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:mod:`string` --- Common string operations
==========================================
.. module:: string
:synopsis: Common string operations.
.. index:: module: re
**Source code:** :source:`Lib/string.py`
--------------
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:
.. data:: ascii_letters
The concatenation of the :const:`ascii_lowercase` and :const:`ascii_uppercase`
constants described below. This value is not locale-dependent.
.. data:: ascii_lowercase
The lowercase letters ``'abcdefghijklmnopqrstuvwxyz'``. This value is not
locale-dependent and will not change.
.. data:: ascii_uppercase
The uppercase letters ``'ABCDEFGHIJKLMNOPQRSTUVWXYZ'``. This value is not
locale-dependent and will not change.
.. data:: digits
The string ``'0123456789'``.
.. data:: hexdigits
The string ``'0123456789abcdefABCDEF'``.
.. data:: letters
The concatenation of the strings :const:`lowercase` and :const:`uppercase`
described below. The specific value is locale-dependent, and will be updated
when :func:`locale.setlocale` is called.
.. data:: lowercase
A string containing all the characters that are considered lowercase letters.
On most systems this is the string ``'abcdefghijklmnopqrstuvwxyz'``. The
specific value is locale-dependent, and will be updated when
:func:`locale.setlocale` is called.
.. data:: octdigits
The string ``'01234567'``.
.. data:: punctuation
String of ASCII characters which are considered punctuation characters in the
``C`` locale.
.. data:: printable
String of characters which are considered printable. This is a combination of
:const:`digits`, :const:`letters`, :const:`punctuation`, and
:const:`whitespace`.
.. data:: uppercase
A string containing all the characters that are considered uppercase letters.
On most systems this is the string ``'ABCDEFGHIJKLMNOPQRSTUVWXYZ'``. The
specific value is locale-dependent, and will be updated when
:func:`locale.setlocale` is called.
.. data:: whitespace
A string containing all characters that are considered whitespace. On most
systems this includes the characters space, tab, linefeed, return, formfeed, and
vertical tab.
.. _new-string-formatting:
String Formatting
-----------------
.. versionadded:: 2.6
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.
.. class:: Formatter
The :class:`Formatter` class has the following public methods:
.. method:: format(format_string, *args, **kwargs)
:meth:`format` is the primary API method. It takes a format string and
an arbitrary set of positional and keyword arguments.
:meth:`format` is just a wrapper that calls :meth:`vformat`.
.. method:: 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 ``**kwds``
syntax. :meth:`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 :class:`Formatter` defines a number of methods that are
intended to be replaced by subclasses:
.. method:: 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 :meth:`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``.
.. method:: get_field(field_name, args, kwargs)
Given *field_name* as returned by :meth:`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
:meth:`vformat`. The return value *used_key* has the same meaning as the
*key* parameter to :meth:`get_value`.
.. method:: 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
:meth:`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
:meth:`get_value` to be called with a *key* argument of 0. The ``name``
attribute will be looked up after :meth:`get_value` returns by calling the
built-in :func:`getattr` function.
If the index or keyword refers to an item that does not exist, then an
:exc:`IndexError` or :exc:`KeyError` should be raised.
.. method:: 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. :meth:`check_unused_args` is assumed to raise an exception if
the check fails.
.. method:: format_field(value, format_spec)
:meth:`format_field` simply calls the global :func:`format` built-in. The
method is provided so that subclasses can override it.
.. method:: convert_field(value, conversion)
Converts the value (returned by :meth:`get_field`) given a conversion type
(as in the tuple returned by the :meth:`parse` method). The default
version understands 's' (str), 'r' (repr) and 'a' (ascii) conversion
types.
.. _formatstrings:
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:
.. productionlist:: sf
replacement_field: "{" [`field_name`] ["!" `conversion`] [":" `format_spec`] "}"
field_name: arg_name ("." `attribute_name` | "[" `element_index` "]")*
arg_name: [`identifier` | `integer`]
attribute_name: `identifier`
element_index: `integer` | `index_string`
index_string: <any source character except "]"> +
conversion: "r" | "s"
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 :ref:`formatspec` 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 :func:`getattr`, while an expression of the form ``'[index]'``
does an index lookup using :func:`__getitem__`.
.. versionchanged:: 2.7
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 :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.
See the :ref:`formatexamples` section for some examples.
.. _formatspec:
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:
.. productionlist:: sf
format_spec: [[`fill`]`align`][`sign`][#][0][`width`][,][.`precision`][`type`]
fill: <a character other than '{' or '}'>
align: "<" | ">" | "=" | "^"
sign: "+" | "-" | " "
width: `integer`
precision: `integer`
type: "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"
The *fill* character can be any character other than '{' or '}'. The presence
of a fill character is signaled by the character following it, 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 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. |
+---------+----------------------------------------------------------+
| ``'^'`` | 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.
.. versionchanged:: 2.7
Added the ``','`` option (see also :pep:`378`).
*width* is a decimal integer defining the minimum field width. If not
specified, then the field width will be determined by the content.
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, :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. |
| | |
| | Positive 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'``. |
+---------+----------------------------------------------------------+
.. _formatexamples:
Format examples
^^^^^^^^^^^^^^^
This section contains examples of the new 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') # 2.7+ 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(object):
... 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 = 19.5
>>> total = 22
>>> 'Correct answers: {:.2%}'.format(points/total)
'Correct answers: 88.64%'
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),
... 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
----------------
.. versionadded:: 2.4
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:
.. class:: Template(template)
The constructor takes a single argument which is the template string.
.. method:: substitute(mapping[, **kws])
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 *kws* are given
and there are duplicates, the placeholders from *kws* take precedence.
.. method:: safe_substitute(mapping[, **kws])
Like :meth:`substitute`, except that if placeholders are missing from
*mapping* and *kws*, instead of raising a :exc:`KeyError` exception, the
original placeholder will appear in the resulting string intact. Also,
unlike with :meth:`substitute`, any other appearances of the ``$`` will
simply return ``$`` instead of raising :exc:`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, :meth:`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.
:class:`Template` instances also provide one public data attribute:
.. 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 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 is ``$``. 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.
.. function:: capwords(s[, sep])
Split the argument into words using :meth:`str.split`, capitalize each word
using :meth:`str.capitalize`, and join the capitalized words using
:meth:`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.
.. function:: maketrans(from, to)
Return a translation table suitable for passing to :func:`translate`, that will
map each character in *from* into the character at the same position in *to*;
*from* and *to* must have the same length.
.. note::
Don't use strings derived from :const:`lowercase` and :const:`uppercase` as
arguments; in some locales, these don't have the same length. For case
conversions, always use :meth:`str.lower` and :meth:`str.upper`.
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. The functions defined in this module are:
.. function:: atof(s)
.. deprecated:: 2.0
Use the :func:`float` built-in function.
.. index:: builtin: float
Convert a string to a floating point number. The string must have the standard
syntax for a floating point literal in Python, optionally preceded by a sign
(``+`` or ``-``). Note that this behaves identical to the built-in function
:func:`float` when passed a string.
.. note::
.. index::
single: NaN
single: Infinity
When passing in a string, values for NaN and Infinity may be returned, depending
on the underlying C library. The specific set of strings accepted which cause
these values to be returned depends entirely on the C library and is known to
vary.
.. function:: atoi(s[, base])
.. deprecated:: 2.0
Use the :func:`int` built-in function.
.. index:: builtin: eval
Convert string *s* to an integer in the given *base*. The string must consist
of one or more digits, optionally preceded by a sign (``+`` or ``-``). The
*base* defaults to 10. If it is 0, a default base is chosen depending on the
leading characters of the string (after stripping the sign): ``0x`` or ``0X``
means 16, ``0`` means 8, anything else means 10. If *base* is 16, a leading
``0x`` or ``0X`` is always accepted, though not required. This behaves
identically to the built-in function :func:`int` when passed a string. (Also
note: for a more flexible interpretation of numeric literals, use the built-in
function :func:`eval`.)
.. function:: atol(s[, base])
.. deprecated:: 2.0
Use the :func:`long` built-in function.
.. index:: builtin: long
Convert string *s* to a long integer in the given *base*. The string must
consist of one or more digits, optionally preceded by a sign (``+`` or ``-``).
The *base* argument has the same meaning as for :func:`atoi`. A trailing ``l``
or ``L`` is not allowed, except if the base is 0. Note that when invoked
without *base* or with *base* set to 10, this behaves identical to the built-in
function :func:`long` when passed a string.
.. function:: capitalize(word)
Return a copy of *word* with only its first character capitalized.
.. function:: expandtabs(s[, tabsize])
Expand tabs in a string replacing them by one or more spaces, depending on the
current column and the given tab size. The column number is reset to zero after
each newline occurring in the string. This doesn't understand other non-printing
characters or escape sequences. The tab size defaults to 8.
.. function:: find(s, sub[, start[,end]])
Return the lowest index in *s* where the substring *sub* is found such that
*sub* is wholly contained in ``s[start:end]``. Return ``-1`` on failure.
Defaults for *start* and *end* and interpretation of negative values is the same
as for slices.
.. function:: rfind(s, sub[, start[, end]])
Like :func:`find` but find the highest index.
.. function:: index(s, sub[, start[, end]])
Like :func:`find` but raise :exc:`ValueError` when the substring is not found.
.. function:: rindex(s, sub[, start[, end]])
Like :func:`rfind` but raise :exc:`ValueError` when the substring is not found.
