[Python-ideas] PEP 380 alternative: A yielding function

Guido van Rossum guido at python.org
Wed Jul 28 16:26:25 CEST 2010

On Tue, Jul 27, 2010 at 9:38 PM, Carl M. Johnson
<cmjohnson.mailinglist at gmail.com> wrote:
> What would this code do:
> def yield_if_true(x):
>     if x:
>         yield_(x)
> def maybe_yield(x):
>    if calculate_property(x):
>        yield_if_true(x)
>    else:
>        return None
> maybe_yield(5)
> When maybe_yield(5) is called different things need to happen
> depending on whether it’s a function or a generator. If it’s a
> generator, it shouldn’t execute calculate_property(x) yet, because
> generators don’t execute their contents until someone says
> next(maybe_yield(5)) (or maybe_yield(5).next() in Py2.x). On the other
> hand, if it’s not a generator but a function, then it should run
> calculate_property right away. You could try starting out as a
> function and then switching to being a generator if anything that you
> call has a call to yield_ inside of it, but that strikes me as
> extremely clumsy and complicated, and it could lead to unpleasant
> surprises if calculate_property has side-effects.
> ISTM there’s no way to do something like PEP-380 without requiring
> that some special keyword is used to indicate that the def is creating
> a generator and not creating a function. There other directions to
> take this besides yield from (for example, we could replace def with
> gen or some such and give return a special meaning inside a gen
> statement), but to try to do it without any kind of keyword at the
> site of the caller means violating "In the face of ambiguity, refuse
> the temptation to guess.”

Well, in a statically typed language the compiler could figure it out
based on the type of the things being called -- "generator-ness" could
be propagated by the type system just like "thows a certain
expression" is in Java.

--Guido van Rossum (python.org/~guido)

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