In codecs.c,  when _PyCodec_Lookup() call normalizestring(), both spaces and hyphens should be convered to underscores. Not convert spaces to hyphens.

see:https://github.com/python/peps/blob/master/pep-0100.txt, Codecs (Coder/Decoders) Lookup

and I will try to fix it.

Hm, there is a bit misleading between desc(https://github.com/python/cpython/blob/master/Python/codecs.c#L53) and the code (https://github.com/python/cpython/blob/master/Python/codecs.c#L74).

The design and code of the following four places need to be consistent,

No.1 https://github.com/python/peps/blob/master/pep-0100.txt#L292
No.2 https://github.com/python/cpython/blob/master/Python/codecs.c#L113
No.3 https://github.com/python/cpython/blob/master/Python/codecs.c#L53  
No.4 https://github.com/python/cpython/blob/master/Python/codecs.c#74

Jordon is right. Conversion has to be to underscores, not hyphens. I guess this bug was introduced when the normalization function was converted to C.

New changeset 20f59fe1f7748ae899aceee4cb560e5e1f528a1f by Victor Stinner (Jordon Xu) in branch 'master':
bpo-37751: Fix codecs.lookup() normalization (GH-15092)
https://github.com/python/cpython/commit/20f59fe1f7748ae899aceee4cb560e5e1f528a1f

Thanks for the fix Jordon Xu.

IMHO this change is not strictly a bugfix, but more like an enhancement. I close the issue.

If you consider that a backport to Python 3.7 and 3.8 is needed, please say so.

Thanks vstinner. I also don't think it's necessary to backport to the old version. Close this issue is fine.