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FWIW it's, I've been thinking further about this recently and I think implementing this feature as builtin methods is the wrong way to approach it.
Instead, I propose the addition of codecs.encode and codecs.decode methods that are type neutral (leaving any type checks entirely up to the codecs themselves), while the str.encode and bytes.decode methods retain their current strict test model related type restrictions.
Also, I now think my previous proposal for nice error messages was massively over-engineered. A much simpler approach is to just replace the status quo:
>>> "".encode("bz2_codec")
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/ncoghlan/devel/py3k/Lib/encodings/bz2_codec.py", line 17, in bz2_encode
 return (bz2.compress(input), len(input))
 File "/home/ncoghlan/devel/py3k/Lib/bz2.py", line 443, in compress
 return comp.compress(data) + comp.flush()
TypeError: 'str' does not support the buffer interface
with a better error with more context like:
UnicodeEncodeError: encoding='bz2_codec', errors='strict', codec_error="TypeError: 'str' does not support the buffer interface"
A similar change would be straightforward on the decoding side.
This would be a good use case for __cause__, but the codec error should still be included in the string representation.