
``string`` --- Common string operations
***************************************

The ``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 *Sequence
Types --- str, bytes, bytearray, list, tuple, range* section, and also
the string-specific methods described in the *String Methods* section.
To output formatted strings, see the *String Formatting* section.
Also, see the ``re`` module for string functions based on regular
expressions.


String constants
================

The constants defined in this module are:

string.ascii_letters

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

string.ascii_lowercase

   The lowercase letters ``'abcdefghijklmnopqrstuvwxyz'``.  This value
   is not locale-dependent and will not change.

string.ascii_uppercase

   The uppercase letters ``'ABCDEFGHIJKLMNOPQRSTUVWXYZ'``.  This value
   is not locale-dependent and will not change.

string.digits

   The string ``'0123456789'``.

string.hexdigits

   The string ``'0123456789abcdefABCDEF'``.

string.octdigits

   The string ``'01234567'``.

string.punctuation

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

string.printable

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

string.whitespace

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


String Formatting
=================

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.

class string.Formatter

   The ``Formatter`` class has the following public methods:

   format(format_string, *args, *kwargs)

      ``format()`` is the primary API method.  It takes a format
      template string, and an arbitrary set of positional and keyword
      argument. ``format()`` is just a wrapper that calls
      ``vformat()``.

   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.  ``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 ``Formatter`` defines a number of methods that are
   intended to be replaced by subclasses:

   parse(format_string)

      Loop over the format_string and return an iterable of tuples
      (*literal_text*, *field_name*, *format_spec*, *conversion*).
      This is used by ``vformat()`` to break the string 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``.

   get_field(field_name, args, kwargs)

      Given *field_name* as returned by ``parse()`` (see above),
      convert it to an object to be formatted.  Returns a tuple (obj,
      used_key).  The default version takes strings of the form
      defined in **PEP 3101**, such as "0[name]" or "label.title".
      *args* and *kwargs* are as passed in to ``vformat()``.  The
      return value *used_key* has the same meaning as the *key*
      parameter to ``get_value()``.

   get_value(key, args, kwargs)

      Retrieve a given field value.  The *key* argument will be either
      an integer or a string.  If it is an integer, it represents the
      index of the positional argument in *args*; if it is a string,
      then it represents a named argument in *kwargs*.

      The *args* parameter is set to the list of positional arguments
      to ``vformat()``, and the *kwargs* parameter is set to the
      dictionary of keyword arguments.

      For compound field names, these functions are only called for
      the first component of the field name; Subsequent components are
      handled through normal attribute and indexing operations.

      So for example, the field expression '0.name' would cause
      ``get_value()`` to be called with a *key* argument of 0.  The
      ``name`` attribute will be looked up after ``get_value()``
      returns by calling the built-in ``getattr()`` function.

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

   check_unused_args(used_args, args, kwargs)

      Implement checking for unused arguments if desired.  The
      arguments to this function is the set of all argument keys that
      were actually referred to in the format string (integers for
      positional arguments, and strings for named arguments), and a
      reference to the *args* and *kwargs* that was passed to vformat.
      The set of unused args can be calculated from these parameters.
      ``check_unused_args()`` is assumed to throw an exception if the
      check fails.

   format_field(value, format_spec)

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

   convert_field(value, conversion)

      Converts the value (returned by ``get_field()``) given a
      conversion type (as in the tuple returned by the ``parse()``
      method.)  The default version understands 'r' (repr) and 's'
      (str) conversion types.


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.)

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        ::= (identifier | integer) ("." attribute_name | "[" element_index "]")*
      attribute_name    ::= identifier
      element_index     ::= integer
      conversion        ::= "r" | "s" | "a"
      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 ``getattr()``,
while an expression of the form ``'[index]'`` does an index lookup
using ``__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
``__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

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 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 *Format String Syntax*.)  They can also be passed directly to the
builtin ``format()`` function.  Each formattable type may define how
the format specification is to be interpreted.

Most built-in types implement the following options for format
specifications, although some of the formatting options are only
supported by the numeric types.

A general convention is that an empty format string (``""``) produces
the same result as if you had called ``str()`` on the value.

The general form of a *standard format specifier* is:

   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.             |
   +-----------+------------------------------------------------------------+

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.

*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 ignored for integer values.

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

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'``.                                       |
   +-----------+------------------------------------------------------------+

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. Infinity and NaN    |
   |           | values are formatted as ``inf``, ``-inf`` and ``nan``,     |
   |           | respectively.                                              |
   +-----------+------------------------------------------------------------+
   | ``'G'``   | General format. Same as ``'g'`` except switches to ``'E'`` |
   |           | if the number gets to 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
``ValueError`` being raised.

The ``string`` module provides a ``Template`` class that implements
these rules.  The methods of ``Template`` are:

class string.Template(template)

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

   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.

   safe_substitute(mapping[, **kws])

      Like ``substitute()``, except that if placeholders are missing
      from *mapping* and *kws*, instead of raising a ``KeyError``
      exception, the original placeholder will appear in the resulting
      string intact.  Also, unlike with ``substitute()``, any other
      appearances of the ``$`` will simply return ``$`` instead of
      raising ``ValueError``.

      While other exceptions may still occur, this method is called
      "safe" because substitutions always tries to return a usable
      string instead of raising an exception.  In another sense,
      ``safe_substitute()`` may be anything other than safe, since it
      will silently ignore malformed templates containing dangling
      delimiters, unmatched braces, or placeholders that are not valid
      Python identifiers.

``Template`` instances also provide one public data attribute:

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 ``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 ``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 objects.
They are not available as string methods.

string.capwords(s)

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

string.maketrans(frm, to)

   Return a translation table suitable for passing to
   ``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.
