If _is_text_encoding may change in Python 3.5, you should add a comment to warn users to not use it and explain its purpose, maybe with a reference to this issue.

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We have talking about a very few codecs:

* base64: bytes => bytes
* bz2: bytes => bytes
* hex: bytes => bytes; decode supports also ASCII string (str) => bytes
* quopri: bytes => bytes
* rot_13: str => str
* uu: bytes => bytes
* zlib: bytes => bytes

I suppose that supporting ASCII string input to the hex decoder is a border effect of its implementation. I don't know if it is expected *for the codec*.

If we simplify the hex decoder to reject str types, all these codecs would have simply one type: same input and output type. Anyway, if you want something based on types, the special case for the hex decoder cannot be expressed with a type nor ABC. "ASCII string" is not a type.

So instead of  _is_text_encoding=False could be transform=bytes or transform=str. (I don't care of the name: transform_type, type, codec_type, data_type, etc.)

I know that bytes is not exact: bytearray, memoryview and any bytes-like object is accepted, but it is a probably enough for now.