an undecorated generator function.

Note that there is no specific slot for any of these methods in the type structure for Python objects in the Python/C API. Extension types wanting to define these methods must provide them as a normal Python accessible method. Compared to the overhead of setting up the runtime context, the overhead of a single class dictionary lookup is negligible.

## 一般别名类型 ¶

``` GenericAlias ``` objects are created by subscripting a class (usually a container), such as ``` list[int] ``` . They are intended primarily for 类型注解 .

``` __getitem__() ``` of the class’ metaclass is present, it will take precedence over the ``` __class_getitem__() ``` defined in the class (see PEP 560 for more details).

``` GenericAlias ``` object acts as a proxy for 一般类型 , implementing parameterized generics - a specific instance of a generic which provides the types for container elements.

The user-exposed type for the ``` GenericAlias ``` object can be accessed from ``` types.GenericAlias ``` and used for ``` isinstance() ``` checks. It can also be used to create ``` GenericAlias ``` objects directly.

``` T[X, Y, ...] ```

```def average(values: list[float]) -> float:
return sum(values) / len(values)
```

Another example for 映射 objects, using a ``` dict ``` , which is a generic type expecting two type parameters representing the key type and the value type. In this example, the function expects a ``` dict ``` with keys of type ``` str ``` and values of type ``` int ``` :

```def send_post_request(url: str, body: dict[str, int]) -> None:
...
```

```>>> isinstance([1, 2], list[str])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: isinstance() argument 2 cannot be a parameterized generic
```

The Python runtime does not enforce 类型注解 . This extends to generic types and their type parameters. When creating an object from a ``` GenericAlias ``` , container elements are not checked against their type. For example, the following code is discouraged, but will run without errors:

```>>> t = list[str]
>>> t([1, 2, 3])
[1, 2, 3]
```

Furthermore, parameterized generics erase type parameters during object creation:

```>>> t = list[str]
>>> type(t)
<class 'types.GenericAlias'>
>>> l = t()
>>> type(l)
<class 'list'>
```

```>>> repr(list[int])
'list[int]'
>>> str(list[int])
'list[int]'
```

``` __getitem__() ``` method of generics will raise an exception to disallow mistakes like ``` dict[str][str] ``` :

```>>> dict[str][str]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: There are no type variables left in dict[str]
```

However, such expressions are valid when 类型变量 are used. The index must have as many elements as there are type variable items in the ``` GenericAlias ``` 对象的 ``` __args__ ``` .

```>>> from typing import TypeVar
>>> Y = TypeVar('Y')
>>> dict[str, Y][int]
dict[str, int]
```

### 标准一般集合 ¶

These standard library collections support parameterized generics.

### 一般别名的特殊属性 ¶

All parameterized generics implement special read-only attributes.

``` genericalias. ``` ``` __origin__ ```

This attribute points at the non-parameterized generic class:

```>>> list[int].__origin__
<class 'list'>
```
``` genericalias. ``` ``` __args__ ```

This attribute is a ``` tuple ``` (possibly of length 1) of generic types passed to the original ``` __class_getitem__() ``` of the generic container:

```>>> dict[str, list[int]].__args__
(<class 'str'>, list[int])
```
``` genericalias. ``` ``` __parameters__ ```

This attribute is a lazily computed tuple (possibly empty) of unique type variables found in ``` __args__ ``` :

```>>> from typing import TypeVar
>>> T = TypeVar('T')
>>> list[T].__parameters__
(~T,)
```

3.9 版新增。

## 其他内置类型 ¶

The interpreter supports several other kinds of objects. Most of these support only one or two operations.

### 模块 ¶

The only special operation on a module is attribute access: ``` m.name ``` ，其中 m is a module and name accesses a name defined in m ’s symbol table. Module attributes can be assigned to. (Note that the ``` import ``` statement is not, strictly speaking, an operation on a module object; ``` import foo ``` does not require a module object named foo to exist, rather it requires an (external) definition for a module named foo somewhere.)

A special attribute of every module is ``` __dict__ ``` . This is the dictionary containing the module’s symbol table. Modifying this dictionary will actually change the module’s symbol table, but direct assignment to the ``` __dict__ ``` attribute is not possible (you can write ``` m.__dict__['a'] = 1 ``` , which defines ``` m.a ``` to be ``` 1 ``` , but you can’t write ``` m.__dict__ = {} ``` ). Modifying ``` __dict__ ``` directly is not recommended.

Modules built into the interpreter are written like this: ``` <module 'sys' (built-in)> ``` . If loaded from a file, they are written as ``` <module 'os' from '/usr/local/lib/pythonX.Y/os.pyc'> ``` .

### 函数 ¶

Function objects are created by function definitions. The only operation on a function object is to call it: ``` func(argument-list) ``` .

