1:mod:`string` --- Common string operations
2==========================================
3
4.. module:: string
5   :synopsis: Common string operations.
6
7**Source code:** :source:`Lib/string.py`
8
9--------------
10
11.. seealso::
12
13   :ref:`textseq`
14
15   :ref:`string-methods`
16
17String constants
18----------------
19
20The constants defined in this module are:
21
22
23.. data:: ascii_letters
24
25   The concatenation of the :const:`ascii_lowercase` and :const:`ascii_uppercase`
26   constants described below.  This value is not locale-dependent.
27
28
29.. data:: ascii_lowercase
30
31   The lowercase letters ``'abcdefghijklmnopqrstuvwxyz'``.  This value is not
32   locale-dependent and will not change.
33
34
35.. data:: ascii_uppercase
36
37   The uppercase letters ``'ABCDEFGHIJKLMNOPQRSTUVWXYZ'``.  This value is not
38   locale-dependent and will not change.
39
40
41.. data:: digits
42
43   The string ``'0123456789'``.
44
45
46.. data:: hexdigits
47
48   The string ``'0123456789abcdefABCDEF'``.
49
50
51.. data:: octdigits
52
53   The string ``'01234567'``.
54
55
56.. data:: punctuation
57
58   String of ASCII characters which are considered punctuation characters
59   in the ``C`` locale.
60
61
62.. data:: printable
63
64   String of ASCII characters which are considered printable.  This is a
65   combination of :const:`digits`, :const:`ascii_letters`, :const:`punctuation`,
66   and :const:`whitespace`.
67
68
69.. data:: whitespace
70
71   A string containing all ASCII characters that are considered whitespace.
72   This includes the characters space, tab, linefeed, return, formfeed, and
73   vertical tab.
74
75
76.. _string-formatting:
77
78Custom String Formatting
79------------------------
80
81The built-in string class provides the ability to do complex variable
82substitutions and value formatting via the :meth:`~str.format` method described in
83:pep:`3101`.  The :class:`Formatter` class in the :mod:`string` module allows
84you to create and customize your own string formatting behaviors using the same
85implementation as the built-in :meth:`~str.format` method.
86
87
88.. class:: Formatter
89
90   The :class:`Formatter` class has the following public methods:
91
92   .. method:: format(format_string, *args, **kwargs)
93
94      The primary API method.  It takes a format string and
95      an arbitrary set of positional and keyword arguments.
96      It is just a wrapper that calls :meth:`vformat`.
97
98      .. versionchanged:: 3.7
99         A format string argument is now :ref:`positional-only
100         <positional-only_parameter>`.
101
102   .. method:: vformat(format_string, args, kwargs)
103
104      This function does the actual work of formatting.  It is exposed as a
105      separate function for cases where you want to pass in a predefined
106      dictionary of arguments, rather than unpacking and repacking the
107      dictionary as individual arguments using the ``*args`` and ``**kwargs``
108      syntax.  :meth:`vformat` does the work of breaking up the format string
109      into character data and replacement fields.  It calls the various
110      methods described below.
111
112   In addition, the :class:`Formatter` defines a number of methods that are
113   intended to be replaced by subclasses:
114
115   .. method:: parse(format_string)
116
117      Loop over the format_string and return an iterable of tuples
118      (*literal_text*, *field_name*, *format_spec*, *conversion*).  This is used
119      by :meth:`vformat` to break the string into either literal text, or
120      replacement fields.
121
122      The values in the tuple conceptually represent a span of literal text
123      followed by a single replacement field.  If there is no literal text
124      (which can happen if two replacement fields occur consecutively), then
125      *literal_text* will be a zero-length string.  If there is no replacement
126      field, then the values of *field_name*, *format_spec* and *conversion*
127      will be ``None``.
128
129   .. method:: get_field(field_name, args, kwargs)
130
131      Given *field_name* as returned by :meth:`parse` (see above), convert it to
132      an object to be formatted.  Returns a tuple (obj, used_key).  The default
133      version takes strings of the form defined in :pep:`3101`, such as
134      "0[name]" or "label.title".  *args* and *kwargs* are as passed in to
135      :meth:`vformat`.  The return value *used_key* has the same meaning as the
136      *key* parameter to :meth:`get_value`.
