string.rst



:mod:`string` --- Common string operations
The :mod:`string` module contains a number of useful constants and
classes, as well as some deprecated legacy functions that are also
available as methods on strings. In addition, Python's built-in string
classes support the sequence type methods described in the
:ref:`typesseq` section, and also the string-specific methods described
in the :ref:`string-methods` section. To output formatted strings use
template strings or the % operator described in the
:ref:`string-formatting` section. Also, see the :mod:`re` module for
string functions based on regular expressions.

String constants
The constants defined in this module are:

String Formatting
The built-in str and unicode classes provide the ability
to do complex variable substitutions and value formatting via the
:meth:`str.format` method described in PEP 3101.  The :class:`Formatter`
class in the :mod:`string` module allows you to create and customize your own
string formatting behaviors using the same implementation as the built-in
:meth:`format` method.
The :class:`Formatter` class has the following public methods:
In addition, the :class:`Formatter` defines a number of methods that are
intended to be replaced by subclasses:

Format String Syntax
The :meth:`str.format` method and the :class:`Formatter` class share the same
syntax for format strings (although in the case of :class:`Formatter`,
subclasses can define their own format string syntax.)
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:


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.
The field_name itself begins with an arg_name that is either 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.
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 :func:`getattr`, while an expression of the form '[index]'
does an index lookup using :func:`__getitem__`.
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 :meth:`__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 :meth:`__format__`, the normal formatting logic
is bypassed.
Two conversion flags are currently supported: '!s' which calls :func:`str`
on the value, and '!r' which calls :func:`repr`.
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

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 can contain only a field name; conversion flags
and format specifications are 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.
For example, suppose you wanted to have a replacement field whose field width is
determined by another variable:
"A man with two {0:{1}}".format("noses", 10)

This would first evaluate the inner replacement field, making the format string
effectively:
"A man with two {0:10}"

Then the outer replacement field would be evaluated, producing:
"noses     "

Which is substituted into the string, yielding:
"A man with two noses     "

(The extra space is because we specified a field width of 10, and because left
alignment is the default for strings.)

Format Specification Mini-Language
"Format specifications" are used within replacement fields contained within a
format string to define how individual values are presented (see
:ref:`formatstrings`.)  They can also be passed directly to the built-in
:func:`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 :func:`str` on the value. A
non-empty format string typically modifies the result.
The general form of a standard format specifier is:
The fill character can be any character other than '}' (which signifies the
end of the field).  The presence of a fill character is signaled by the next
character, which must be one of the alignment options. If the second character
of format_spec is not a valid alignment option, then it is assumed that both
the fill character and the alignment option are absent.
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.)


'>'
Forces the field to be right-aligned within the
available space.


'='
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.


'^'
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 is only valid for integers, and only for binary, octal, or
hexadecimal output.  If present, it specifies that the output will be prefixed
by '0b', '0o', or '0x', respectively.
The ',' option signals the use of a comma for a thousands separator.
For a locale aware separator, use the 'n' integer presentation type
instead.
width is a decimal integer defining the minimum field width.  If not
specified, then the field width will be determined by the content.
If the width field is preceded by a zero ('0') character, this enables
zero-padding.  This is equivalent to an alignment type of '=' and a fill
character of '0'.
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, :func:`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.


'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.


'F'
Fixed point. Same as 'f'.


'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.
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 -4 <= exp < p, 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.
Postive and negative infinity, positive and negative
zero, and nans, are formatted as inf, -inf,
0, -0 and nan respectively, regardless of
the precision.
A precision of 0 is treated as equivalent to a
precision of 1.


'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
The same as 'g'.


Template strings
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" must spell a Python
identifier.  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 :exc:`ValueError`
being raised.
The :mod:`string` module provides a :class:`Template` class that implements
these rules.  The methods of :class:`Template` are:
The constructor takes a single argument which is the template string.
:class:`Template` instances also provide one public data attribute:
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 $100').substitute(d)
Traceback (most recent call last):
[...]
ValueError: Invalid placeholder in string: line 1, col 10
>>> Template('$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 :class:`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 $.  Note that this should not be a regular
expression, as the implementation will call :meth:`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]*.

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


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

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

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

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


String functions
The following functions are available to operate on string and Unicode objects.
They are not available as string methods.

Deprecated string functions
The following list of functions are also defined as methods of string and
Unicode objects; see section :ref:`string-methods` for more information on
those.  You should consider these functions as deprecated, although they will
not be removed until Python 3.0.  The functions defined in this module are: