[3.6] bpo-31900: Fix localeconv() encoding for LC_NUMERIC (#4174) (#5192)

* Add _Py_GetLocaleconvNumeric() function: decode decimal_point and
  thousands_sep fields of localeconv() from the LC_NUMERIC encoding,
  rather than decoding from the LC_CTYPE encoding.
* Modify locale.localeconv() and "n" formatter of str.format() (for
  int, float and complex to use _Py_GetLocaleconvNumeric()
  internally.

(cherry picked from commit cb064fc2321ce8673fe365e9ef60445a27657f54)

2 files changed, 24 insertions, 0 deletions

diff --git a/Doc/library/locale.rst b/Doc/library/locale.rst
index b04442b..9a0c570 100644
--- a/Doc/library/locale.rst
+++ b/Doc/library/locale.rst
@@ -147,6 +147,16 @@ The :mod:`locale` module defines the following exception and functions:
    | ``CHAR_MAX`` | Nothing is specified in this locale.    |
    +--------------+-----------------------------------------+
 
+   The function sets temporarily the ``LC_CTYPE`` locale to the ``LC_NUMERIC``
+   locale to decode ``decimal_point`` and ``thousands_sep`` byte strings if
+   they are non-ASCII or longer than 1 byte, and the ``LC_NUMERIC`` locale is
+   different than the ``LC_CTYPE`` locale. This temporary change affects other
+   threads.
+
+   .. versionchanged:: 3.6.5
+      The function now sets temporarily the ``LC_CTYPE`` locale to the
+      ``LC_NUMERIC`` locale in some cases.
+
 
 .. function:: nl_langinfo(option)
 
diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
index 196a4c0..d8a1647 100644
--- a/Doc/library/stdtypes.rst
+++ b/Doc/library/stdtypes.rst
@@ -1599,6 +1599,20 @@ expression support in the :mod:`re` module).
    See :ref:`formatstrings` for a description of the various formatting options
    that can be specified in format strings.
 
+   .. note::
+      When formatting a number (:class:`int`, :class:`float`, :class:`float`
+      and subclasses) with the ``n`` type (ex: ``'{:n}'.format(1234)``), the
+      function sets temporarily the ``LC_CTYPE`` locale to the ``LC_NUMERIC``
+      locale to decode ``decimal_point`` and ``thousands_sep`` fields of
+      :c:func:`localeconv` if they are non-ASCII or longer than 1 byte, and the
+      ``LC_NUMERIC`` locale is different than the ``LC_CTYPE`` locale. This
+      temporary change affects other threads.
+
+   .. versionchanged:: 3.6.5
+      When formatting a number with the ``n`` type, the function sets
+      temporarily the ``LC_CTYPE`` locale to the ``LC_NUMERIC`` locale in some
+      cases.
+
 
 .. method:: str.format_map(mapping)