There are really two flavors of function objects: built-in functions and user-defined functions. Both support the same operation (to call the function), but the implementation is different, hence the different object types.

### 方法 ¶

Methods are functions that are called using the attribute notation. There are two flavors: built-in methods (such as ``` append() ``` on lists) and class instance methods. Built-in methods are described with the types that support them.

If you access a method (a function defined in a class namespace) through an instance, you get a special object: a bound method (also called instance method ) object. When called, it will add the ``` self ``` argument to the argument list. Bound methods have two special read-only attributes: ``` m.__self__ ``` is the object on which the method operates, and ``` m.__func__ ``` is the function implementing the method. Calling ``` m(arg-1, arg-2, ..., arg-n) ``` is completely equivalent to calling ``` m.__func__(m.__self__, arg-1, arg-2, ..., arg-n) ``` .

Like function objects, bound method objects support getting arbitrary attributes. However, since method attributes are actually stored on the underlying function object ( ``` meth.__func__ ``` ), setting method attributes on bound methods is disallowed. Attempting to set an attribute on a method results in an ``` AttributeError ``` being raised. In order to set a method attribute, you need to explicitly set it on the underlying function object:

```>>> class C:
...     def method(self):
...         pass
...
>>> c = C()
>>> c.method.whoami = 'my name is method'  # can't set on the method
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'method' object has no attribute 'whoami'
>>> c.method.__func__.whoami = 'my name is method'
>>> c.method.whoami
'my name is method'
```

### 代码对象 ¶

Code objects are used by the implementation to represent “pseudo-compiled” executable Python code such as a function body. They differ from function objects because they don’t contain a reference to their global execution environment. Code objects are returned by the built-in ``` compile() ``` function and can be extracted from function objects through their ``` __code__ ``` attribute. See also the ``` code ``` 模块。

A code object can be executed or evaluated by passing it (instead of a source string) to the ``` exec() ``` or ``` eval() ``` 内置函数。

### 类型对象 ¶

Type objects represent the various object types. An object’s type is accessed by the built-in function ``` type() ``` . There are no special operations on types. The standard module ``` types ``` defines names for all standard built-in types.

Types are written like this: ``` <class 'int'> ``` .

### Null 对象 ¶

This object is returned by functions that don’t explicitly return a value. It supports no special operations. There is exactly one null object, named ``` None ``` (a built-in name). ``` type(None)() ``` produces the same singleton.

### 省略对象 ¶

This object is commonly used by slicing (see 切片 ). It supports no special operations. There is exactly one ellipsis object, named ``` Ellipsis ``` (a built-in name). ``` type(Ellipsis)() ``` produces the ``` Ellipsis ``` singleton.

### 未实现对象 ¶

This object is returned from comparisons and binary operations when they are asked to operate on types they don’t support. See 比较 for more information. There is exactly one ``` NotImplemented ``` 对象。 ``` type(NotImplemented)() ``` produces the singleton instance.

### 布尔值 ¶

Boolean values are the two constant objects ``` False ``` and ``` True ``` . They are used to represent truth values (although other values can also be considered false or true). In numeric contexts (for example when used as the argument to an arithmetic operator), they behave like the integers 0 and 1, respectively. The built-in function ``` bool() ``` can be used to convert any value to a Boolean, if the value can be interpreted as a truth value (see section 真值测试 above).

They are written as ``` False ``` and ``` True ``` ，分别。

## 特殊属性 ¶

`object.` ``` __dict__ ```

A dictionary or other mapping object used to store an object’s (writable) attributes.

`instance.` ``` __class__ ```

`class.` ``` __bases__ ```

The tuple of base classes of a class object.

``` definition. ``` ``` __name__ ```

``` definition. ``` ``` __qualname__ ```

3.3 版新增。

`class.` ``` __mro__ ```

This attribute is a tuple of classes that are considered when looking for base classes during method resolution.

`class.` ``` mro ``` ( )

This method can be overridden by a metaclass to customize the method resolution order for its instances. It is called at class instantiation, and its result is stored in ``` __mro__ ``` .

`class.` ``` __subclasses__ ``` ( )

Each class keeps a list of weak references to its immediate subclasses. This method returns a list of all those references still alive. The list is in definition order. Example:

```>>> int.__subclasses__()
[<class 'bool'>]
```

Additional information on these special methods may be found in the Python Reference Manual ( 基本定制 ).

As a consequence, the list ``` [1, 2] ``` is considered equal to ``` [1.0, 2.0] ``` , and similarly for tuples.

They must have since the parser can’t tell the type of the operands.

4 ( 1 , 2 , 3 , 4 )

Cased characters are those with general category property being one of “Lu” (Letter, uppercase), “Ll” (Letter, lowercase), or “Lt” (Letter, titlecase).

5 ( 1 , 2 )

To format only a tuple you should therefore provide a singleton tuple whose only element is the tuple to be formatted.