string — 常见字符串操作

源代码: Lib/



string. ascii_letters

串联的 ascii_lowercase and ascii_uppercase 常量的描述见下文。此值不从属区域设置。

string. ascii_lowercase

小写字母 'abcdefghijklmnopqrstuvwxyz' 。此值不从属区域设置且不会改变。

string. ascii_uppercase


string. digits

字符串 '0123456789' .

string. hexdigits

字符串 '0123456789abcdefABCDEF' .

string. octdigits

字符串 '01234567' .

string. punctuation

被认为是标点符号的 ASCII 字符字符串,在 C 区域设置: !"#$%&'()*+,-./:;<=>?@[\]^_`{|}~ .

string. printable

被认为是可打印的 ASCII 字符字符串。 这是组合的 digits , ascii_letters , punctuation ,和 whitespace .

string. whitespace

包含所有被视为空白的 ASCII 字符的字符串。这些字符包括空格、Tab 制表符、换行、回车、换页符及垂直制表符。


The built-in string class provides the ability to do complex variable substitutions and value formatting via the format() 方法描述在 PEP 3101 Formatter 类在 string module allows you to create and customize your own string formatting behaviors using the same implementation as the built-in format() 方法。

class string. Formatter

Formatter 类具有下列公共方法:

format ( format_string , / , *args , **kwargs )

The primary API method. It takes a format string and an arbitrary set of positional and keyword arguments. It is just a wrapper that calls vformat() .

3.7 版改变: A format string argument is now 仅位置 .

vformat ( format_string , args , kwargs )

This function does the actual work of formatting. It is exposed as a separate function for cases where you want to pass in a predefined dictionary of arguments, rather than unpacking and repacking the dictionary as individual arguments using the *args and **kwargs 句法。 vformat() does the work of breaking up the format string into character data and replacement fields. It calls the various methods described below.

此外, Formatter defines a number of methods that are intended to be replaced by subclasses:

parse ( format_string )

Loop over the format_string and return an iterable of tuples ( literal_text , field_name , format_spec , conversion ). This is used by vformat() to break the string into either literal text, or replacement fields.

The values in the tuple conceptually represent a span of literal text followed by a single replacement field. If there is no literal text (which can happen if two replacement fields occur consecutively), then literal_text will be a zero-length string. If there is no replacement field, then the values of field_name , format_spec and conversion 将是 None .

get_field ( field_name , args , kwargs )

Given field_name as returned by parse() (see above), convert it to an object to be formatted. Returns a tuple (obj, used_key). The default version takes strings of the form defined in PEP 3101 , such as “0[name]” or “label.title”. args and kwargs are as passed in to vformat() . The return value used_key has the same meaning as the key 参数到 get_value() .

get_value ( key , args , kwargs )

检索给定字段值。 key argument will be either an integer or a string. If it is an integer, it represents the index of the positional argument in args ; if it is a string, then it represents a named argument in kwargs .

args parameter is set to the list of positional arguments to vformat() ,和 kwargs parameter is set to the dictionary of keyword arguments.

For compound field names, these functions are only called for the first component of the field name; subsequent components are handled through normal attribute and indexing operations.

So for example, the field expression ‘’ would cause get_value() to be called with a key argument of 0. The name attribute will be looked up after get_value() returns by calling the built-in getattr() 函数。

If the index or keyword refers to an item that does not exist, then an IndexError or KeyError should be raised.

check_unused_args ( used_args , args , kwargs )

Implement checking for unused arguments if desired. The arguments to this function is the set of all argument keys that were actually referred to in the format string (integers for positional arguments, and strings for named arguments), and a reference to the args and kwargs that was passed to vformat. The set of unused args can be calculated from these parameters. check_unused_args() is assumed to raise an exception if the check fails.

format_field ( value , format_spec )

format_field() 只需调用全局 format() 内置。提供此方法以便子类可以覆盖它。

convert_field ( value , conversion )

转换值 (返回通过 get_field() ) given a conversion type (as in the tuple returned by the parse() method). The default version understands ‘s’ (str), ‘r’ (repr) and ‘a’ (ascii) conversion types.


str.format() 方法和 Formatter class share the same syntax for format strings (although in the case of Formatter , subclasses can define their own format string syntax). The syntax is related to that of 格式化字符串文字 , but it is less sophisticated and, in particular, does not support arbitrary expressions.

