string — Common string operations — Python 3.11.0 documentation

Source code: Lib/string.py

A string containing all ASCII characters that are considered whitespace. This includes the characters space, tab, linefeed, return, formfeed, and vertical tab.

String of ASCII characters which are considered printable. This is a combination of digits , ascii_letters , punctuation , and whitespace .

String of ASCII characters which are considered punctuation characters in the C locale: !”#$%&'()*+,-./:;<=>?@[\]^_`{|}~ .

The uppercase letters ‘ABCDEFGHIJKLMNOPQRSTUVWXYZ’ . This value is not locale-dependent and will not change.

The lowercase letters ‘abcdefghijklmnopqrstuvwxyz’ . This value is not locale-dependent and will not change.

The concatenation of the ascii_lowercase and ascii_uppercase constants described below. This value is not locale-dependent.

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.

format_field() simply calls the global format() built-in. The method is provided so that subclasses can override it.

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.

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

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.

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.

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.

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.

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() .

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 .

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.

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

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.

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() .

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.

Format String Syntax¶

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 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.
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 for str.format(),
so '{} {}'.format(a, b) is equivalent to '{0} {1}'.format(a, b).

Changed in version 3.4: The positional argument specifiers can be omitted for Formatter.

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 Specification Mini-Language¶

“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 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 standard format specifier 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" | "%"

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 for
numbers 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 '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.

Changed in version 3.11: Added the 'z' option (see also PEP 682).

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 and complex
types. For integers, when binary, octal, or hexadecimal output
is used, this option adds the respective prefix '0b', '0o',
'0x', or '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.

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 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 '='.

Changed in version 3.10: Preceding the width field by '0' no longer affects the default
alignment for strings.

The 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.

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.
In case '#' is specified, the prefix '0x' will
be upper-cased to '0X' as well.

'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 float and
Decimal values are:

Type

Meaning

'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 'e' and
precision p-1 would have exponent exp. Then,
if m <= exp < p, where 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.

'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

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.

Format examples¶

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
following 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
-
5
j
>>> 
(
'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