[Python-ideas] A "local" pseudo-function

[Tim Peters]

[MRAB]
>> Any binding that's not specified as local is bound in the parent scope:

[Tim]
> Reverse-engineering the example following, is this a fair way of
> making that more precise?
>
> Given a binding-target name N in scope S, N is bound in scope T, where
> T is the closest-containing scope (which may be S itself) for which T
> is either
>
> 1. established by a "local:" block that declares name N
>
> or
>
> 2. not established by a "local: block

Here's an example where I don't know what the consequences of "the
rules" should be:

def f():
    a = 10
    local a:
        def showa():
            print("a is", a)
        showa() # 10
        a = 20
        showa() # 20
        a = 30
    showa() # 10

The comments show what the output would be under the "nothing about
scope rules change" meaning.  They're all obvious (since there is is
no new scope then - it's all function-local).

But under the other meaning ...?

The twist here is that `def` is an executable statement in Python, and
is a "binding site" for the name of the function being defined.  So
despite that `showa` appears to be defined in a new nested lexical
scope, it's _actually_ bound as a function-local name.  That's bound
to be surprising to people from other languages:  "I defined it in a
nested lexical scope, but the name is still visible after that scope
ends?".

I don't know what the first `showa()` is intended to do.  Presumably
`a` is unbound at the start of the new nested scope?  So raises
NameError?  If so, comment that line out so we can make progress ;-)

It seems clear that the second `showa()` will display 20 under any reading.

But the third?  Now we're out of the `local a:` scope, but call a
function whose textual definition was inside that scope.  What does
`showa()` do now to find a's value?  f's local `a` had nothing to do
with the `a` in the nested scope, so presumably it shouldn't display
10 now.  What should it do?
Does the final state of the nested scope's locals need to preserved so
that showa() can display 30 instead?  Or ...?

Not necessarily complaining  - just fleshing out a bit my earlier
claim that a world of semantics need to be defined if anything akin to
a "real scope" is desired.