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

Ethan Furman ethan at stoneleaf.us
Fri Nov 24 21:09:24 EST 2017

On 11/24/2017 04:22 PM, Guido van Rossum wrote:

The more I hear about this topic, the more I think that await, yield and yield from should all be banned from occurring in all comprehensions and generator expressions. That's not much different from disallowing return or break.

For me, the deciding factor would be the the affect upon:

  1. the containing function (if any); and
  2. the result of the genexp/comprehension

I think of generator expressions / comprehensions as self-contained units of code that will have no (side-)effects upon the containing function and/or surrounding code (and a containing function is not necessary), and that each will return the type expressed by the syntax.

In other words:

[ l for l in ...] -> list {s for s in ...] -> set {k:v for k, v in ...] -> dict (g for g in ...] -> genexp

Since 'yield' and 'yield from' invalidate those, I'm in favor of declaring them syntax errors.

If 'await' does not invalidate the above constraints then I'm fine with allowing them.

-- Ethan

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