23.2. "locale" — Internationalization services
**********************************************

**Source code:** Lib/locale.py

======================================================================

The "locale" module opens access to the POSIX locale database and
functionality. The POSIX locale mechanism allows programmers to deal
with certain cultural issues in an application, without requiring the
programmer to know all the specifics of each country where the
software is executed.

The "locale" module is implemented on top of the "_locale" module,
which in turn uses an ANSI C locale implementation if available.

The "locale" module defines the following exception and functions:

exception locale.Error

   Exception raised when the locale passed to "setlocale()" is not
   recognized.

locale.setlocale(category, locale=None)

   If *locale* is given and not "None", "setlocale()" modifies the
   locale setting for the *category*. The available categories are
   listed in the data description below. *locale* may be a string, or
   an iterable of two strings (language code and encoding). If it’s an
   iterable, it’s converted to a locale name using the locale aliasing
   engine. An empty string specifies the user’s default settings. If
   the modification of the locale fails, the exception "Error" is
   raised. If successful, the new locale setting is returned.

   If *locale* is omitted or "None", the current setting for
   *category* is returned.

   "setlocale()" is not thread-safe on most systems. Applications
   typically start with a call of

      import locale
      locale.setlocale(locale.LC_ALL, '')

   This sets the locale for all categories to the user’s default
   setting (typically specified in the "LANG" environment variable).
   If the locale is not changed thereafter, using multithreading
   should not cause problems.

locale.localeconv()

   Returns the database of the local conventions as a dictionary. This
   dictionary has the following strings as keys:

   +------------------------+---------------------------------------+----------------------------------+
   | Category               | Key                                   | Meaning                          |
   |========================|=======================================|==================================|
   | "LC_NUMERIC"           | "'decimal_point'"                     | Decimal point character.         |
   +------------------------+---------------------------------------+----------------------------------+
   |                        | "'grouping'"                          | Sequence of numbers specifying   |
   |                        |                                       | which relative positions the     |
   |                        |                                       | "'thousands_sep'" is expected.   |
   |                        |                                       | If the sequence is terminated    |
   |                        |                                       | with "CHAR_MAX", no further      |
   |                        |                                       | grouping is performed. If the    |
   |                        |                                       | sequence terminates with a "0",  |
   |                        |                                       | the last group size is           |
   |                        |                                       | repeatedly used.                 |
   +------------------------+---------------------------------------+----------------------------------+
   |                        | "'thousands_sep'"                     | Character used between groups.   |
   +------------------------+---------------------------------------+----------------------------------+
   | "LC_MONETARY"          | "'int_curr_symbol'"                   | International currency symbol.   |
   +------------------------+---------------------------------------+----------------------------------+
   |                        | "'currency_symbol'"                   | Local currency symbol.           |
   +------------------------+---------------------------------------+----------------------------------+
   |                        | "'p_cs_precedes/n_cs_precedes'"       | Whether the currency symbol      |
   |                        |                                       | precedes the value (for positive |
   |                        |                                       | resp. negative values).          |
   +------------------------+---------------------------------------+----------------------------------+
   |                        | "'p_sep_by_space/n_sep_by_space'"     | Whether the currency symbol is   |
   |                        |                                       | separated from the value  by a   |
   |                        |                                       | space (for positive resp.        |
   |                        |                                       | negative values).                |
   +------------------------+---------------------------------------+----------------------------------+
   |                        | "'mon_decimal_point'"                 | Decimal point used for monetary  |
   |                        |                                       | values.                          |
   +------------------------+---------------------------------------+----------------------------------+
   |                        | "'frac_digits'"                       | Number of fractional digits used |
   |                        |                                       | in local formatting of monetary  |
   |                        |                                       | values.                          |
   +------------------------+---------------------------------------+----------------------------------+
   |                        | "'int_frac_digits'"                   | Number of fractional digits used |
   |                        |                                       | in international formatting of   |
   |                        |                                       | monetary values.                 |
   +------------------------+---------------------------------------+----------------------------------+
   |                        | "'mon_thousands_sep'"                 | Group separator used for         |
   |                        |                                       | monetary values.                 |
   +------------------------+---------------------------------------+----------------------------------+
   |                        | "'mon_grouping'"                      | Equivalent to "'grouping'", used |
   |                        |                                       | for monetary values.             |
   +------------------------+---------------------------------------+----------------------------------+
   |                        | "'positive_sign'"                     | Symbol used to annotate a        |
   |                        |                                       | positive monetary value.         |
   +------------------------+---------------------------------------+----------------------------------+
   |                        | "'negative_sign'"                     | Symbol used to annotate a        |
   |                        |                                       | negative monetary value.         |
   +------------------------+---------------------------------------+----------------------------------+
   |                        | "'p_sign_posn/n_sign_posn'"           | The position of the sign (for    |
   |                        |                                       | positive resp. negative values), |
   |                        |                                       | see below.                       |
   +------------------------+---------------------------------------+----------------------------------+

   All numeric values can be set to "CHAR_MAX" to indicate that there
   is no value specified in this locale.

