Add test and API note for locale grouping in ScalarFormatter.

Author: QuLogic

doc/api/next_api_changes/behavior/8987-Z.rst

@@ -0,0 +1,6 @@
+``ScalarFormatter`` *useLocale* option obeys grouping
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When the `~.ScalarFormatter` option *useLocale* is enabled (or
+:rc:`axes.formatter.use_locale` is *True*) and the configured locale uses
+grouping, a separator will be added as described in `locale.format_string`.

lib/matplotlib/tests/test_ticker.py

@@ -1,6 +1,7 @@
 from contextlib import nullcontext
-import re
 import itertools
+import locale
+import re
 
 import numpy as np
 from numpy.testing import assert_almost_equal, assert_array_equal
@@ -546,6 +547,21 @@ def test_use_offset(self, use_offset):
             tmp_form = mticker.ScalarFormatter()
             assert use_offset == tmp_form.get_useOffset()
 
+    def test_use_locale(self):
+        conv = locale.localeconv()
+        sep = conv['thousands_sep']
+        if not sep or conv['grouping'][-1:] in ([], [locale.CHAR_MAX]):
+            pytest.skip('Locale does not apply grouping')  # pragma: no cover
+
+        with mpl.rc_context({'axes.formatter.use_locale': True}):
+            tmp_form = mticker.ScalarFormatter()
+            assert tmp_form.get_useLocale()
+
+            tmp_form.create_dummy_axis()
+            tmp_form.set_bounds(0, 10)
+            tmp_form.set_locs([1, 2, 3])
+            assert sep in tmp_form(1e9)
+
     @pytest.mark.parametrize(
         'sci_type, scilimits, lim, orderOfMag, fewticks', scilimits_data)
     def test_scilimits(self, sci_type, scilimits, lim, orderOfMag, fewticks):