137
138   .. method:: get_value(key, args, kwargs)
139
140      Retrieve a given field value.  The *key* argument will be either an
141      integer or a string.  If it is an integer, it represents the index of the
142      positional argument in *args*; if it is a string, then it represents a
143      named argument in *kwargs*.
144
145      The *args* parameter is set to the list of positional arguments to
146      :meth:`vformat`, and the *kwargs* parameter is set to the dictionary of
147      keyword arguments.
148
149      For compound field names, these functions are only called for the first
150      component of the field name; Subsequent components are handled through
151      normal attribute and indexing operations.
152
153      So for example, the field expression '0.name' would cause
154      :meth:`get_value` to be called with a *key* argument of 0.  The ``name``
155      attribute will be looked up after :meth:`get_value` returns by calling the
156      built-in :func:`getattr` function.
157
158      If the index or keyword refers to an item that does not exist, then an
159      :exc:`IndexError` or :exc:`KeyError` should be raised.
160
161   .. method:: check_unused_args(used_args, args, kwargs)
162
163      Implement checking for unused arguments if desired.  The arguments to this
164      function is the set of all argument keys that were actually referred to in
165      the format string (integers for positional arguments, and strings for
166      named arguments), and a reference to the *args* and *kwargs* that was
167      passed to vformat.  The set of unused args can be calculated from these
168      parameters.  :meth:`check_unused_args` is assumed to raise an exception if
169      the check fails.
170
171   .. method:: format_field(value, format_spec)
172
173      :meth:`format_field` simply calls the global :func:`format` built-in.  The
174      method is provided so that subclasses can override it.
175
176   .. method:: convert_field(value, conversion)
177
178      Converts the value (returned by :meth:`get_field`) given a conversion type
179      (as in the tuple returned by the :meth:`parse` method).  The default
180      version understands 's' (str), 'r' (repr) and 'a' (ascii) conversion
181      types.
182
183
184.. _formatstrings:
185
186Format String Syntax
187--------------------
188
189The :meth:`str.format` method and the :class:`Formatter` class share the same
190syntax for format strings (although in the case of :class:`Formatter`,
191subclasses can define their own format string syntax).  The syntax is
192related to that of :ref:`formatted string literals <f-strings>`, but
193there are differences.
194
195.. index::
196   single: {} (curly brackets); in string formatting
197   single: . (dot); in string formatting
198   single: [] (square brackets); in string formatting
199   single: ! (exclamation); in string formatting
200   single: : (colon); in string formatting
201
202Format strings contain "replacement fields" surrounded by curly braces ``{}``.
203Anything that is not contained in braces is considered literal text, which is
204copied unchanged to the output.  If you need to include a brace character in the
205literal text, it can be escaped by doubling: ``{{`` and ``}}``.
206
207The grammar for a replacement field is as follows:
208
209   .. productionlist:: sf
210      replacement_field: "{" [`field_name`] ["!" `conversion`] [":" `format_spec`] "}"
211      field_name: arg_name ("." `attribute_name` | "[" `element_index` "]")*
212      arg_name: [`identifier` | `digit`+]
213      attribute_name: `identifier`
214      element_index: `digit`+ | `index_string`
215      index_string: <any source character except "]"> +
216      conversion: "r" | "s" | "a"
217      format_spec: <described in the next section>
218
219In less formal terms, the replacement field can start with a *field_name* that specifies
220the object whose value is to be formatted and inserted
221into the output instead of the replacement field.
222The *field_name* is optionally followed by a  *conversion* field, which is
223preceded by an exclamation point ``'!'``, and a *format_spec*, which is preceded
224by a colon ``':'``.  These specify a non-default format for the replacement value.
225
226See also the :ref:`formatspec` section.
227
228The *field_name* itself begins with an *arg_name* that is either a number or a
229keyword.  If it's a number, it refers to a positional argument, and if it's a keyword,
230it refers to a named keyword argument.  If the numerical arg_names in a format string
231are 0, 1, 2, ... in sequence, they can all be omitted (not just some)
232and the numbers 0, 1, 2, ... will be automatically inserted in that order.
233Because *arg_name* is not quote-delimited, it is not possible to specify arbitrary
234dictionary keys (e.g., the strings ``'10'`` or ``':-]'``) within a format string.
235The *arg_name* can be followed by any number of index or
236attribute expressions. An expression of the form ``'.name'`` selects the named
237attribute using :func:`getattr`, while an expression of the form ``'[index]'``
238does an index lookup using :func:`__getitem__`.