   The possible values for "'p_sign_posn'" and "'n_sign_posn'" are
   given below.

   +----------------+-------------------------------------------+
   | Value          | Explanation                               |
   |================|===========================================|
   | "0"            | Currency and value are surrounded by      |
   |                | parentheses.                              |
   +----------------+-------------------------------------------+
   | "1"            | The sign should precede the value and     |
   |                | currency symbol.                          |
   +----------------+-------------------------------------------+
   | "2"            | The sign should follow the value and      |
   |                | currency symbol.                          |
   +----------------+-------------------------------------------+
   | "3"            | The sign should immediately precede the   |
   |                | value.                                    |
   +----------------+-------------------------------------------+
   | "4"            | The sign should immediately follow the    |
   |                | value.                                    |
   +----------------+-------------------------------------------+
   | "CHAR_MAX"     | Nothing is specified in this locale.      |
   +----------------+-------------------------------------------+

   The function sets temporarily the "LC_CTYPE" locale to the
   "LC_NUMERIC" locale or the "LC_MONETARY" locale if locales are
   different and numeric or monetary strings are non-ASCII. This
   temporary change affects other threads.

   Changed in version 3.6.5: The function now sets temporarily the
   "LC_CTYPE" locale to the "LC_NUMERIC" locale in some cases.

locale.nl_langinfo(option)

   Return some locale-specific information as a string.  This function
   is not available on all systems, and the set of possible options
   might also vary across platforms.  The possible argument values are
   numbers, for which symbolic constants are available in the locale
   module.

   The "nl_langinfo()" function accepts one of the following keys.
   Most descriptions are taken from the corresponding description in
   the GNU C library.

   locale.CODESET

      Get a string with the name of the character encoding used in the
      selected locale.

   locale.D_T_FMT

      Get a string that can be used as a format string for
      "time.strftime()" to represent date and time in a locale-
      specific way.

   locale.D_FMT

      Get a string that can be used as a format string for
      "time.strftime()" to represent a date in a locale-specific way.

   locale.T_FMT

      Get a string that can be used as a format string for
      "time.strftime()" to represent a time in a locale-specific way.

   locale.T_FMT_AMPM

      Get a format string for "time.strftime()" to represent time in
      the am/pm format.

   DAY_1 ... DAY_7

      Get the name of the n-th day of the week.

      Note:

        This follows the US convention of "DAY_1" being Sunday, not
        the international convention (ISO 8601) that Monday is the
        first day of the week.

   ABDAY_1 ... ABDAY_7

      Get the abbreviated name of the n-th day of the week.

   MON_1 ... MON_12

      Get the name of the n-th month.

   ABMON_1 ... ABMON_12

      Get the abbreviated name of the n-th month.

   locale.RADIXCHAR

      Get the radix character (decimal dot, decimal comma, etc.).

   locale.THOUSEP

      Get the separator character for thousands (groups of three
      digits).

   locale.YESEXPR

      Get a regular expression that can be used with the regex
      function to recognize a positive response to a yes/no question.

      Note:

        The expression is in the syntax suitable for the "regex()"
        function from the C library, which might differ from the
        syntax used in "re".

   locale.NOEXPR

      Get a regular expression that can be used with the regex(3)
      function to recognize a negative response to a yes/no question.

   locale.CRNCYSTR

      Get the currency symbol, preceded by “-” if the symbol should
      appear before the value, “+” if the symbol should appear after
      the value, or “.” if the symbol should replace the radix
      character.

   locale.ERA

      Get a string that represents the era used in the current locale.

      Most locales do not define this value.  An example of a locale
      which does define this value is the Japanese one.  In Japan, the
      traditional representation of dates includes the name of the era
      corresponding to the then-emperor’s reign.

      Normally it should not be necessary to use this value directly.
      Specifying the "E" modifier in their format strings causes the
      "time.strftime()" function to use this information.  The format
      of the returned string is not specified, and therefore you
      should not assume knowledge of it on different systems.

   locale.ERA_D_T_FMT

      Get a format string for "time.strftime()" to represent date and
      time in a locale-specific era-based way.

   locale.ERA_D_FMT

      Get a format string for "time.strftime()" to represent a date in
      a locale-specific era-based way.

   locale.ERA_T_FMT

      Get a format string for "time.strftime()" to represent a time in
      a locale-specific era-based way.

   locale.ALT_DIGITS

      Get a representation of up to 100 values used to represent the
      values 0 to 99.

locale.getdefaultlocale([envvars])

   Tries to determine the default locale settings and returns them as
   a tuple of the form "(language code, encoding)".