Format strings contain “replacement fields” surrounded by curly braces {} . Anything that is not contained in braces is considered literal text, which is copied unchanged to the output. If you need to include a brace character in the literal text, it can be escaped by doubling: {{ and }} .

The grammar for a replacement field is as follows:

replacement_field ::=  "{" [field_name] ["!" conversion] [":" format_spec] "}"
field_name        ::=  arg_name ("." attribute_name | "[" element_index "]")*
arg_name          ::=  [identifier | digit+]
attribute_name    ::=  identifier
element_index     ::=  digit+ | index_string
index_string      ::=  <any source character except "]"> +
conversion        ::=  "r" | "s" | "a"
format_spec       ::=  <described in the next section>

In less formal terms, the replacement field can start with a field_name that specifies the object whose value is to be formatted and inserted into the output instead of the replacement field. field_name is optionally followed by a conversion field, which is preceded by an exclamation point '!' ,和 format_spec , which is preceded by a colon ':' . These specify a non-default format for the replacement value.

另请参阅 格式规范的迷你语言 章节。

field_name itself begins with an arg_name that is either a number or a keyword. If it’s a number, it refers to a positional argument, and if it’s a keyword, it refers to a named keyword argument. If the numerical arg_names in a format string are 0, 1, 2, … in sequence, they can all be omitted (not just some) and the numbers 0, 1, 2, … will be automatically inserted in that order. Because arg_name is not quote-delimited, it is not possible to specify arbitrary dictionary keys (e.g., the strings '10' or ':-]' ) within a format string. arg_name can be followed by any number of index or attribute expressions. An expression of the form '.name' selects the named attribute using getattr() , while an expression of the form '[index]' does an index lookup using __getitem__() .

3.1 版改变: The positional argument specifiers can be omitted for str.format() , so '{} {}'.format(a, b) 相当于 '{0} {1}'.format(a, b) .

3.4 版改变: The positional argument specifiers can be omitted for Formatter .


"First, thou shalt count to {0}"  # References first positional argument
"Bring me a {}"                   # Implicitly references the first positional argument
"From {} to {}"                   # Same as "From {0} to {1}"
"My quest is {name}"              # References keyword argument 'name'
"Weight in tons {0.weight}"       # 'weight' attribute of first positional arg
"Units destroyed: {players[0]}"   # First element of keyword argument 'players'.

conversion field causes a type coercion before formatting. Normally, the job of formatting a value is done by the __format__() method of the value itself. However, in some cases it is desirable to force a type to be formatted as a string, overriding its own definition of formatting. By converting the value to a string before calling __format__() , the normal formatting logic is bypassed.

Three conversion flags are currently supported: '!s' which calls str() on the value, '!r' which calls repr() and '!a' which calls ascii() .


"Harold's a clever {0!s}"        # Calls str() on the argument first
"Bring out the holy {name!r}"    # Calls repr() on the argument first
"More {!a}"                      # Calls ascii() on the argument first

format_spec field contains a specification of how the value should be presented, including such details as field width, alignment, padding, decimal precision and so on. Each value type can define its own “formatting mini-language” or interpretation of the format_spec .

Most built-in types support a common formatting mini-language, which is described in the next section.

A format_spec field can also include nested replacement fields within it. These nested replacement fields may contain a field name, conversion flag and format specification, but deeper nesting is not allowed. The replacement fields within the format_spec are substituted before the format_spec string is interpreted. This allows the formatting of a value to be dynamically specified.

格式范例 section for some examples.


“Format specifications” are used within replacement fields contained within a format string to define how individual values are presented (see 格式字符串语法 and 格式化字符串文字 ). They can also be passed directly to the built-in format() function. Each formattable type may define how the format specification is to be interpreted.

