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bpo-46659: Update the test on the mbcs codec alias #31168

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Feb 6, 2022
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bpo-46659: Update the test on the mbcs codec alias #31168

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f424c82
bpo-46659: Fix the mbcs codec alias
vstinner Feb 6, 2022
6a4c886
Update test_codecs.test_basics()
vstinner Feb 6, 2022
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7 changes: 4 additions & 3 deletions Lib/encodings/__init__.py
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Original file line number Diff line number Diff line change
Expand Up @@ -152,9 +152,6 @@ def search_function(encoding):
# Return the registry entry
return entry

# Register the search_function in the Python codec registry
codecs.register(search_function)

if sys.platform == 'win32':
def _alias_mbcs(encoding):
try:
Expand All @@ -167,4 +164,8 @@ def _alias_mbcs(encoding):
# Imports may fail while we are shutting down
pass

# It must be registered before search_function()
codecs.register(_alias_mbcs)

# Register the search_function in the Python codec registry
codecs.register(search_function)
17 changes: 11 additions & 6 deletions Lib/test/test_codecs.py
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Expand Up @@ -1904,7 +1904,10 @@ def test_basics(self):
name += "_codec"
elif encoding == "latin_1":
name = "latin_1"
self.assertEqual(encoding.replace("_", "-"), name.replace("_", "-"))
# Skip the mbcs alias on Windows
if name != "mbcs":
self.assertEqual(encoding.replace("_", "-"),
name.replace("_", "-"))

(b, size) = codecs.getencoder(encoding)(s)
self.assertEqual(size, len(s), "encoding=%r" % encoding)
Expand Down Expand Up @@ -3188,11 +3191,13 @@ def test_incremental(self):
self.assertEqual(decoded, ('abc', 3))

def test_mbcs_alias(self):
# Check that looking up our 'default' codepage will return
# mbcs when we don't have a more specific one available
with mock.patch('_winapi.GetACP', return_value=123):
codec = codecs.lookup('cp123')
self.assertEqual(codec.name, 'mbcs')
# On Windows, the encoding name must be the ANSI code page
encoding = locale.getpreferredencoding(False)
self.assertTrue(encoding.startswith('cp'), encoding)
Comment on lines +3194 to +3196
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This will fail if PYTHONUTF8 is set in the environment, because it overrides getpreferredencoding(False) and _get_locale_encoding().


# The encodings module create a "mbcs" alias to the ANSI code page
codec = codecs.lookup(encoding)
self.assertEqual(codec.name, "mbcs")
Comment on lines +3198 to +3200
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@eryksun eryksun Feb 6, 2022 •
edited
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This was never true before. With 1252 as my ANSI code page, I checked codecs.lookup('cp1252') in 2.7, 3.4, 3.5, 3.6, 3.9, and 3.10, and none of them return the "mbcs" encoding. It's not equivalent, and not supposed to be. The implementation of "cp1252" should be cross-platform, regardless of whether we're on a Windows system with 1252 as the ANSI code page, as opposed to a Windows system with some other ANSI code page, or a Linux or macOS system.

The differences are that "mbcs" maps every byte, whereas our code-page encodings do not map undefined bytes, and the "replace" handler of "mbcs" uses a best-fit mapping (e.g. "α" -> "a") when encoding text, instead of mapping all undefined characters to "?".

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This issue is worse than what I expected, I created https://bugs.python.org/issue46668 to discuss it.


@support.bigmemtest(size=2**31, memuse=7, dry_run=False)
def test_large_input(self, size):
Expand Down
10 changes: 0 additions & 10 deletions Lib/test/test_site.py
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Expand Up @@ -456,16 +456,6 @@ def test_setting_help(self):
# 'help' should be set in builtins
self.assertTrue(hasattr(builtins, "help"))

def test_aliasing_mbcs(self):
if sys.platform == "win32":
import locale
if locale.getdefaultlocale()[1].startswith('cp'):
for value in encodings.aliases.aliases.values():
if value == "mbcs":
break
else:
self.fail("did not alias mbcs")

def test_sitecustomize_executed(self):
# If sitecustomize is available, it should have been imported.
if "sitecustomize" not in sys.modules:
Expand Down