[Python-ideas] x=(yield from) confusion [was:Yet another alternative name for yield-from]

[Guido van Rossum]

On Sat, Apr 11, 2009 at 5:32 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> Jacob Holm wrote:
>> I am not really sure about this.  It looks very much like an
>> implementation detail to me.  On the other hand, the ability to replace
>> the methods mid-flight might give us a way to implement the
>> call-wrappers with minimal overhead.  Since the current patches don't
>> actually do any caching, this is something I should actually be able to
>> try.
>
> The part that makes me nervous is the fact that the PEP as it stands
> gives the green light to an implementation having different bound method
> caching behaviour between for loops and the yield-from expression.
>
> That goes against Guido's request that the degenerate case of yield-from
> have the same semantics as:
>
>  for x in subiter:
>    yield x
>
> Since the language reference is actually silent on the topic of caching
> the bound method when iterating over an object, I would phrase it along
> the following lines:
>
>  - if for loops in a Python implementation cache next(), then yield-from
> in that implementation should also cache next()
>  - if yield-from caches next(), it should also cache sent() and throw()
>  - Since CPython for loops don't cache the bound method for next(), it
> won't cache the methods used by yield-from either

In ceval.c, the FOR_ITER opcode expects the iterator on top of the
stack and calls (v->ob_type->tp_iternext)(v).

You tell me whether that is caching or not. :-)

> Who knows, maybe Guido will actually clarify the matter for us when he
> gets back from his vacation :)

Or sooner. :-)

> P.S. Speaking of vacations, I'll also be offline for the next week or so
> (starting tomorrow), and then my internet access for Python activities
> will be sketchy for another couple of weeks as I move house. So I won't
> be able to contribute much more to this discussion.

Good, let this be a general trend. :-)

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)