> I think it may even make sense to build the filtering
> options into codecs.lookup() itself:
> 
>   def lookup(encoding, decoded_format=None,  encoded_format=None):
>       info = _lookup(encoding) # The existing codec lookup algorithm
>       if ((decoded_format is not None and decoded_format !=
> info.decoded_format) or
>           (encoded_format is not None and encoded_format !=
> info.encoded_format)):
>           raise CodecLookupError(info, decoded_format, encoded_format)

lookup('rot13') should fail with a lookup error to keep backward 
compatibility. You can just change the default values to:

def lookup(encoding, decoded_format='text',  encoded_format='binary'): ...

If you patch lookup, what about the following functions?

- getencoder()
- getdecoder()
- getincrementalencoder()
- getincrementaldecoder()
- getread()
- getwriter()
- itereencode()