On 22 April 2018 at 02:31, Steven D'Aprano <st...@pearwood.info> wrote:
> 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 | ncogh...@gmail.com | Brisbane, Australia
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