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

Ethan Furman ethan at stoneleaf.us
Wed Nov 22 16🔞10 EST 2017

On 11/22/2017 11:01 AM, Ivan Levkivskyi wrote:

I think how to formulate these rules more "precisely" so that they will be all consistent even if there is a yield inside. The key idea is that neither comprehensions nor generator expressions should create a function scope surrounding the expr in (expr for ind in iterable) and [expr for ind in iterable]. (this still can be some hidden implementation detail)

So as Serhiy proposed in one of his first posts any comprehensions and generator expressions with yield are not valid outside functions. If such comprehension or generator expression is inside a function, then it makes it a generator, and all the yields are yielded from that generator, for example:

Whether it's inside or outside a function should be irrelevant -- a comprehension / generator expression should have no influence on the type of the resulting function (and at least synchronous comprehensions / generator expressions should be able to live outside of a function).

def fungen(): return list((yield i) for i in range(3))

The return says it's returning a list, so that's what it should be returning.

should work as following:

g = funcgen() g.send(42) 0 g.send(43) 1 g.send(44) 2 try: g.send(45) except StopIteration as e: assert e.value == [42, 43, 44] And exactly the same with def funcomp(): [(yield i) for i in range(3)] I hope with this we can close the no-async part of the problem. Currently this is not how it works, and should be fixed. Do you agree?

No.

NB: If we go with making yield inside a comprehension / generator expression a SyntaxError then this subthread can die.

-- Ethan

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