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codecs.charmap_decode behaves differently with native and user string as decode table.
>>> import codecs
>>> print(ascii(codecs.charmap_decode(b'\x00', 'replace', '\uFFFE')))
('\ufffd', 1)
>>> class S(str): pass
... 
>>> print(ascii(codecs.charmap_decode(b'\x00', 'replace', S('\uFFFE'))))
('\ufffe', 1)
It's because charmap decoder (function PyUnicode_DecodeCharmap in Objects/unicodeobject.c) uses different algorithms for exact strings and for other.
We need to fix it? If yes, what should return `codecs.charmap_decode(b'\x00', 'replace', {0:'\uFFFE'})`? What should return `codecs.charmap_decode(b'\x00', 'replace', {0:0xFFFE})`?