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

Jelle Zijlstra jelle.zijlstra at gmail.com
Wed Nov 22 13:54:01 EST 2017

2017-11-22 9:58 GMT-08:00 Guido van Rossum <guido at python.org>:

Wow, 44 messages in 4 hours. That must be some kind of record.

If/when there's an action item, can someone summarize for me? The main disagreement seems to be about what this code should do:

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

Currently, this makes g into a generator, not a list. Everybody seems to agree this is nonintuitive and should be changed.

One proposal is to make it so g gets assigned a list, and the yield happens in the enclosing scope (so the enclosing function would have to be a generator). This was the way things worked in Python 2, I believe.

Another proposal is to make this code a syntax error, because it's confusing either way. (For what it's worth, that would be my preference.)

There is related discussion about the semantics of list comprehensions versus calling list() on a generator expression, and of async semantics, but I don't think there's any clear point of action there.

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

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