string — 常见字符串操作 ¶

字符串常量 ¶

此模块中定义的常量：

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 ¶

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

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。

自定义字符串格式化 ¶

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 ‘0.name’ 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.

格式字符串语法 ¶

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__() .

一些简单格式字符串范例：

"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() .

一些范例：

"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.

见 格式范例 section for some examples.

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" | "%"
							

>>> '{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
'abracadabra'
									

>>> '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'
									

>>> "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
'***********centered***********'
									

>>> '{:+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'
									

>>> # 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)
'1,234,567,890'
									

>>> 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)
...
'left<<<<<<<<<<<<'
'^^^^^center^^^^^'
'>>>>>>>>>>>right'
>>>
>>> octets = [192, 168, 0, 1]
>>> '{:02X}{:02X}{:02X}{:02X}'.format(*octets)
'C0A80001'
>>> int(_, 16)
3232235521
>>>
>>> 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
									

模板字符串 ¶

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 ) ¶

构造函数接受单模板字符串自变量。

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 ) ¶

像 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：

template ¶

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.

  注意

  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.

帮手函数 ¶

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.