Most built-in types implement the following options for format specifications, although some of the formatting options are only supported by the numeric types.

A general convention is that an empty format specification produces the same result as if you had called str() on the value. A non-empty format specification typically modifies the result.

The general form of a 标准格式说明符 is:

format_spec     ::=  [[fill]align][sign][#][0][width][grouping_option][.precision][type]
fill            ::=  <any character>
align           ::=  "<" | ">" | "=" | "^"
sign            ::=  "+" | "-" | " "
width           ::=  digit+
grouping_option ::=  "_" | ","
precision       ::=  digit+
type            ::=  "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"

若有效 align 值被指定,它的前面可以加 fill character that can be any character and defaults to a space if omitted. It is not possible to use a literal curly brace (” { } ) 作为 fill 字符在 格式化字符串文字 或当使用 str.format() method. However, it is possible to insert a curly brace with a nested replacement field. This limitation doesn’t affect the format() 函数。





Forces the field to be left-aligned within the available space (this is the default for most objects).


Forces the field to be right-aligned within the available space (this is the default for numbers).


Forces the padding to be placed after the sign (if any) but before the digits. This is used for printing fields in the form ‘+000000120’. This alignment option is only valid for numeric types. It becomes the default when ‘0’ immediately precedes the field width.


Forces the field to be centered within the available space.

Note that unless a minimum field width is defined, the field width will always be the same size as the data to fill it, so that the alignment option has no meaning in this case.

sign option is only valid for number types, and can be one of the following:




indicates that a sign should be used for both positive as well as negative numbers.


indicates that a sign should be used only for negative numbers (this is the default behavior).


indicates that a leading space should be used on positive numbers, and a minus sign on negative numbers.

'#' option causes the “alternate form” to be used for the conversion. The alternate form is defined differently for different types. This option is only valid for integer, float and complex types. For integers, when binary, octal, or hexadecimal output is used, this option adds the respective prefix '0b' , '0o' , '0x' ,或 '0X' to the output value. For float and complex the alternate form causes the result of the conversion to always contain a decimal-point character, even if no digits follow it. Normally, a decimal-point character appears in the result of these conversions only if a digit follows it. In addition, for 'g' and 'G' conversions, trailing zeros are not removed from the result.

',' option signals the use of a comma for a thousands separator. For a locale aware separator, use the 'n' integer presentation type instead.

3.1 版改变: 添加 ',' 选项 (另请参阅 PEP 378 ).

'_' option signals the use of an underscore for a thousands separator for floating point presentation types and for integer presentation type 'd' . For integer presentation types 'b' , 'o' , 'x' ,和 'X' , underscores will be inserted every 4 digits. For other presentation types, specifying this option is an error.

3.6 版改变: 添加 '_' 选项 (另请参阅 PEP 515 ).

width is a decimal integer defining the minimum total field width, including any prefixes, separators, and other formatting characters. If not specified, then the field width will be determined by the content.

When no explicit alignment is given, preceding the width field by a zero ( '0' ) character enables sign-aware zero-padding for numeric types. This is equivalent to a fill character of '0' with an alignment type of '=' .

precision is a decimal number indicating how many digits should be displayed after the decimal point for a floating point value formatted with 'f' and 'F' , or before and after the decimal point for a floating point value formatted with 'g' or 'G' . For non-number types the field indicates the maximum field size - in other words, how many characters will be used from the field content. The precision is not allowed for integer values.

Finally, the type determines how the data should be presented.

The available string presentation types are:






如同 's' .

The available integer presentation types are:




Binary format. Outputs the number in base 2.


Character. Converts the integer to the corresponding unicode character before printing.


Decimal Integer. Outputs the number in base 10.


Octal format. Outputs the number in base 8.


Hex format. Outputs the number in base 16, using lower-case letters for the digits above 9.


Hex format. Outputs the number in base 16, using upper-case letters for the digits above 9. In case '#' is specified, the prefix '0x' will be upper-cased to '0X' as well.


Number. This is the same as 'd' , except that it uses the current locale setting to insert the appropriate number separator characters.


如同 'd' .

