[Python-Dev] PEP 572 semantics (original) (raw)

Chris Angelico rosuav at gmail.com
Wed Jul 4 20:25:17 EDT 2018

On Thu, Jul 5, 2018 at 10:20 AM, Steve Dower <steve.dower at python.org> wrote:

On 04Jul2018 1518, Tim Peters wrote:

The only new thing is specifying the scope of a, where "local to f" means exactly the same thing as for any other name local to a function today. So far as the PEP semantics go, it doesn't even matter whether an implementation does implement some form of closure as such. It just has to provide the visible semantics of lexically nested scopes with indefinite extent, by whatever means it likes best. That's what "local to f" means (and has meant all along - well, since lexically nested scopes were first introduced). In that case, please provide more examples of how it should work when the assignment expression appears to define a variable in a scope that is not on the call stack.

Using an assignment expression in a comprehension is the same as using an assignment expression outside a comprehension at the exact same point in the code. I'm not sure what else needs to be explained further. It's assignment, so it creates a name at that scope.

For example, what should be returned from this function?

>>> A = 0 >>> def f(x): ... if x: ... [A := i for i in [1]] ... return A

A would be local to f(). This will either return 1 or raise UnboundLocalError; it will never return 0.

ChrisA

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