string.rst

:mod:`string` --- Common string operations
==========================================

.. module:: string
   :synopsis: Common string operations.


.. index:: module: re

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, see 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:: octdigits

   The string ``'01234567'``.


.. data:: punctuation

   String of ASCII characters which are considered punctuation characters
   in the ``C`` locale.


.. data:: printable

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


.. data:: whitespace

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


.. _string-formatting:

String Formatting
-----------------

Starting in Python 3.0, the built-in string class provides the ability to do
complex variable substitutions and value formatting via the :func:`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 template
      string, and an arbitrary set of positional and keyword argument.
      :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 template
      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 in to 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 throw 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 'r' (repr) and 's' (str) 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: (`identifier` | `integer`) ("." `attribute_name` | "[" element_index "]")*
      attribute_name: `identifier`
      element_index: `integer`
      conversion: "r" | "s"
      format_spec: <described in the next section>
      
In less formal terms, the replacement field starts with a *field_name*, which
can either be a number (for a positional argument), or an identifier (for
keyword arguments).  Following this is an optional *conversion* field, which is
preceded by an exclamation point ``'!'``, and a *format_spec*, which is preceded
by a colon ``':'``.

The *field_name* itself begins with 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.  This 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
   "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 it's 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 subsitituted 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.)


.. _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 builtin
: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.

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 '}'>
   align: "<" | ">" | "=" | "^"
   sign: "+" | "-" | " "
   width: `integer`
   precision: `integer`
   type: "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "x" | "X" | "%"
   
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.  |
   +---------+----------------------------------------------------------+

*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.  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 ignored for
integer values.

Finally, the *type* determines how the data should be presented.

The available integer presentation types are:

   +---------+----------------------------------------------------------+
   | Type    | Meaning                                                  |
   +=========+==========================================================+
   | ``'b'`` | Binary. 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.                     |
   +---------+----------------------------------------------------------+
   | None    | the same as ``'d'``                                      |
   +---------+----------------------------------------------------------+
                                                                         
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. This prints the number as a fixed-point  |
   |         | number, unless the number is too large, in which case    |
   |         | it switches to ``'e'`` exponent notation.                |
   +---------+----------------------------------------------------------+
   | ``'G'`` | General format. Same as ``'g'`` except switches to       |
   |         | ``'E'`` if the number gets to large.                     |
   +---------+----------------------------------------------------------+
   | ``'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:

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:


.. class:: Template(template)

   The constructor takes a single argument which is the template string.


.. method:: Template.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:: Template.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:: string.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 ``$``.  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)

   Split the argument into words using :func:`split`, capitalize each word using
   :func:`capitalize`, and join the capitalized words using :func:`join`.  Note
   that this replaces runs of whitespace characters by a single space, and removes
   leading and trailing whitespace.


.. function:: maketrans(frm, to)

   Return a translation table suitable for passing to :meth:`bytes.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.