bpo-37751 changed codecs.lookup() in a subtle way: non-ASCII characters are now ignored, whereas they were copied unmodified previously.

I would prefer that codecs.lookup() and encodings.normalize_encoding() behave the same. Either always ignore or always copy.

Moreover, it seems like there is no test on how the encoding names are normalized in codecs.register(). I recall that using codecs.register() in an unit test causes troubles since there is no API to unregister a search function. Maybe we should just add a private function for test in _testcapi.

Serhiy Storchaka wrote an example on my PR:
https://github.com/python/cpython/pull/17997/files

> There are other differences. For example, normalize_encoding("КОИ-8") returns "кои_8", but codecs.lookup normalizes it to "8".

> The comment in the sources is also not correct.

> I would prefer that codecs.lookup() and encodings.normalize_encoding() behave the same. Either always ignore or always copy.

How about calling `encodings.normalize_encoding() in codecs.normalizestring()` to keep same behavior?(I create PR18845)

> Maybe we should just add a private function for test in _testcapi

I can try to add some test cases in next weekend ;)

> How about calling `encodings.normalize_encoding() in codecs.normalizestring()` to keep same behavior?(I create PR18845)

I have try this idea, but it make the testcase of test_io.py failed because some object will call `codecs.Lookup()` in `__del__()`.-->extension module will be cleaned before calling `__del__().`

> I would prefer that codecs.lookup() and encodings.normalize_encoding() behave the same. Either always ignore or always copy.

I try to add a `_Py_normalize_unicode_encoding()` in unicodeobject.c to support non-ASCII encoding names' normalization(PR18987), but this PR caused many testcases failed.

For example:

In master:
python3.9 -c "print('a\xac\u1234\u20ac\u8000\U0010ffff'.encode('iso-8859-15', 'namereplace'))"
result:
b'a\xac\\N{ETHIOPIC SYLLABLE SEE}\xa4\\N{CJK UNIFIED IDEOGRAPH-8000}\\U0010ffff'

after PR18987:
./python -c "print('a\xac\u1234\u20ac\u8000\U0010ffff'.encode('iso-8859-15', 'namereplace'))"
result:
b'a\xac\\N{ETHIOPIC SYLLABLE SEE}\\N{EURO SIGN}\\N{CJK UNIFIED IDEOGRAPH-8000}\\U0010ffff'

> Maybe we should just add a private function for test in _testcapi.

Oh, there have a problem with this idea:
struct _is is defined in internal/pycore_pystate.h.
_testcapi must test the public Python C API, not CPython internal C API

> _testcapi must test the public Python C API, not CPython internal C API

Use _testinternalcapi in this case.

> Use _testinternalcapi in this case.
oh, forgive me. I don't atttention this extension module before :(

See also bpo-37751 and my PR 17997.

New changeset 3f342376ab0da3b4c8a38a27edfafba8e8cdf52d by Hai Shi in branch 'master':
bpo-39337: Add a test case for normalizing of codec names (GH-19069)
https://github.com/python/cpython/commit/3f342376ab0da3b4c8a38a27edfafba8e8cdf52d

New changeset c5b049b91ca50c615f9a5425055c2b79a82ac547 by Hai Shi in branch 'master':
bpo-39337: encodings.normalize_encoding() now ignores non-ASCII characters (GH-22219)
https://github.com/python/cpython/commit/c5b049b91ca50c615f9a5425055c2b79a82ac547

Thanks Hai Shi for the change and for new codecs and encodings tests.

Thanks for everyone's continus review :)