Nick Coghlan added the comment:
The full input/output type specifications can't be implemented sensibly without
also defining at least a ByteSequence ABC. While I think it's a good idea in
the long run, there's no feasible way to design such a system in the time
remaining before the Python 3.4 feature freeze.
However, we could do something much simpler as a blacklist API:
def is_unicode_codec(name):
"""Returns true if this is the name of a known Unicode text encoding"""
def set_as_non_unicode(name):
"""Indicates that the named codec is not a Unicode codec"""
And then the codecs module would just maintain a set internally of all the
names explicitly flagged as non-unicode.
Such an API remains useful even if the input/output type support is added in
Python 3.5 (since "codecs.is_unicode_codec(name)" is a bit simpler thing to
explain than the exact type restrictions).
Alternatively, implementing just the "encodes_to" and "decodes_to" attributes
would be enough for str.encode, bytes.decode and bytearray.decode to reject
known bad encodings early, leaving the input type checks to the codecs for now
(since it is correctly defining "encode_from" and "decode_from" for many stdlib
codecs that would need the ByteSequence ABC).
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<http://bugs.python.org/issue19619>
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