239
240.. versionchanged:: 3.1
241   The positional argument specifiers can be omitted for :meth:`str.format`,
242   so ``'{} {}'.format(a, b)`` is equivalent to ``'{0} {1}'.format(a, b)``.
243
244.. versionchanged:: 3.4
245   The positional argument specifiers can be omitted for :class:`Formatter`.
246
247Some simple format string examples::
248
249   "First, thou shalt count to {0}"  # References first positional argument
250   "Bring me a {}"                   # Implicitly references the first positional argument
251   "From {} to {}"                   # Same as "From {0} to {1}"
252   "My quest is {name}"              # References keyword argument 'name'
253   "Weight in tons {0.weight}"       # 'weight' attribute of first positional arg
254   "Units destroyed: {players[0]}"   # First element of keyword argument 'players'.
255
256The *conversion* field causes a type coercion before formatting.  Normally, the
257job of formatting a value is done by the :meth:`__format__` method of the value
258itself.  However, in some cases it is desirable to force a type to be formatted
259as a string, overriding its own definition of formatting.  By converting the
260value to a string before calling :meth:`__format__`, the normal formatting logic
261is bypassed.
262
263Three conversion flags are currently supported: ``'!s'`` which calls :func:`str`
264on the value, ``'!r'`` which calls :func:`repr` and ``'!a'`` which calls
265:func:`ascii`.
266
267Some examples::
268
269   "Harold's a clever {0!s}"        # Calls str() on the argument first
270   "Bring out the holy {name!r}"    # Calls repr() on the argument first
271   "More {!a}"                      # Calls ascii() on the argument first
272
273The *format_spec* field contains a specification of how the value should be
274presented, including such details as field width, alignment, padding, decimal
275precision and so on.  Each value type can define its own "formatting
276mini-language" or interpretation of the *format_spec*.
277
278Most built-in types support a common formatting mini-language, which is
279described in the next section.
280
281A *format_spec* field can also include nested replacement fields within it.
282These nested replacement fields may contain a field name, conversion flag
283and format specification, but deeper nesting is
284not allowed.  The replacement fields within the
285format_spec are substituted before the *format_spec* string is interpreted.
286This allows the formatting of a value to be dynamically specified.
287
288See the :ref:`formatexamples` section for some examples.
289
290
291.. _formatspec:
292
293Format Specification Mini-Language
294^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
295
296"Format specifications" are used within replacement fields contained within a
297format string to define how individual values are presented (see
298:ref:`formatstrings` and :ref:`f-strings`).
299They can also be passed directly to the built-in
300:func:`format` function.  Each formattable type may define how the format
301specification is to be interpreted.
302
303Most built-in types implement the following options for format specifications,
304although some of the formatting options are only supported by the numeric types.
305
306A general convention is that an empty format string (``""``) produces
307the same result as if you had called :func:`str` on the value. A
308non-empty format string typically modifies the result.
309
310The general form of a *standard format specifier* is:
311
312.. productionlist:: sf
313   format_spec: [[`fill`]`align`][`sign`][#][0][`width`][`grouping_option`][.`precision`][`type`]
314   fill: <any character>
315   align: "<" | ">" | "=" | "^"
316   sign: "+" | "-" | " "
317   width: `digit`+
318   grouping_option: "_" | ","
319   precision: `digit`+
320   type: "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"
321
322If a valid *align* value is specified, it can be preceded by a *fill*
323character that can be any character and defaults to a space if omitted.
324It is not possible to use a literal curly brace ("``{``" or "``}``") as
325the *fill* character in a :ref:`formatted string literal
326<f-strings>` or when using the :meth:`str.format`
327method.  However, it is possible to insert a curly brace
328with a nested replacement field.  This limitation doesn't
329affect the :func:`format` function.