In addition to the above presentation types, integers can be formatted with the floating point presentation types listed below (except 'n' and None ). When doing so, float() is used to convert the integer to a floating point number before formatting.

The available presentation types for float and Decimal values are:




Scientific notation. For a given precision p , formats the number in scientific notation with the letter ‘e’ separating the coefficient from the exponent. The coefficient has one digit before and p digits after the decimal point, for a total of p + 1 significant digits. With no precision given, uses a precision of 6 digits after the decimal point for float , and shows all coefficient digits for Decimal . If no digits follow the decimal point, the decimal point is also removed unless the # option is used.


Scientific notation. Same as 'e' except it uses an upper case ‘E’ as the separator character.


Fixed-point notation. For a given precision p , formats the number as a decimal number with exactly p digits following the decimal point. With no precision given, uses a precision of 6 digits after the decimal point for float , and uses a precision large enough to show all coefficient digits for Decimal . If no digits follow the decimal point, the decimal point is also removed unless the # option is used.


Fixed-point notation. Same as 'f' , but converts nan to NAN and inf to INF .


General format. For a given precision p >= 1 , this rounds the number to p significant digits and then formats the result in either fixed-point format or in scientific notation, depending on its magnitude. A precision of 0 is treated as equivalent to a precision of 1 .

The precise rules are as follows: suppose that the result formatted with presentation type 'e' and precision p-1 would have exponent exp . Then, if m <= exp < p ,其中 m is -4 for floats and -6 for Decimals , the number is formatted with presentation type 'f' and precision p-1-exp . Otherwise, the number is formatted with presentation type 'e' and precision p-1 . In both cases insignificant trailing zeros are removed from the significand, and the decimal point is also removed if there are no remaining digits following it, unless the '#' option is used.

With no precision given, uses a precision of 6 significant digits for float . For Decimal , the coefficient of the result is formed from the coefficient digits of the value; scientific notation is used for values smaller than 1e-6 in absolute value and values where the place value of the least significant digit is larger than 1, and fixed-point notation is used otherwise.

Positive and negative infinity, positive and negative zero, and nans, are formatted as inf , -inf , 0 , -0 and nan respectively, regardless of the precision.


General format. Same as 'g' except switches to 'E' if the number gets too large. The representations of infinity and NaN are uppercased, too.


Number. This is the same as 'g' , except that it uses the current locale setting to insert the appropriate number separator characters.


Percentage. Multiplies the number by 100 and displays in fixed ( 'f' ) format, followed by a percent sign.


For float this is the same as 'g' , except that when fixed-point notation is used to format the result, it always includes at least one digit past the decimal point. The precision used is as large as needed to represent the given value faithfully.

For Decimal , this is the same as either 'g' or 'G' depending on the value of context.capitals for the current decimal context.

The overall effect is to match the output of str() as altered by the other format modifiers.


This section contains examples of the str.format() syntax and comparison with the old % 格式化。

In most of the cases the syntax is similar to the old % -formatting, with the addition of the {} and with : used instead of % 。例如, '%03.2f' 可被翻译成 '{:03.2f}' .

The new format syntax also supports new and different options, shown in the following examples.


>>> '{0}, {1}, {2}'.format('a', 'b', 'c')
'a, b, c'
>>> '{}, {}, {}'.format('a', 'b', 'c')  # 3.1+ only
'a, b, c'
>>> '{2}, {1}, {0}'.format('a', 'b', 'c')
'c, b, a'
>>> '{2}, {1}, {0}'.format(*'abc')      # unpacking argument sequence
'c, b, a'
>>> '{0}{1}{0}'.format('abra', 'cad')   # arguments' indices can be repeated


>>> 'Coordinates: {latitude}, {longitude}'.format(latitude='37.24N', longitude='-115.81W')
'Coordinates: 37.24N, -115.81W'
>>> coord = {'latitude': '37.24N', 'longitude': '-115.81W'}
>>> 'Coordinates: {latitude}, {longitude}'.format(**coord)
'Coordinates: 37.24N, -115.81W'


