[issue19543] Add -3 warnings for codec convenience method changes

[Nick Coghlan]

Nick Coghlan added the comment:

For the warnings, it's actually bytes.decode() (et al) that are expected to return str, and str.encode() that's expected to return bytes. The codecs themselves remain free to do what they want (hence the recommendation to use codecs.encode() and codecs.decode() to invoke arbitrary codecs without the type constraints of the builtins). Aside from that, those 2 warnings and the unicode.decode() one look good.

However, the "bytes.encode()" warning is the one we determined couldn't be treated as a warning as "text".encode("ascii") is valid in both Python 2 & 3, it just does a str->str conversion in 2.x, and a str -> bytes conversion in 3.x.

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<http://bugs.python.org/issue19543>
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