330
331The meaning of the various alignment options is as follows:
332
333   .. index::
334      single: < (less); in string formatting
335      single: > (greater); in string formatting
336      single: = (equals); in string formatting
337      single: ^ (caret); in string formatting
338
339   +---------+----------------------------------------------------------+
340   | Option  | Meaning                                                  |
341   +=========+==========================================================+
342   | ``'<'`` | Forces the field to be left-aligned within the available |
343   |         | space (this is the default for most objects).            |
344   +---------+----------------------------------------------------------+
345   | ``'>'`` | Forces the field to be right-aligned within the          |
346   |         | available space (this is the default for numbers).       |
347   +---------+----------------------------------------------------------+
348   | ``'='`` | Forces the padding to be placed after the sign (if any)  |
349   |         | but before the digits.  This is used for printing fields |
350   |         | in the form '+000000120'. This alignment option is only  |
351   |         | valid for numeric types.  It becomes the default when '0'|
352   |         | immediately precedes the field width.                    |
353   +---------+----------------------------------------------------------+
354   | ``'^'`` | Forces the field to be centered within the available     |
355   |         | space.                                                   |
356   +---------+----------------------------------------------------------+
357
358Note that unless a minimum field width is defined, the field width will always
359be the same size as the data to fill it, so that the alignment option has no
360meaning in this case.
361
362The *sign* option is only valid for number types, and can be one of the
363following:
364
365   .. index::
366      single: + (plus); in string formatting
367      single: - (minus); in string formatting
368      single: space; in string formatting
369
370   +---------+----------------------------------------------------------+
371   | Option  | Meaning                                                  |
372   +=========+==========================================================+
373   | ``'+'`` | indicates that a sign should be used for both            |
374   |         | positive as well as negative numbers.                    |
375   +---------+----------------------------------------------------------+
376   | ``'-'`` | indicates that a sign should be used only for negative   |
377   |         | numbers (this is the default behavior).                  |
378   +---------+----------------------------------------------------------+
379   | space   | indicates that a leading space should be used on         |
380   |         | positive numbers, and a minus sign on negative numbers.  |
381   +---------+----------------------------------------------------------+
382
383
384.. index:: single: # (hash); in string formatting
385
386The ``'#'`` option causes the "alternate form" to be used for the
387conversion.  The alternate form is defined differently for different
388types.  This option is only valid for integer, float, complex and
389Decimal types. For integers, when binary, octal, or hexadecimal output
390is used, this option adds the prefix respective ``'0b'``, ``'0o'``, or
391``'0x'`` to the output value. For floats, complex and Decimal the
392alternate form causes the result of the conversion to always contain a
393decimal-point character, even if no digits follow it. Normally, a
394decimal-point character appears in the result of these conversions
395only if a digit follows it. In addition, for ``'g'`` and ``'G'``
396conversions, trailing zeros are not removed from the result.
397
398.. index:: single: , (comma); in string formatting
399
400The ``','`` option signals the use of a comma for a thousands separator.
401For a locale aware separator, use the ``'n'`` integer presentation type
402instead.
403
404.. versionchanged:: 3.1
405   Added the ``','`` option (see also :pep:`378`).
406
407.. index:: single: _ (underscore); in string formatting
408
409The ``'_'`` option signals the use of an underscore for a thousands
410separator for floating point presentation types and for integer
411presentation type ``'d'``.  For integer presentation types ``'b'``,
412``'o'``, ``'x'``, and ``'X'``, underscores will be inserted every 4
413digits.  For other presentation types, specifying this option is an
414error.
415
416.. versionchanged:: 3.6
417   Added the ``'_'`` option (see also :pep:`515`).
418
419*width* is a decimal integer defining the minimum field width.  If not
420specified, then the field width will be determined by the content.
421
422When no explicit alignment is given, preceding the *width* field by a zero
423(``'0'``) character enables
424sign-aware zero-padding for numeric types.  This is equivalent to a *fill*
425character of ``'0'`` with an *alignment* type of ``'='``.
426
427The *precision* is a decimal number indicating how many digits should be
428displayed after the decimal point for a floating point value formatted with
429``'f'`` and ``'F'``, or before and after the decimal point for a floating point
430value formatted with ``'g'`` or ``'G'``.  For non-number types the field
431indicates the maximum field size - in other words, how many characters will be
432used from the field content. The *precision* is not allowed for integer values.
433
434Finally, the *type* determines how the data should be presented.