>>> c = 3-5j
>>> ('The complex number {0} is formed from the real part {0.real} '
...  'and the imaginary part {0.imag}.').format(c)
'The complex number (3-5j) is formed from the real part 3.0 and the imaginary part -5.0.'
>>> class Point:
...     def __init__(self, x, y):
...         self.x, self.y = x, y
...     def __str__(self):
...         return 'Point({self.x}, {self.y})'.format(self=self)
>>> str(Point(4, 2))
'Point(4, 2)'


>>> coord = (3, 5)
>>> 'X: {0[0]};  Y: {0[1]}'.format(coord)
'X: 3;  Y: 5'

替换 %s and %r :

>>> "repr() shows quotes: {!r}; str() doesn't: {!s}".format('test1', 'test2')
"repr() shows quotes: 'test1'; str() doesn't: test2"


>>> '{:<30}'.format('left aligned')
'left aligned                  '
>>> '{:>30}'.format('right aligned')
'                 right aligned'
>>> '{:^30}'.format('centered')
'           centered           '
>>> '{:*^30}'.format('centered')  # use '*' as a fill char

替换 %+f , %-f ,和 % f 并指定正负号:

>>> '{:+f}; {:+f}'.format(3.14, -3.14)  # show it always
'+3.140000; -3.140000'
>>> '{: f}; {: f}'.format(3.14, -3.14)  # show a space for positive numbers
' 3.140000; -3.140000'
>>> '{:-f}; {:-f}'.format(3.14, -3.14)  # show only the minus -- same as '{:f}; {:f}'
'3.140000; -3.140000'

替换 %x and %o 并把值转换成不同的基数:

>>> # format also supports binary numbers
>>> "int: {0:d};  hex: {0:x};  oct: {0:o};  bin: {0:b}".format(42)
'int: 42;  hex: 2a;  oct: 52;  bin: 101010'
>>> # with 0x, 0o, or 0b as prefix:
>>> "int: {0:d};  hex: {0:#x};  oct: {0:#o};  bin: {0:#b}".format(42)
'int: 42;  hex: 0x2a;  oct: 0o52;  bin: 0b101010'


>>> '{:,}'.format(1234567890)


>>> points = 19
>>> total = 22
>>> 'Correct answers: {:.2%}'.format(points/total)
'Correct answers: 86.36%'


>>> import datetime
>>> d = datetime.datetime(2010, 7, 4, 12, 15, 58)
>>> '{:%Y-%m-%d %H:%M:%S}'.format(d)
'2010-07-04 12:15:58'


>>> for align, text in zip('<^>', ['left', 'center', 'right']):
...     '{0:{fill}{align}16}'.format(text, fill=align, align=align)
>>> octets = [192, 168, 0, 1]
>>> '{:02X}{:02X}{:02X}{:02X}'.format(*octets)
>>> int(_, 16)
>>> width = 5
>>> for num in range(5,12):
...     for base in 'dXob':
...         print('{0:{width}{base}}'.format(num, base=base, width=width), end=' ')
...     print()
    5     5     5   101
    6     6     6   110
    7     7     7   111
    8     8    10  1000
    9     9    11  1001
   10     A    12  1010
   11     B    13  1011


Template strings provide simpler string substitutions as described in PEP 292 . A primary use case for template strings is for internationalization (i18n) since in that context, the simpler syntax and functionality makes it easier to translate than other built-in string formatting facilities in Python. As an example of a library built on template strings for i18n, see the flufl.i18n 包。

Template 字符串支持 $ -based substitutions, using the following rules:

  • $$ is an escape; it is replaced with a single $ .

  • $identifier names a substitution placeholder matching a mapping key of "identifier" . By default, "identifier" is restricted to any case-insensitive ASCII alphanumeric string (including underscores) that starts with an underscore or ASCII letter. The first non-identifier character after the $ character terminates this placeholder specification.

  • ${identifier} 相当于 $identifier . It is required when valid identifier characters follow the placeholder but are not part of the placeholder, such as "${noun}ification" .

