[Python-Dev] Checking that PEP 558 (defined semantics for locals()) handles assignment expressions (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Thu Jul 5 18:06:23 EDT 2018

On Thu., 5 Jul. 2018, 1:21 am Guido van Rossum, <guido at python.org> wrote:

Correct, there shouldn't be any additional corner cases for your PEP due to inline assignments. We're not introducing new scopes nor other runtime mechanisms; the target of an inline assignment is either a global or a cell (nonlocal) defined at a specific outer level.

Cool. I'll still review the PEP to see if it makes sense to mention this as a side note anywhere, but it may turn out to make more sense to simply not mention it at all.

What I wish we had (quite independent from PEP 572) is a way in a debugger to evaluate a comprehension that references a local in the current stack frame. E.g. here's something I had in a pdb session yesterday:

(Pdb) p [getattr(context.context, x) for x in dir(context.context)] *** NameError: name 'context' is not defined (Pdb) global cc; cc = context (Pdb) p [getattr(cc.context, x) for x in dir(cc.context)] [<class 'mypy.nodes.CallExpr'>, ............] (Pdb) The first attempt failed because the outer context was a local variable in some function, and pdb uses eval() to evaluate expressions.

Perhaps pdb should be passing something like "ChainMap(frame.f_locals, frame.f_globals)" to the eval call as its global namespace when the current frame uses optimized local variable references? That way even lexically nested scopes inside the eval call will all be able to see the current local variables via dynamic name lookup, even though they still won't see them as lexical closure references.

Cheers, Nick. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20180706/0891c47b/attachment.html>

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