[Python-Dev] Tricky way of of creating a generator via a comprehension expression (original) (raw)

Stephen J. Turnbull turnbull.stephen.fw at u.tsukuba.ac.jp
Thu Nov 23 01:12:46 EST 2017

Greg Ewing writes:

Consider this:

def g():
   return ((yield i) for i in range(10))

Presumably the yield should turn g into a generator, but... then what? My brain is hurting trying to figure out what it should do.

I don't understand why you presume that. The generator expression doesn't do that anywhere else. My model is that implicitly the generator expression is creating a function that becomes a generator factory, which is implicitly called to return the iterable generator object, which contains the yield. Because the call takes place implicitly = at compile time, all the containing function "sees" is an iterable (which happens to be a generator object). "Look Ma, no yields left!" And then g returns the generator object.

What am I missing?

In other words, g above is equivalent to

def g():
    def _g():
        for i in range(10):
            # the outer yield is the usual implicit yield from the
            # expansion of the generator expression, and the inner
            # yield is explicit in your code.
            yield (yield i)
    return _g()

(modulo some issues of leaking identifiers). I have not figured out why either your g or my g does what it does, but they do the same thing.

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