.. function:: count(s, sub[, start[, end]])
Return the number of (non-overlapping) occurrences of substring *sub* in string
``s[start:end]``. Defaults for *start* and *end* and interpretation of negative
values are the same as for slices.
.. function:: lower(s)
Return a copy of *s*, but with upper case letters converted to lower case.
.. function:: split(s[, sep[, maxsplit]])
Return a list of the words of the string *s*. If the optional second argument
*sep* is absent or ``None``, the words are separated by arbitrary strings of
whitespace characters (space, tab, newline, return, formfeed). If the second
argument *sep* is present and not ``None``, it specifies a string to be used as
the word separator. The returned list will then have one more item than the
number of non-overlapping occurrences of the separator in the string.
If *maxsplit* is given, at most *maxsplit* number of splits occur, and the
remainder of the string is returned as the final element of the list (thus,
the list will have at most ``maxsplit+1`` elements). If *maxsplit* is not
specified or ``-1``, then there is no limit on the number of splits (all
possible splits are made).
The behavior of split on an empty string depends on the value of *sep*. If *sep*
is not specified, or specified as ``None``, the result will be an empty list.
If *sep* is specified as any string, the result will be a list containing one
element which is an empty string.
.. function:: rsplit(s[, sep[, maxsplit]])
Return a list of the words of the string *s*, scanning *s* from the end. To all
intents and purposes, the resulting list of words is the same as returned by
:func:`split`, except when the optional third argument *maxsplit* is explicitly
specified and nonzero. If *maxsplit* is given, at most *maxsplit* number of
splits -- the *rightmost* ones -- occur, and the remainder of the string is
returned as the first element of the list (thus, the list will have at most
``maxsplit+1`` elements).
.. versionadded:: 2.4
.. function:: splitfields(s[, sep[, maxsplit]])
This function behaves identically to :func:`split`. (In the past, :func:`split`
was only used with one argument, while :func:`splitfields` was only used with
two arguments.)
.. function:: join(words[, sep])
Concatenate a list or tuple of words with intervening occurrences of *sep*.
The default value for *sep* is a single space character. It is always true that
``string.join(string.split(s, sep), sep)`` equals *s*.
.. function:: joinfields(words[, sep])
This function behaves identically to :func:`join`. (In the past, :func:`join`
was only used with one argument, while :func:`joinfields` was only used with two
arguments.) Note that there is no :meth:`joinfields` method on string objects;
use the :meth:`join` method instead.
.. function:: lstrip(s[, chars])
Return a copy of the string with leading characters removed. If *chars* is
omitted or ``None``, whitespace characters are removed. If given and not
``None``, *chars* must be a string; the characters in the string will be
stripped from the beginning of the string this method is called on.
.. versionchanged:: 2.2.3
The *chars* parameter was added. The *chars* parameter cannot be passed in
earlier 2.2 versions.
.. function:: rstrip(s[, chars])
Return a copy of the string with trailing characters removed. If *chars* is
omitted or ``None``, whitespace characters are removed. If given and not
``None``, *chars* must be a string; the characters in the string will be
stripped from the end of the string this method is called on.
.. versionchanged:: 2.2.3
The *chars* parameter was added. The *chars* parameter cannot be passed in
earlier 2.2 versions.
.. function:: strip(s[, chars])
Return a copy of the string with leading and trailing characters removed. If
*chars* is omitted or ``None``, whitespace characters are removed. If given and
not ``None``, *chars* must be a string; the characters in the string will be
stripped from the both ends of the string this method is called on.
.. versionchanged:: 2.2.3
The *chars* parameter was added. The *chars* parameter cannot be passed in
earlier 2.2 versions.
.. function:: swapcase(s)
Return a copy of *s*, but with lower case letters converted to upper case and
vice versa.
.. function:: translate(s, table[, deletechars])
Delete all characters from *s* that are in *deletechars* (if present), and then
translate the characters using *table*, which must be a 256-character string
giving the translation for each character value, indexed by its ordinal. If
*table* is ``None``, then only the character deletion step is performed.
.. function:: upper(s)
Return a copy of *s*, but with lower case letters converted to upper case.
.. function:: ljust(s, width[, fillchar])
rjust(s, width[, fillchar])
center(s, width[, fillchar])
These functions respectively left-justify, right-justify and center a string in
a field of given width. They return a string that is at least *width*
characters wide, created by padding the string *s* with the character *fillchar*
(default is a space) until the given width on the right, left or both sides.
The string is never truncated.
.. function:: zfill(s, width)
Pad a numeric string *s* on the left with zero digits until the
given *width* is reached. Strings starting with a sign are handled
correctly.
.. function:: replace(s, old, new[, maxreplace])
Return a copy of string *s* with all occurrences of substring *old* replaced
by *new*. If the optional argument *maxreplace* is given, the first
*maxreplace* occurrences are replaced.
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