   According to POSIX, a program which has not called
   "setlocale(LC_ALL, '')" runs using the portable "'C'" locale.
   Calling "setlocale(LC_ALL, '')" lets it use the default locale as
   defined by the "LANG" variable.  Since we do not want to interfere
   with the current locale setting we thus emulate the behavior in the
   way described above.

   To maintain compatibility with other platforms, not only the "LANG"
   variable is tested, but a list of variables given as envvars
   parameter.  The first found to be defined will be used.  *envvars*
   defaults to the search path used in GNU gettext; it must always
   contain the variable name "'LANG'".  The GNU gettext search path
   contains "'LC_ALL'", "'LC_CTYPE'", "'LANG'" and "'LANGUAGE'", in
   that order.

   Except for the code "'C'", the language code corresponds to **RFC
   1766**. *language code* and *encoding* may be "None" if their
   values cannot be determined.

locale.getlocale(category=LC_CTYPE)

   Returns the current setting for the given locale category as
   sequence containing *language code*, *encoding*. *category* may be
   one of the "LC_*" values except "LC_ALL".  It defaults to
   "LC_CTYPE".

   Except for the code "'C'", the language code corresponds to **RFC
   1766**. *language code* and *encoding* may be "None" if their
   values cannot be determined.

locale.getpreferredencoding(do_setlocale=True)

   Return the encoding used for text data, according to user
   preferences.  User preferences are expressed differently on
   different systems, and might not be available programmatically on
   some systems, so this function only returns a guess.

   On some systems, it is necessary to invoke "setlocale()" to obtain
   the user preferences, so this function is not thread-safe. If
   invoking setlocale is not necessary or desired, *do_setlocale*
   should be set to "False".

locale.normalize(localename)

   Returns a normalized locale code for the given locale name.  The
   returned locale code is formatted for use with "setlocale()".  If
   normalization fails, the original name is returned unchanged.

   If the given encoding is not known, the function defaults to the
   default encoding for the locale code just like "setlocale()".

locale.resetlocale(category=LC_ALL)

   Sets the locale for *category* to the default setting.

   The default setting is determined by calling "getdefaultlocale()".
   *category* defaults to "LC_ALL".

locale.strcoll(string1, string2)

   Compares two strings according to the current "LC_COLLATE" setting.
   As any other compare function, returns a negative, or a positive
   value, or "0", depending on whether *string1* collates before or
   after *string2* or is equal to it.

locale.strxfrm(string)

   Transforms a string to one that can be used in locale-aware
   comparisons.  For example, "strxfrm(s1) < strxfrm(s2)" is
   equivalent to "strcoll(s1, s2) < 0".  This function can be used
   when the same string is compared repeatedly, e.g. when collating a
   sequence of strings.

locale.format(format, val, grouping=False, monetary=False)

   Formats a number *val* according to the current "LC_NUMERIC"
   setting. The format follows the conventions of the "%" operator.
   For floating point values, the decimal point is modified if
   appropriate.  If *grouping* is true, also takes the grouping into
   account.

   If *monetary* is true, the conversion uses monetary thousands
   separator and grouping strings.

   Please note that this function will only work for exactly one %char
   specifier. For whole format strings, use "format_string()".

locale.format_string(format, val, grouping=False)

   Processes formatting specifiers as in "format % val", but takes the
   current locale settings into account.

locale.currency(val, symbol=True, grouping=False, international=False)

   Formats a number *val* according to the current "LC_MONETARY"
   settings.

   The returned string includes the currency symbol if *symbol* is
   true, which is the default. If *grouping* is true (which is not the
   default), grouping is done with the value. If *international* is
   true (which is not the default), the international currency symbol
   is used.

   Note that this function will not work with the ‘C’ locale, so you
   have to set a locale via "setlocale()" first.

locale.str(float)

   Formats a floating point number using the same format as the built-
   in function "str(float)", but takes the decimal point into account.

locale.delocalize(string)

   Converts a string into a normalized number string, following the
   "LC_NUMERIC" settings.