435
436The available string presentation types are:
437
438   +---------+----------------------------------------------------------+
439   | Type    | Meaning                                                  |
440   +=========+==========================================================+
441   | ``'s'`` | String format. This is the default type for strings and  |
442   |         | may be omitted.                                          |
443   +---------+----------------------------------------------------------+
444   | None    | The same as ``'s'``.                                     |
445   +---------+----------------------------------------------------------+
446
447The available integer presentation types are:
448
449   +---------+----------------------------------------------------------+
450   | Type    | Meaning                                                  |
451   +=========+==========================================================+
452   | ``'b'`` | Binary format. Outputs the number in base 2.             |
453   +---------+----------------------------------------------------------+
454   | ``'c'`` | Character. Converts the integer to the corresponding     |
455   |         | unicode character before printing.                       |
456   +---------+----------------------------------------------------------+
457   | ``'d'`` | Decimal Integer. Outputs the number in base 10.          |
458   +---------+----------------------------------------------------------+
459   | ``'o'`` | Octal format. Outputs the number in base 8.              |
460   +---------+----------------------------------------------------------+
461   | ``'x'`` | Hex format. Outputs the number in base 16, using         |
462   |         | lower-case letters for the digits above 9.               |
463   +---------+----------------------------------------------------------+
464   | ``'X'`` | Hex format. Outputs the number in base 16, using         |
465   |         | upper-case letters for the digits above 9.               |
466   +---------+----------------------------------------------------------+
467   | ``'n'`` | Number. This is the same as ``'d'``, except that it uses |
468   |         | the current locale setting to insert the appropriate     |
469   |         | number separator characters.                             |
470   +---------+----------------------------------------------------------+
471   | None    | The same as ``'d'``.                                     |
472   +---------+----------------------------------------------------------+
473
474In addition to the above presentation types, integers can be formatted
475with the floating point presentation types listed below (except
476``'n'`` and ``None``). When doing so, :func:`float` is used to convert the
477integer to a floating point number before formatting.
478
479The available presentation types for floating point and decimal values are:
480
481   +---------+----------------------------------------------------------+
482   | Type    | Meaning                                                  |
483   +=========+==========================================================+
484   | ``'e'`` | Exponent notation. Prints the number in scientific       |
485   |         | notation using the letter 'e' to indicate the exponent.  |
486   |         | The default precision is ``6``.                          |
487   +---------+----------------------------------------------------------+
488   | ``'E'`` | Exponent notation. Same as ``'e'`` except it uses an     |
489   |         | upper case 'E' as the separator character.               |
490   +---------+----------------------------------------------------------+
491   | ``'f'`` | Fixed-point notation. Displays the number as a           |
492   |         | fixed-point number.  The default precision is ``6``.     |
493   +---------+----------------------------------------------------------+
494   | ``'F'`` | Fixed-point notation. Same as ``'f'``, but converts      |
495   |         | ``nan`` to  ``NAN`` and ``inf`` to ``INF``.              |
496   +---------+----------------------------------------------------------+
497   | ``'g'`` | General format.  For a given precision ``p >= 1``,       |
498   |         | this rounds the number to ``p`` significant digits and   |
499   |         | then formats the result in either fixed-point format     |
500   |         | or in scientific notation, depending on its magnitude.   |
501   |         |                                                          |
502   |         | The precise rules are as follows: suppose that the       |
503   |         | result formatted with presentation type ``'e'`` and      |
504   |         | precision ``p-1`` would have exponent ``exp``.  Then     |
505   |         | if ``-4 <= exp < p``, the number is formatted            |
506   |         | with presentation type ``'f'`` and precision             |
507   |         | ``p-1-exp``.  Otherwise, the number is formatted         |
508   |         | with presentation type ``'e'`` and precision ``p-1``.    |
509   |         | In both cases insignificant trailing zeros are removed   |
510   |         | from the significand, and the decimal point is also      |
511   |         | removed if there are no remaining digits following it.   |
512   |         |                                                          |
513   |         | Positive and negative infinity, positive and negative    |
514   |         | zero, and nans, are formatted as ``inf``, ``-inf``,      |
515   |         | ``0``, ``-0`` and ``nan`` respectively, regardless of    |
516   |         | the precision.                                           |
517   |         |                                                          |
518   |         | A precision of ``0`` is treated as equivalent to a       |
519   |         | precision of ``1``.  The default precision is ``6``.     |
520   +---------+----------------------------------------------------------+
521   | ``'G'`` | General format. Same as ``'g'`` except switches to       |
522   |         | ``'E'`` if the number gets too large. The                |
523   |         | representations of infinity and NaN are uppercased, too. |
524   +---------+----------------------------------------------------------+
525   | ``'n'`` | Number. This is the same as ``'g'``, except that it uses |
526   |         | the current locale setting to insert the appropriate     |
527   |         | number separator characters.                             |
528   +---------+----------------------------------------------------------+
529   | ``'%'`` | Percentage. Multiplies the number by 100 and displays    |
530   |         | in fixed (``'f'``) format, followed by a percent sign.   |
531   +---------+----------------------------------------------------------+
532   | None    | Similar to ``'g'``, except that fixed-point notation,    |
533   |         | when used, has at least one digit past the decimal point.|
534   |         | The default precision is as high as needed to represent  |
535   |         | the particular value. The overall effect is to match the |
536   |         | output of :func:`str` as altered by the other format     |
537   |         | modifiers.                                               |
538   +---------+----------------------------------------------------------+
539
540
541.. _formatexamples:
542
543Format examples
544^^^^^^^^^^^^^^^
545
546This section contains examples of the :meth:`str.format` syntax and
547comparison with the old ``%``-formatting.
