Marc-Andre Lemburg added the comment:
On 16.11.2013 10:16, Nick Coghlan wrote:
>
> 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.
That doesn't look flexible enough to cover the various different
input/output types.
> 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).
The original idea we discussed some time ago was to add a mapping
or list attribute to CodecInfo which lists all supported type
combinations.
The codecs module could then make this information available through
a simple type check API (which also caches the lookups for performance
reasons), e.g.
codecs.types_supported(encoding, input_type, output_type) -> boolean.
Returns True/False depending on whether the codec for
encoding supports the given input and output types.
Usage:
if not codecs.types_support(encoding, str, bytes):
# not a Unicode -> 8-bit codec
...
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<http://bugs.python.org/issue19619>
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