Any other appearance of $ in the string will result in a ValueError being raised.

string 模块提供 Template class that implements these rules. The methods of Template 是:

class string. Template ( template )


substitute ( mapping={} , / , **kwds )

Performs the template substitution, returning a new string. 映射 is any dictionary-like object with keys that match the placeholders in the template. Alternatively, you can provide keyword arguments, where the keywords are the placeholders. When both 映射 and kwds are given and there are duplicates, the placeholders from kwds take precedence.

safe_substitute ( mapping={} , / , **kwds )

substitute() , except that if placeholders are missing from 映射 and kwds , instead of raising a KeyError exception, the original placeholder will appear in the resulting string intact. Also, unlike with substitute() , any other appearances of the $ will simply return $ instead of raising ValueError .

While other exceptions may still occur, this method is called “safe” because it always tries to return a usable string instead of raising an exception. In another sense, safe_substitute() may be anything other than safe, since it will silently ignore malformed templates containing dangling delimiters, unmatched braces, or placeholders that are not valid Python identifiers.

Template 实例还提供一公共数据属性:


This is the object passed to the constructor’s template argument. In general, you shouldn’t change it, but read-only access is not enforced.

这里是如何使用 Template 的范例:

>>> from string import Template
>>> s = Template('$who likes $what')
>>> s.substitute(who='tim', what='kung pao')
'tim likes kung pao'
>>> d = dict(who='tim')
>>> Template('Give $who $100').substitute(d)
Traceback (most recent call last):
ValueError: Invalid placeholder in string: line 1, col 11
>>> Template('$who likes $what').substitute(d)
Traceback (most recent call last):
KeyError: 'what'
>>> Template('$who likes $what').safe_substitute(d)
'tim likes $what'

Advanced usage: you can derive subclasses of Template to customize the placeholder syntax, delimiter character, or the entire regular expression used to parse template strings. To do this, you can override these class attributes:

  • delimiter – This is the literal string describing a placeholder introducing delimiter. The default value is $ . Note that this should not be a regular expression, as the implementation will call re.escape() on this string as needed. Note further that you cannot change the delimiter after class creation (i.e. a different delimiter must be set in the subclass’s class namespace).

  • idpattern – This is the regular expression describing the pattern for non-braced placeholders. The default value is the regular expression (?a:[_a-z][_a-z0-9]*) . If this is given and braceidpattern is None this pattern will also apply to braced placeholders.


    Since default flags is re.IGNORECASE , pattern [a-z] can match with some non-ASCII characters. That’s why we use the local a flag here.

    3.7 版改变: braceidpattern can be used to define separate patterns used inside and outside the braces.

  • braceidpattern – This is like idpattern but describes the pattern for braced placeholders. Defaults to None which means to fall back to idpattern (i.e. the same pattern is used both inside and outside braces). If given, this allows you to define different patterns for braced and unbraced placeholders.

    3.7 版新增。

  • flags – The regular expression flags that will be applied when compiling the regular expression used for recognizing substitutions. The default value is re.IGNORECASE 。注意, re.VERBOSE will always be added to the flags, so custom idpattern s must follow conventions for verbose regular expressions.

    3.2 版新增。

Alternatively, you can provide the entire regular expression pattern by overriding the class attribute pattern . If you do this, the value must be a regular expression object with four named capturing groups. The capturing groups correspond to the rules given above, along with the invalid placeholder rule:

  • escaped – This group matches the escape sequence, e.g. $$ , in the default pattern.

  • named – This group matches the unbraced placeholder name; it should not include the delimiter in capturing group.

  • braced – This group matches the brace enclosed placeholder name; it should not include either the delimiter or braces in the capturing group.

  • invalid – This group matches any other delimiter pattern (usually a single delimiter), and it should appear last in the regular expression.


string. capwords ( s , sep=None )

把自变量拆分成单词,使用 str.split() ,大写每单词使用 str.capitalize() , and join the capitalized words using str.join() . If the optional second argument sep is absent or None , runs of whitespace characters are replaced by a single space and leading and trailing whitespace are removed, otherwise sep is used to split and join the words.