   New in version 3.5.

locale.atof(string)

   Converts a string to a floating point number, following the
   "LC_NUMERIC" settings.

locale.atoi(string)

   Converts a string to an integer, following the "LC_NUMERIC"
   conventions.

locale.LC_CTYPE

   Locale category for the character type functions.  Depending on the
   settings of this category, the functions of module "string" dealing
   with case change their behaviour.

locale.LC_COLLATE

   Locale category for sorting strings.  The functions "strcoll()" and
   "strxfrm()" of the "locale" module are affected.

locale.LC_TIME

   Locale category for the formatting of time.  The function
   "time.strftime()" follows these conventions.

locale.LC_MONETARY

   Locale category for formatting of monetary values.  The available
   options are available from the "localeconv()" function.

locale.LC_MESSAGES

   Locale category for message display. Python currently does not
   support application specific locale-aware messages.  Messages
   displayed by the operating system, like those returned by
   "os.strerror()" might be affected by this category.

locale.LC_NUMERIC

   Locale category for formatting numbers.  The functions "format()",
   "atoi()", "atof()" and "str()" of the "locale" module are affected
   by that category.  All other numeric formatting operations are not
   affected.

locale.LC_ALL

   Combination of all locale settings.  If this flag is used when the
   locale is changed, setting the locale for all categories is
   attempted. If that fails for any category, no category is changed
   at all.  When the locale is retrieved using this flag, a string
   indicating the setting for all categories is returned. This string
   can be later used to restore the settings.

locale.CHAR_MAX

   This is a symbolic constant used for different values returned by
   "localeconv()".

Example:

   >>> import locale
   >>> loc = locale.getlocale()  # get current locale
   # use German locale; name might vary with platform
   >>> locale.setlocale(locale.LC_ALL, 'de_DE')
   >>> locale.strcoll('f\xe4n', 'foo')  # compare a string containing an umlaut
   >>> locale.setlocale(locale.LC_ALL, '')   # use user's preferred locale
   >>> locale.setlocale(locale.LC_ALL, 'C')  # use default (C) locale
   >>> locale.setlocale(locale.LC_ALL, loc)  # restore saved locale


23.2.1. Background, details, hints, tips and caveats
====================================================

The C standard defines the locale as a program-wide property that may
be relatively expensive to change.  On top of that, some
implementation are broken in such a way that frequent locale changes
may cause core dumps.  This makes the locale somewhat painful to use
correctly.

Initially, when a program is started, the locale is the "C" locale, no
matter what the user’s preferred locale is.  There is one exception:
the "LC_CTYPE" category is changed at startup to set the current
locale encoding to the user’s preferred locale encoding. The program
must explicitly say that it wants the user’s preferred locale settings
for other categories by calling "setlocale(LC_ALL, '')".

It is generally a bad idea to call "setlocale()" in some library
routine, since as a side effect it affects the entire program.  Saving
and restoring it is almost as bad: it is expensive and affects other
threads that happen to run before the settings have been restored.

If, when coding a module for general use, you need a locale
independent version of an operation that is affected by the locale
(such as certain formats used with "time.strftime()"), you will have
to find a way to do it without using the standard library routine.
Even better is convincing yourself that using locale settings is okay.
Only as a last resort should you document that your module is not
compatible with non-"C" locale settings.

The only way to perform numeric operations according to the locale is
to use the special functions defined by this module: "atof()",
"atoi()", "format()", "str()".

There is no way to perform case conversions and character
classifications according to the locale.  For (Unicode) text strings
these are done according to the character value only, while for byte
strings, the conversions and classifications are done according to the
ASCII value of the byte, and bytes whose high bit is set (i.e., non-
ASCII bytes) are never converted or considered part of a character
class such as letter or whitespace.


23.2.2. For extension writers and programs that embed Python
============================================================

Extension modules should never call "setlocale()", except to find out
what the current locale is.  But since the return value can only be
used portably to restore it, that is not very useful (except perhaps
to find out whether or not the locale is "C").

When Python code uses the "locale" module to change the locale, this
also affects the embedding application.  If the embedding application
doesn’t want this to happen, it should remove the "_locale" extension
module (which does all the work) from the table of built-in modules in
the "config.c" file, and make sure that the "_locale" module is not
accessible as a shared library.


23.2.3. Access to message catalogs
==================================

locale.gettext(msg)

locale.dgettext(domain, msg)

locale.dcgettext(domain, msg, category)

locale.textdomain(domain)

locale.bindtextdomain(domain, dir)

The locale module exposes the C library’s gettext interface on systems
that provide this interface.  It consists of the functions
"gettext()", "dgettext()", "dcgettext()", "textdomain()",
"bindtextdomain()", and "bind_textdomain_codeset()".  These are
similar to the same functions in the "gettext" module, but use the C
library’s binary format for message catalogs, and the C library’s
search algorithms for locating message catalogs.

Python applications should normally find no need to invoke these
functions, and should use "gettext" instead.  A known exception to
this rule are applications that link with additional C libraries which
internally invoke "gettext()" or "dcgettext()".  For these
applications, it may be necessary to bind the text domain, so that the
libraries can properly locate their message catalogs.
