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

Ethan Furman ethan at stoneleaf.us
Wed Nov 22 12:37:55 EST 2017

On 11/22/2017 05:03 AM, Serhiy Storchaka wrote:

From https://stackoverflow.com/questions/45190729/differences-between-generator-comprehension-expressions.

g = [(yield i) for i in range(3)] Syntactically this looks like a list comprehension, and g should be a list, right? But actually it is a generator. This code is equivalent to the following code: def makelist(it): result = [] for i in it: result.append(yield i) return result g = makelist(iter(range(3))) Due to "yield" in the expression makelist() is not a function returning a list, but a generator function returning a generator.

The [] syntax says g should be list. Seems to me we could do either of:

  1. raise if the returned object is not a list;
  2. wrap a returned object in a list if it isn't one already;

In other words, (2) would make

g = [(yield i) for i in range(3)]

and

g = [((yield i) for i in range(3))]

be the same.

I have no idea how either of those solutions would interact with async/await.

-- Ethan

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