[Python-Dev] Minimal 'stackless' PEP using generators?

[Clark C. Evans]

On Mon, Aug 23, 2004 at 01:18:28PM -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