On Thu, Oct 20, 2011 at 8:34 AM, STINNER Victor <report@bugs.python.org> wrote:
>> str.transform('bz2') ==> CodecLookupError
>
> A lookup error is surprising here. It may be a TypeError instead. The bz2 can be used with .transform, but not on str. So:

No, it's the same concept as the other cases - we found a codec with
the requested name, but it's not the kind of codec we wanted in the
current context (i.e. str.transform). It may be that the problem is
the user has a str when they expected to have a bytearray or a bytes
object, but there's no way for the codec lookup process to know that.

>  - Lookup error if the codec cannot be used with encode/decode or transform/untransform
>  - Type error if the value type is invalid

There's no way for str.transform to tell the difference between "I
asked for the wrong codec" and "I expected to have a bytes object
here, not a str object". That's why I think we need to think in terms
of format checks rather than type checks.

> (CodecLookupError doesn't exist, you propose to define a new exception who inherits from LookupError?)

Yeah, and I'd get that to handle the process of creating the nice
error messages. 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)

Then the various encode, decode and transform methods can just pass
the appropriate arguments to 'codecs.lookup' without all having to
reimplement the format checking logic.