[Python-Dev] bytes.from_hex() [Was: PEP 332 revival in coordination with pep 349?]

[M.-A. Lemburg]

Martin v. Löwis wrote:
> M.-A. Lemburg wrote:
>>>> True. However, note that the .encode()/.decode() methods on
>>>> strings and Unicode narrow down the possible return types.
>>>> The corresponding .bytes methods should only allow bytes and
>>>> Unicode.
>>> I forgot that: what is the rationale for that restriction?
>>
>> To assure that only those types can be returned from those
>> methods, ie. instances of basestring, which in return permits
>> type inference for those methods.
> 
> Hmm. So it for type inference????
> Where is that documented?

Somewhere in the python-dev mailing list archives ;-)

Seriously, we should probably add this to the documentation.

> This looks pretty inconsistent. Either codecs can give arbitrary
> return types, then .encode/.decode should also be allowed to
> give arbitrary return types, or codecs should be restricted.

No.

As I've said before: the .encode() and .decode() methods
are convenience methods to interface to codecs which take
string/Unicode on input and create string/Unicode output.

> What's the point of first allowing a wide interface, and then
> narrowing it?

The codec interface is an abstract interface. It is a flexible
enough to allow codecs to define possible input and output
types while being strict about the method names and signatures.

Much like the file interface in Python, the copy protocol
or the pickle interface.

> Also, if type inference is the goal, what is the point in allowing
> two result types?

I'm not sure I understand the question: type inference is about
being able to infer the types of (among other things) function
return objects. This is what the restriction guarantees - much
like int() guarantees that you get either an integer or a long.

-- 
Marc-Andre Lemburg
eGenix.com

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