548
549In most of the cases the syntax is similar to the old ``%``-formatting, with the
550addition of the ``{}`` and with ``:`` used instead of ``%``.
551For example, ``'%03.2f'`` can be translated to ``'{:03.2f}'``.
552
553The new format syntax also supports new and different options, shown in the
554following examples.
555
556Accessing arguments by position::
557
558   >>> '{0}, {1}, {2}'.format('a', 'b', 'c')
559   'a, b, c'
560   >>> '{}, {}, {}'.format('a', 'b', 'c')  # 3.1+ only
561   'a, b, c'
562   >>> '{2}, {1}, {0}'.format('a', 'b', 'c')
563   'c, b, a'
564   >>> '{2}, {1}, {0}'.format(*'abc')      # unpacking argument sequence
565   'c, b, a'
566   >>> '{0}{1}{0}'.format('abra', 'cad')   # arguments' indices can be repeated
567   'abracadabra'
568
569Accessing arguments by name::
570
571   >>> 'Coordinates: {latitude}, {longitude}'.format(latitude='37.24N', longitude='-115.81W')
572   'Coordinates: 37.24N, -115.81W'
573   >>> coord = {'latitude': '37.24N', 'longitude': '-115.81W'}
574   >>> 'Coordinates: {latitude}, {longitude}'.format(**coord)
575   'Coordinates: 37.24N, -115.81W'
576
577Accessing arguments' attributes::
578
579   >>> c = 3-5j
580   >>> ('The complex number {0} is formed from the real part {0.real} '
581   ...  'and the imaginary part {0.imag}.').format(c)
582   'The complex number (3-5j) is formed from the real part 3.0 and the imaginary part -5.0.'
583   >>> class Point:
584   ...     def __init__(self, x, y):
585   ...         self.x, self.y = x, y
586   ...     def __str__(self):
587   ...         return 'Point({self.x}, {self.y})'.format(self=self)
588   ...
589   >>> str(Point(4, 2))
590   'Point(4, 2)'
591
592Accessing arguments' items::
593
594   >>> coord = (3, 5)
595   >>> 'X: {0[0]};  Y: {0[1]}'.format(coord)
596   'X: 3;  Y: 5'
597
598Replacing ``%s`` and ``%r``::
599
600   >>> "repr() shows quotes: {!r}; str() doesn't: {!s}".format('test1', 'test2')
601   "repr() shows quotes: 'test1'; str() doesn't: test2"
602
603Aligning the text and specifying a width::
604
605   >>> '{:<30}'.format('left aligned')
606   'left aligned                  '
607   >>> '{:>30}'.format('right aligned')
608   '                 right aligned'
609   >>> '{:^30}'.format('centered')
610   '           centered           '
611   >>> '{:*^30}'.format('centered')  # use '*' as a fill char
612   '***********centered***********'
613
614Replacing ``%+f``, ``%-f``, and ``% f`` and specifying a sign::
615
616   >>> '{:+f}; {:+f}'.format(3.14, -3.14)  # show it always
617   '+3.140000; -3.140000'
618   >>> '{: f}; {: f}'.format(3.14, -3.14)  # show a space for positive numbers
619   ' 3.140000; -3.140000'
620   >>> '{:-f}; {:-f}'.format(3.14, -3.14)  # show only the minus -- same as '{:f}; {:f}'
621   '3.140000; -3.140000'
622
623Replacing ``%x`` and ``%o`` and converting the value to different bases::
624
625   >>> # format also supports binary numbers
626   >>> "int: {0:d};  hex: {0:x};  oct: {0:o};  bin: {0:b}".format(42)
627   'int: 42;  hex: 2a;  oct: 52;  bin: 101010'
628   >>> # with 0x, 0o, or 0b as prefix:
629   >>> "int: {0:d};  hex: {0:#x};  oct: {0:#o};  bin: {0:#b}".format(42)
630   'int: 42;  hex: 0x2a;  oct: 0o52;  bin: 0b101010'
631
632Using the comma as a thousands separator::
633
634   >>> '{:,}'.format(1234567890)
635   '1,234,567,890'
636
637Expressing a percentage::
638
639   >>> points = 19
640   >>> total = 22
641   >>> 'Correct answers: {:.2%}'.format(points/total)
642   'Correct answers: 86.36%'
643
644Using type-specific formatting::
645
646   >>> import datetime
647   >>> d = datetime.datetime(2010, 7, 4, 12, 15, 58)
648   >>> '{:%Y-%m-%d %H:%M:%S}'.format(d)
649   '2010-07-04 12:15:58'
650
651Nesting arguments and more complex examples::
652
653   >>> for align, text in zip('<^>', ['left', 'center', 'right']):
654   ...     '{0:{fill}{align}16}'.format(text, fill=align, align=align)
655   ...
