[Python-Dev] Minimal 'stackless' PEP using generators? (original) (raw)

Clark C. Evans cce at clarkevans.com
Mon Aug 23 20:33:15 CEST 2004

On Mon, Aug 23, 2004 at 01🔞28PM -0400, Phillip J. Eby wrote: | It would be nice if there were some way to "accept" data and exceptions | within a generator that didn't require the 'events.resume' hack, e.g.: || result = yield aDeferred || would be really nice, especially if 'result' could cause an exception to | be raised. I was hoping that this was something along the lines of what | you were proposing.

Perhaps it would be nice to add an alternative syntax to call a generator when you are expecting exactly one value.

def generator():
    yield 'one value'
def consumer():
     value = generator()

This, when combined with the previous proposal would give:

>>> def top():
>>>     yield cooperate
>>>     yield "one value"
>>>
>>> def middle():
>>>     """ intermediate generator _only_ sees 'one value' """
>>>     bing = top()
>>>     # do something with bing
>>>     yield bing
>>>
>>> def lower():
>>>     """ this is not a generator, so it sees cooperate """
>>>     for x in middle():
>>>         print x
>>>
>>> lower()

cooperate
one value

| Perhaps there should be a "simple coroutines" PEP, that doesn't try to | extend generators into coroutines, but instead treats coroutines as a | first-class animal that just happens to be implemented using some of the | same techniques "under the hood".

The problem is maintaining a 'stack' of generators requires that intermediate generators in the stack need a tedious hack (in both peek.event and twisted.flow) for them to work. If we can have python allow messages to be sent from the top-most generator to the inner-most non-generator (via yield cooperate, or some other magic), this difficulty is resolved -- intermediate generators can then be written in a operational style, without icky hacks for managing deferred execution.

Full-blown corountines arn't necessary. A small tweak to generators will do.

Best,

Clark

More information about the Python-Dev mailing list