string
— 常见字符串操作
¶
源代码： Lib/string.py
另请参阅
此模块中定义的常量：
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。
小写字母
'abcdefghijklmnopqrstuvwxyz'
。此值不从属区域设置且不会改变。
大写字母
'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
。此值不从属区域设置且不会改变。
字符串
'0123456789'
.
字符串
'0123456789abcdefABCDEF'
.
字符串
'01234567'
.
被认为是标点符号的 ASCII 字符字符串，在
C
区域设置：
!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~
.
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
.
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。
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 .
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:
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
.
Given
field_name
如返回通过
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
参数用于
get_value()
.
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.
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()
只需调用全局
format()
内置。提供此方法以便子类可以覆盖它。
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
类共享相同格式字符串句法 (尽管在
Formatter
，子类可以定义它们自己的格式字符串句法)。句法相关
格式化字符串文字
，但它不太完备，尤其不支持任意表达式。
格式字符串包含 "置换字段" 环绕通过花括号
{}
. 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
}}
.
置换字段的语法如下：
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. The
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. The
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：
可以省略位置自变量说明符对于
str.format()
，所以
'{} {}'.format(a, b)
相当于
'{0} {1}'.format(a, b)
.
3.4 版改变：
可以省略位置自变量说明符对于
Formatter
.
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'
调用
str()
on the value,
'!r'
调用
repr()
and
'!a'
调用
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 章节举的一些范例。
“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.
大多数内置类型为格式规范实现了下列选项，尽管某些格式选项只支持数值类型。
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.
一般形式的 标准格式说明符 is:
format_spec ::= [[fill]align][sign][z][#][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 (”
{
或
}
) 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。
各对齐选项的含义如下：
| 选项 | 含义 |
|---|---|
'<'
|
强制字段在可用空间内左对齐 (这是大多数对象的默认值)。 |
'>'
|
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 for numbers when ‘0’ immediately precedes the field width. |
'^'
|
强制字段在可用空间内居中。 |
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:
| 选项 | 含义 |
|---|---|
'+'
|
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). |
| space | indicates that a leading space should be used on positive numbers, and a minus sign on negative numbers. |
'z'
option coerces negative zero floating-point values to positive zero after rounding to the format precision. This option is only valid for floating-point presentation types.
3.11 版改变：
添加
'z'
选项 (另请参阅
PEP 682
).
'#'
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.
','
选项表示使用逗号作为千位分隔符。对于区域设置感知分隔符，使用
'n'
整数表示类型代替。
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'
。对于整数表示类型
'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 是定义最小总字段宽度 (包括任何前缀、分隔符及其它格式化字符) 的十进制整数。若未指定，则字段宽度将由内容确定。
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
'='
.
3.10 版改变：
Preceding the
width
field by
'0'
no longer affects the default alignment for strings.
precision
is a decimal integer indicating how many digits should be displayed after the decimal point for presentation types
'f'
and
'F'
, or before and after the decimal point for presentation types
'g'
or
'G'
. For string presentation 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 presentation types.
最后， type 确定应如何呈现数据。
可用的字符串表示类型：
| 类型 | 含义 |
|---|---|
's'
|
字符串格式。这是用于字符串的默认类型，且可以省略。 |
| None |
如同
's'
.
|
可用的整数表示类型：
| 类型 | 含义 |
|---|---|
'b'
|
Binary format. Outputs the number in base 2. |
'c'
|
Character. Converts the integer to the corresponding unicode character before printing. |
'd'
|
Decimal Integer. Outputs the number in base 10. |
'o'
|
Octal format. Outputs the number in base 8. |
'x'
|
Hex format. Outputs the number in base 16, using lower-case letters for the digits above 9. |
'X'
|
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'
还。
|
'n'
|
数字。这如同
'd'
, except that it uses the current locale setting to insert the appropriate number separator characters.
|
| None |
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.
可用的表示类型对于
float
and
Decimal
值是：
| 类型 | 含义 |
|---|---|
'e'
|
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.
|
'E'
|
Scientific notation. Same as
'e'
except it uses an upper case ‘E’ as the separator character.
|
'f'
|
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.
|
'F'
|
Fixed-point notation. Same as
'f'
, but converts
nan
to
NAN
and
inf
to
INF
.
|
'g'
|
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
With no precision given, uses a precision of
Positive and negative infinity, positive and negative zero, and nans, are formatted as
|
'G'
|
General format. Same as
'g'
except switches to
'E'
if the number gets too large. The representations of infinity and NaN are uppercased, too.
|
'n'
|
数字。这如同
'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.
|
| None |
For
float
这如同
'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
The overall effect is to match the output of
|
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
{}
和采用
:
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.
按位置访问自变量：
>>> '{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'
按名称访问自变量：
>>> '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'
访问自变量的属性：
>>> 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)'
访问自变量的项：
>>> 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"
对齐文本并指定宽度：
>>> '{:<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***********'
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'
使用逗号作为千位分隔符：
>>> '{:,}'.format(1234567890) '1,234,567,890'
表达百分比：
>>> points = 19 >>> total = 22 >>> 'Correct answers: {:.2%}'.format(points/total) 'Correct answers: 86.36%'
使用特定类型格式：
>>> 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'
嵌套自变量和更复杂范例：
>>> 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
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:
$$
是转义；它被替换采用单个
$
.
$identifier
names a substitution placeholder matching a mapping key of
"identifier"
。默认情况下，
"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"
.
任何其它外观的
$
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：
构造函数接受单模板字符串自变量。
Performs the template substitution, returning a new string. mapping 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 mapping and kwds are given and there are duplicates, the placeholders from kwds take precedence.
像
substitute()
, except that if placeholders are missing from
mapping
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
$
而不是引发
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.
Returns false if the template has invalid placeholders that will cause
substitute()
to raise
ValueError
.
3.11 版新增。
Returns a list of the valid identifiers in the template, in the order they first appear, ignoring any invalid identifiers.
3.11 版新增。
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.
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.
命名 – 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.
The methods on this class will raise
ValueError
if the pattern matches the template without one of these named groups matching.
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.