656   'left<<<<<<<<<<<<'
657   '^^^^^center^^^^^'
658   '>>>>>>>>>>>right'
659   >>>
660   >>> octets = [192, 168, 0, 1]
661   >>> '{:02X}{:02X}{:02X}{:02X}'.format(*octets)
662   'C0A80001'
663   >>> int(_, 16)
664   3232235521
665   >>>
666   >>> width = 5
667   >>> for num in range(5,12): #doctest: +NORMALIZE_WHITESPACE
668   ...     for base in 'dXob':
669   ...         print('{0:{width}{base}}'.format(num, base=base, width=width), end=' ')
670   ...     print()
671   ...
672       5     5     5   101
673       6     6     6   110
674       7     7     7   111
675       8     8    10  1000
676       9     9    11  1001
677      10     A    12  1010
678      11     B    13  1011
679
680
681
682.. _template-strings:
683
684Template strings
685----------------
686
687Template strings provide simpler string substitutions as described in
688:pep:`292`.  A primary use case for template strings is for
689internationalization (i18n) since in that context, the simpler syntax and
690functionality makes it easier to translate than other built-in string
691formatting facilities in Python.  As an example of a library built on template
692strings for i18n, see the
693`flufl.i18n <http://flufli18n.readthedocs.io/en/latest/>`_ package.
694
695.. index:: single: $ (dollar); in template strings
696
697Template strings support ``$``-based substitutions, using the following rules:
698
699* ``$$`` is an escape; it is replaced with a single ``$``.
700
701* ``$identifier`` names a substitution placeholder matching a mapping key of
702  ``"identifier"``.  By default, ``"identifier"`` is restricted to any
703  case-insensitive ASCII alphanumeric string (including underscores) that
704  starts with an underscore or ASCII letter.  The first non-identifier
705  character after the ``$`` character terminates this placeholder
706  specification.
707
708* ``${identifier}`` is equivalent to ``$identifier``.  It is required when
709  valid identifier characters follow the placeholder but are not part of the
710  placeholder, such as ``"${noun}ification"``.
711
712Any other appearance of ``$`` in the string will result in a :exc:`ValueError`
713being raised.
714
715The :mod:`string` module provides a :class:`Template` class that implements
716these rules.  The methods of :class:`Template` are:
717
718
719.. class:: Template(template)
720
721   The constructor takes a single argument which is the template string.
722
723
724   .. method:: substitute(mapping, **kwds)
725
726      Performs the template substitution, returning a new string.  *mapping* is
727      any dictionary-like object with keys that match the placeholders in the
728      template.  Alternatively, you can provide keyword arguments, where the
729      keywords are the placeholders.  When both *mapping* and *kwds* are given
730      and there are duplicates, the placeholders from *kwds* take precedence.
731
732
733   .. method:: safe_substitute(mapping, **kwds)
734
735      Like :meth:`substitute`, except that if placeholders are missing from
736      *mapping* and *kwds*, instead of raising a :exc:`KeyError` exception, the
737      original placeholder will appear in the resulting string intact.  Also,
738      unlike with :meth:`substitute`, any other appearances of the ``$`` will
739      simply return ``$`` instead of raising :exc:`ValueError`.
