Specify that style files are utf-8.

I don't think other encodings really worked previously, so there's no
deprecation period.

Author: anntzer

doc/api/next_api_changes/behavior/23031-AL.rst

@@ -0,0 +1,5 @@
+The encoding of style file is now specified to be utf-8
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It has been impossible to import Matplotlib with a non UTF-8 compatible locale
+encoding because we read the style library at import time.  This change is
+formalizing and documenting the status quo so there is no deprecation period.

lib/matplotlib/__init__.py

@@ -754,10 +754,7 @@ def _open_file_or_url(fname):
             yield (line.decode('utf-8') for line in f)
     else:
         fname = os.path.expanduser(fname)
-        encoding = locale.getpreferredencoding(do_setlocale=False)
-        if encoding is None:
-            encoding = "utf-8"
-        with open(fname, encoding=encoding) as f:
+        with open(fname, encoding='utf-8') as f:
             yield f
 
 
@@ -802,11 +799,8 @@ def _rc_params_in_file(fname, transform=lambda x: x, fail_on_error=False):
                                  fname, line_no, line.rstrip('\n'))
                 rc_temp[key] = (val, line, line_no)
         except UnicodeDecodeError:
-            _log.warning('Cannot decode configuration file %s with encoding '
-                         '%s, check LANG and LC_* variables.',
-                         fname,
-                         locale.getpreferredencoding(do_setlocale=False)
-                         or 'utf-8 (default)')
+            _log.warning('Cannot decode configuration file %r as utf-8.',
+                         fname)
             raise
 
     config = RcParams()

lib/matplotlib/mpl-data/matplotlibrc

@@ -42,6 +42,8 @@
 ## String values may optionally be enclosed in double quotes, which allows
 ## using the comment character # in the string.
 ##
+## This file (and other style files) must be encoded as utf-8.
+##
 ## Matplotlib configuration are currently divided into following parts:
 ##     - BACKENDS
 ##     - LINES

lib/matplotlib/tests/test_rcparams.py

@@ -39,7 +39,7 @@ def test_rcparams(tmpdir):
     linewidth = mpl.rcParams['lines.linewidth']
 
     rcpath = Path(tmpdir) / 'test_rcparams.rc'
-    rcpath.write_text('lines.linewidth: 33')
+    rcpath.write_text('lines.linewidth: 33', encoding='utf-8')
 
     # test context given dictionary
     with mpl.rc_context(rc={'text.usetex': not usetex}):
@@ -191,7 +191,7 @@ def test_axes_titlecolor_rcparams():
 
 def test_Issue_1713(tmpdir):
     rcpath = Path(tmpdir) / 'test_rcparams.rc'
-    rcpath.write_text('timezone: UTC', encoding='UTF-32-BE')
+    rcpath.write_text('timezone: UTC', encoding='utf-8')
     with mock.patch('locale.getpreferredencoding', return_value='UTF-32-BE'):
         rc = mpl.rc_params_from_file(rcpath, True, False)
     assert rc.get('timezone') == 'UTC'

lib/matplotlib/tests/test_style.py

@@ -26,7 +26,8 @@ def temp_style(style_name, settings=None):
         with TemporaryDirectory() as tmpdir:
             # Write style settings to file in the tmpdir.
             Path(tmpdir, temp_file).write_text(
-                "\n".join("{}: {}".format(k, v) for k, v in settings.items()))
+                "\n".join("{}: {}".format(k, v) for k, v in settings.items()),
+                encoding="utf-8")
             # Add tmpdir to style path and reload so we can access this style.
             USER_LIBRARY_PATHS.append(tmpdir)
             style.reload_library()
@@ -59,7 +60,7 @@ def test_use():
 
 def test_use_url(tmpdir):
     path = Path(tmpdir, 'file')
-    path.write_text('axes.facecolor: adeade')
+    path.write_text('axes.facecolor: adeade', encoding='utf-8')
     with temp_style('test', DUMMY_SETTINGS):
         url = ('file:'
                + ('///' if sys.platform == 'win32' else '')
@@ -72,7 +73,7 @@ def test_single_path(tmpdir):
     mpl.rcParams[PARAM] = 'gray'
     temp_file = f'text.{STYLE_EXTENSION}'
     path = Path(tmpdir, temp_file)
-    path.write_text(f'{PARAM} : {VALUE}')
+    path.write_text(f'{PARAM} : {VALUE}', encoding='utf-8')
     with style.context(path):
         assert mpl.rcParams[PARAM] == VALUE
     assert mpl.rcParams[PARAM] == 'gray'

setup.py

@@ -194,15 +194,15 @@ def update_matplotlibrc(path):
     # line.  Otherwise, use the default `##backend: Agg` which has no effect
     # even after decommenting, which allows _auto_backend_sentinel to be filled
     # in at import time.
-    template_lines = path.read_text().splitlines(True)
+    template_lines = path.read_text(encoding="utf-8").splitlines(True)
     backend_line_idx, = [  # Also asserts that there is a single such line.
         idx for idx, line in enumerate(template_lines)
         if "#backend:" in line]
     template_lines[backend_line_idx] = (
         "#backend: {}\n".format(setupext.options["backend"])
         if setupext.options["backend"]
         else "##backend: Agg\n")
-    path.write_text("".join(template_lines))
+    path.write_text("".join(template_lines), encoding="utf-8")
 
 
 class BuildPy(setuptools.command.build_py.build_py):