[Python-Dev] Tricky way of of creating a generator via a comprehension expression (original) (raw)
Guido van Rossum guido at python.org
Thu Nov 23 11:08:03 EST 2017
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This thread is still going over the speed limit. Don't commit anything without my explicit approval.
I know one thing for sure. The choice to make all comprehensions functions was quite intentional (even though alternatives were also discussed) and the extra scope is now part of the language definition. It can't be removed as a "bug fix". One thing that can be done without a PEP is making yield inside a comprehension a syntax error, since I don't think that was considered when the function scopes were introduced.
A problem with dropping the "function-ness" of the comprehension by renaming the variables (as Ivan/Serhiy's plan seems to be?) would be what does it look like in the debugger -- can I still step through the loop and print the values of expressions? Or do I have to know the "renamed" names?
And my mind boggles when considering a generator expression containing yield that is returned from a function. I tried this and cannot say I expected the outcome:
def f(): return ((yield i) for i in range(3)) print(list(f()))
In both Python 2 and Python 3 this prints
[0, None, 1, None, 2, None]
Even if there's a totally logical explanation for that, I still don't like it, and I think yield in a comprehension should be banned. From this it follows that we should also simply ban yield from comprehensions.
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