740
741      While other exceptions may still occur, this method is called "safe"
742      because it always tries to return a usable string instead of
743      raising an exception.  In another sense, :meth:`safe_substitute` may be
744      anything other than safe, since it will silently ignore malformed
745      templates containing dangling delimiters, unmatched braces, or
746      placeholders that are not valid Python identifiers.
747
748   :class:`Template` instances also provide one public data attribute:
749
750   .. attribute:: template
751
752      This is the object passed to the constructor's *template* argument.  In
753      general, you shouldn't change it, but read-only access is not enforced.
754
755Here is an example of how to use a Template::
756
757   >>> from string import Template
758   >>> s = Template('$who likes $what')
759   >>> s.substitute(who='tim', what='kung pao')
760   'tim likes kung pao'
761   >>> d = dict(who='tim')
762   >>> Template('Give $who $100').substitute(d)
763   Traceback (most recent call last):
764   ...
765   ValueError: Invalid placeholder in string: line 1, col 11
766   >>> Template('$who likes $what').substitute(d)
767   Traceback (most recent call last):
768   ...
769   KeyError: 'what'
770   >>> Template('$who likes $what').safe_substitute(d)
771   'tim likes $what'
772
773Advanced usage: you can derive subclasses of :class:`Template` to customize
774the placeholder syntax, delimiter character, or the entire regular expression
775used to parse template strings.  To do this, you can override these class
776attributes:
777
778* *delimiter* -- This is the literal string describing a placeholder
779  introducing delimiter.  The default value is ``$``.  Note that this should
780  *not* be a regular expression, as the implementation will call
781  :meth:`re.escape` on this string as needed.  Note further that you cannot
782  change the delimiter after class creation (i.e. a different delimiter must
783  be set in the subclass's class namespace).
784
785* *idpattern* -- This is the regular expression describing the pattern for
786  non-braced placeholders.  The default value is the regular expression
787  ``(?a:[_a-z][_a-z0-9]*)``.  If this is given and *braceidpattern* is
788  ``None`` this pattern will also apply to braced placeholders.
789
790  .. note::
791
792     Since default *flags* is ``re.IGNORECASE``, pattern ``[a-z]`` can match
793     with some non-ASCII characters. That's why we use the local ``a`` flag
794     here.
795
796  .. versionchanged:: 3.7
797     *braceidpattern* can be used to define separate patterns used inside and
798     outside the braces.
799
800* *braceidpattern* -- This is like *idpattern* but describes the pattern for
801  braced placeholders.  Defaults to ``None`` which means to fall back to
802  *idpattern* (i.e. the same pattern is used both inside and outside braces).
803  If given, this allows you to define different patterns for braced and
804  unbraced placeholders.
805
806  .. versionadded:: 3.7
807
808* *flags* -- The regular expression flags that will be applied when compiling
809  the regular expression used for recognizing substitutions.  The default value
810  is ``re.IGNORECASE``.  Note that ``re.VERBOSE`` will always be added to the
811  flags, so custom *idpattern*\ s must follow conventions for verbose regular
812  expressions.
813
814  .. versionadded:: 3.2
815
816Alternatively, you can provide the entire regular expression pattern by
817overriding the class attribute *pattern*.  If you do this, the value must be a
818regular expression object with four named capturing groups.  The capturing
819groups correspond to the rules given above, along with the invalid placeholder
820rule:
821
822* *escaped* -- This group matches the escape sequence, e.g. ``$$``, in the
823  default pattern.
824
825* *named* -- This group matches the unbraced placeholder name; it should not
826  include the delimiter in capturing group.
827
828* *braced* -- This group matches the brace enclosed placeholder name; it should
829  not include either the delimiter or braces in the capturing group.
830
831* *invalid* -- This group matches any other delimiter pattern (usually a single
832  delimiter), and it should appear last in the regular expression.
833
834
835Helper functions
836----------------
837
838.. function:: capwords(s, sep=None)
839
840   Split the argument into words using :meth:`str.split`, capitalize each word
841   using :meth:`str.capitalize`, and join the capitalized words using
842   :meth:`str.join`.  If the optional second argument *sep* is absent
843   or ``None``, runs of whitespace characters are replaced by a single space
844   and leading and trailing whitespace are removed, otherwise *sep* is used to
845   split and join the words.
846