[Python-ideas] Cofunctions: It's alive! Its alive!

Guido van Rossum guido at python.org
Wed Aug 11 05:01:49 CEST 2010

On Tue, Aug 10, 2010 at 7:48 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On Wed, Aug 11, 2010 at 10:49 AM, Greg Ewing
> <greg.ewing at canterbury.ac.nz> wrote:
>>> - The syntax worries me. Your PEP suggests that cocall binds tightly
>>> to an atom. That would mean that if the cofunction is really a
>>> comethod, you'd have to parenthesis it,
>> No, if you examine the grammar in the PEP you'll see that
>> the atom can be followed by a subset of the trailers allowed
>> after atoms in other contexts, so it's possible to write
>> things like
>>   x = cocall foo.blarg[42].stuff(y)
>> which parses as
>>   x = cocall (foo.blarg[42].stuff)(y)
> I would expect the grammatical rules for cocall expressions to be
> similar to those for yield expressions. And if they weren't, I'd want
> to hear a really good excuse for the inconsistency :)

This I can explain -- a cocall *must* be a call, syntactically, so
that it can take the callable, check that it has a __cocall__ method,
and call it with the given argument list. But I have to say, I don't
really like it, it's very odd syntax (even worse than decorators).

> Also, a possible trick to make a @cofunction decorator work:
> class cofunction:
>    # Obviously missing a bunch of stuff to tidy up the metadata
>    def __init__(self, f):
>        self._f = f
>    def __cocall__(*args, **kwds):
>        self, *args = *args
>        return yield from self._f(*args, **kwds)
> Cofunctions then wouldn't even *have* a __call__ slot, so you couldn't
> call them normally by mistake, and ordinary functions wouldn't define
> __cocall__ so you couldn't invadvertently use them with the new
> keyword.
> Cheers,
> Nick.
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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--Guido van Rossum (python.org/~guido)

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