On 22 April 2018 at 02:31, Steven D'Aprano
This is not entirely unprecedented in Python: it is analogous (although not identical) to binding default values to parameters:
def running_total(items, total=total): # Here total is local to the function, but the default # is taken from the surrounding scope.
The stronger precedent for "look up elsewhere until first use" is class scopes: >>> x = "global" >>> class C: ... print(x) ... x = "class attribute to be" ... print(x) ... global class attribute to be However, that has its own quirks, in that it ignores function scopes entirely: >>> def f(): ... x = "function local" ... class C: ... print(x) ... x = "class attribute to be" ... print(x) ... >>> f() global class attribute to be Whereas if you don't rebind the name in the class body, the class scope can see the function local as you'd expect: >>> def f2(): ... x = "function local" ... class C: ... print(x) ... >>> f2() function local While I haven't explicitly researched the full history, my assumption is that references from class scopes prior to a local name rebinding are an edge case that https://www.python.org/dev/peps/pep-0227/ didn't fully account for, so they retain their original pre-PEP-227 behaviour. Cheers, Nick. P.S. It may be becoming clearer why the earlier iterations of PEP 572 proposed sublocal scoping semantics for the new name binding expression: it not only gives greater differentiation from traditional assignments and limits the potential for obviously unwanted side effects like accidentally clobbering a name that's already in use, it also sidesteps a lot of these quirky name resolution issues that arise when you use full local name bindings. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia