string — Common string operations¶Source code: Lib/string.py
The constants defined in this module are:
string.ascii_letters¶The concatenation of the ascii_lowercase and ascii_uppercase
constants described below. This value is not locale-dependent.
string.ascii_lowercase¶The lowercase letters 'abcdefghijklmnopqrstuvwxyz'. This value is not
locale-dependent and will not change.
string.ascii_uppercase¶The uppercase letters 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'. This value is not
locale-dependent and will not change.
string.digits¶The string '0123456789'.
string.hexdigits¶The string '0123456789abcdefABCDEF'.
string.octdigits¶The string '01234567'.
string.punctuation¶String of ASCII characters which are considered punctuation characters
in the C locale.
string.printable¶String of ASCII characters which are considered printable. This is a
combination of digits, ascii_letters, punctuation,
and whitespace.
string.whitespace¶A string containing all ASCII characters that are considered whitespace. This includes the characters space, tab, linefeed, return, formfeed, and vertical tab.
The built-in string class provides the ability to do complex variable
substitutions and value formatting via the format() method described in
PEP 3101. The Formatter class in the string module allows
you to create and customize your own string formatting behaviors using the same
implementation as the built-in format() method.
string.Formatter¶The Formatter class has the following public methods:
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().
Deprecated since version 3.5: Passing a format string as keyword argument format_string has been deprecated.
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
syntax. vformat() does the work of breaking up the format string
into character data and replacement fields. It calls the various
methods described below.
In addition, the 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
will be 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 parameter to get_value().
get_value(key, args, kwargs)¶Retrieve a given field value. The 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.
The args parameter is set to the list of positional arguments to
vformat(), and the 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() function.
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() simply calls the global format() built-in. The
method is provided so that subclasses can override it.
convert_field(value, conversion)¶Converts the value (returned by 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.
The str.format() method and the 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 formatted string literals, but
there are differences.
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|integer] attribute_name ::=identifierelement_index ::=integer|index_stringindex_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 '!', and a format_spec, which is preceded
by a colon ':'. These specify a non-default format for the replacement value.
See also the Format Specification Mini-Language section.
The 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__().
Changed in version 3.1: The positional argument specifiers can be omitted, so '{} {}' is
equivalent to '{0} {1}'.
Some simple format string examples:
"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'.
The 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().
Some examples:
"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
The 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.
See the Format examples section for some examples.
“Format specifications” are used within replacement fields contained within a
format string to define how individual values are presented (see
Format String Syntax and Formatted string literals).
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 string ("") produces
the same result as if you had called str() on the value. A
non-empty format string typically modifies the result.
The general form of a standard format specifier is:
format_spec ::= [[fill]align][sign][#][0][width][grouping_option][.precision][type] fill ::= <any character> align ::= "<" | ">" | "=" | "^" sign ::= "+" | "-" | " " width ::=integergrouping_option ::= "_" | "," precision ::=integertype ::= "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"
If a valid align value is specified, it can be preceded by a fill
character that can be any character and defaults to a space if omitted.
It is not possible to use a literal curly brace (“{” or “}”) as
the fill character in a formatted string literal or when using the str.format()
method. However, it is possible to insert a curly brace
with a nested replacement field. This limitation doesn’t
affect the format() function.
The meaning of the various alignment options is as follows:
Option Meaning '<'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.
The sign option is only valid for number types, and can be one of the following:
Option Meaning '+'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.
The '#' 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, complex and
Decimal types. For integers, when binary, octal, or hexadecimal output
is used, this option adds the prefix respective '0b', '0o', or
'0x' to the output value. For floats, complex and Decimal 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.
The ',' option signals the use of a comma for a thousands separator.
For a locale aware separator, use the 'n' integer presentation type
instead.
Changed in version 3.1: Added the ',' option (see also PEP 378).
The '_' 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', and 'X', underscores will be inserted every 4
digits. For other presentation types, specifying this option is an
error.
Changed in version 3.6: Added the '_' option (see also PEP 515).
width is a decimal integer defining the minimum field width. 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 '='.
