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

Steve Dower steve.dower at python.org
Wed Jul 4 20:20:46 EDT 2018

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.

Whether intentional or not, there will be changes to how and when names are resolved. The specification should provide enough information to determine the preferred behaviour, so we can tell the difference between intention changes and implementation bugs.

For example, what should be returned from this function?

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

As far as I can tell, the closest current equivalent will not compile:

A = 0 def f(x): ... if x: ... def g(): ... nonlocal A ... A = 1 ... g() ... return A ... File "", line 4 SyntaxError: no binding for nonlocal 'A' found

Is this the equivalent behaviour you want? Or do you want an UnboundLocalError when calling f(0)? Or do you want the global A to be returned? How should we approach decision making about these cases as we implement this? The PEP does not provide enough information for me to choose the right behaviour here, and I argue that it should.

Cheers, Steve

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