On Tue, Dec 7, 2021 at 6:16 AM Brendan Barnwell <brenb...@brenbarn.net> wrote:
>
> On 2021-12-05 08:14, Chris Angelico wrote:
> > Closures cannot be executed without a context. Consider:
> >
> > def f(x=lambda: (a:=[])):
> > if isinstance(x, FunctionType): x = x()
> > print(a)
> >
> > Here's the problem: The name 'a' should be in the context of f, but
> > that context*does not exist* until f starts executing.
>
> Frankly, I would consider this another disadvantage of late-bound
> arguments as defined under your proposal. I do not want argument
> defaults to be able to have the side effect of creating additional local
> variables in the function. (There is also the question of whether they
> could assign in this manner to names already used by other arguments, so
> that one argument's default could potentially override the default of
> another.)
>
Unless it's otherwise denied, all valid forms of expression should be
valid in an argument default. They are already valid in early-bound
defaults:
>>> def f():
... def g(a=(b:=42)): ...
... return locals()
...
>>> f()
{'b': 42, 'g': <function f.<locals>.g at 0x7f9a7083ef00>}
Since early-bound defaults are evaluated at definition time, the
assignment happens in the outer function. If anything, it's more
logical for it to be a part of the inner function, but for early-bound
defaults, that's not possible.
Though I wouldn't recommend that people actually *do* this sort of
thing. Legal doesn't mean recommended. But if someone does, it needs
to behave logically and correctly.
ChrisA
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