The 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:
Type Meaning 's'String format. This is the default type for strings and may be omitted. None The same as 's'.
The available integer presentation types are:
Type Meaning '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. 'n'Number. This is the same as 'd', except that it uses the current locale setting to insert the appropriate number separator characters.None The same as '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 floating point and decimal values are:
Type Meaning 'e'Exponent notation. Prints the number in scientific notation using the letter ‘e’ to indicate the exponent. The default precision is 6.'E'Exponent notation. Same as 'e'except it uses an upper case ‘E’ as the separator character.'f'Fixed point. Displays the number as a fixed-point number. The default precision is 6.'F'Fixed point. Same as 'f', but convertsnantoNANandinftoINF.'g'General format. For a given precision
p >= 1, this rounds the number topsignificant digits and then formats the result in either fixed-point format or in scientific notation, depending on its magnitude.The precise rules are as follows: suppose that the result formatted with presentation type
'e'and precisionp-1would have exponentexp. Then if-4 <= exp < p, the number is formatted with presentation type'f'and precisionp-1-exp. Otherwise, the number is formatted with presentation type'e'and precisionp-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.Positive and negative infinity, positive and negative zero, and nans, are formatted as
inf,-inf,0,-0andnanrespectively, regardless of the precision.A precision of
0is treated as equivalent to a precision of1. The default precision is6.'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'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.None Similar to 'g', except that fixed-point notation, when used, has at least one digit past the decimal point. The default precision is as high as needed to represent the particular value. The overall effect is to match the output ofstr()as altered by the other format modifiers.
This section contains examples of the str.format() syntax and
comparison with the old %-formatting.
In most of the cases the syntax is similar to the old %-formatting, with the
addition of the {} and with : used instead of %.
For example, '%03.2f' can be translated to '{:03.2f}'.
The new format syntax also supports new and different options, shown in the follow examples.
Accessing arguments by position:
>>> '{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'
Accessing arguments by name:
>>> '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'
Accessing arguments’ attributes:
>>> 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)'
Accessing arguments’ items:
>>> coord = (3, 5)
>>> 'X: {0[0]}; Y: {0[1]}'.format(coord)
'X: 3; Y: 5'
Replacing %s and %r:
>>> "repr() shows quotes: {!r}; str() doesn't: {!s}".format('test1', 'test2')
"repr() shows quotes: 'test1'; str() doesn't: test2"
Aligning the text and specifying a width:
>>> '{:<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***********'
Replacing %+f, %-f, and % f and specifying a sign:
>>> '{:+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'
Replacing %x and %o and converting the value to different bases:
>>> # 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'
Using the comma as a thousands separator:
>>> '{:,}'.format(1234567890)
'1,234,567,890'
Expressing a percentage:
>>> points = 19
>>> total = 22
>>> 'Correct answers: {:.2%}'.format(points/total)
'Correct answers: 86.36%'
Using type-specific formatting:
>>> 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'
Nesting arguments and more complex examples:
>>> 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
Templates provide simpler string substitutions as described in PEP 292.
Instead of the normal %-based substitutions, Templates support $-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} is equivalent to $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.
The string module provides a Template class that implements
these rules. The methods of Template are:
string.Template(template)¶The constructor takes a single argument which is the template string.
substitute(mapping, **kwds)¶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.
safe_substitute(mapping, **kwds)¶Like 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 $ instead of raising ValueError.
While other exceptions may still occur, this method is called “safe”
because substitutions 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 instances also provide one public data attribute:
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.
Here is an example of how to use a Template:
>>> 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.
idpattern – This is the regular expression describing the pattern for
non-braced placeholders (the braces will be added automatically as
appropriate). The default value is the regular expression
[_a-z][_a-z0-9]*.
flags – The regular expression flags that will be applied when compiling
the regular expression used for recognizing substitutions. The default value
is re.IGNORECASE. Note that re.VERBOSE will always be added to the
flags, so custom idpatterns must follow conventions for verbose regular
expressions.
New in version 3.2.
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:
$$, in the
default pattern.string.capwords(s, sep=None)¶Split the argument into words using str.split(), capitalize each word
using 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.