[Python-ideas] Sublocal scoping at its simplest

[Matt Arcidy]

On Sat, Apr 28, 2018, 20:16 Chris Angelico <rosuav at gmail.com> wrote:

> There's been a lot of talk about sublocal scopes, within and without
> the context of PEP 572. I'd like to propose what I believe is the
> simplest form of sublocal scopes, and use it to simplify one specific
> special case in Python.
>
> There are no syntactic changes, and only a very slight semantic change.
>
> def f():
>     e = 2.71828
>     try:
>         1/0
>     except Exception as e:
>         print(e)
>     print(e)
> f()
>
> The current behaviour of the 'except... as' statement is as follows:
>
> 1) Bind the caught exception to the name 'e', replacing 2.71828
> 2) Execute the suite (printing "Division by zero")
> 3) Set e to None
> 4) Unbind e
>
> Consequently, the final print call raises UnboundLocalError. I propose
> to change the semantics as follows:
>
> 1) Bind the caught exception to a sublocal 'e'
> 2) Execute the suite, with the reference to 'e' seeing the sublocal
> 3) Set the sublocal e to None
> 4) Unbind the sublocal e
>
> At the unindent, the sublocal name will vanish, and the original 'e'
> will reappear. Thus the final print will display 2.71828, just as it
> would if no exception had been raised.
>
>
Does this mean indentation is now a scope, or colons are a scope, or is
that over simplifying?

either seems to be more consistent with the patterns set by class and
function defs, barring keywords.

not sure if relevant but curious.

I think with sublocal scope, reuse of a name makes more sense.  Currently,
if using sensible, descriptive names, it really doesn't make sense to go
from food = apple to food = car as the value between scopes, but it
happens.  And if from fruit = apple to fruit = orange (eg appending a msg
to a base string) it _could_ be nice to restore to apple once finished.

Obviously that's simple enough to do now, I am only illustrating my point.
I know bad code can be written with anything, this is not my point.  It can
be seen as enforcing that, what every nonsense someone writes like
fruit=car, there is at least some continuity of information represented by
the name... till they do it again once out of the sublocal scope of course.

as for the value of this use case, I do not know.


> The above definitions would become language-level specifications. For
> CPython specifically, my proposed implementation would be for the name
> 'e' to be renamed inside the block, creating a separate slot with the
> same name.
>
> With no debates about whether "expr as name" or "name := expr" or
> "local(name=expr)" is better, hopefully we can figure out whether
> sublocal scopes are themselves a useful feature :)
>
> ChrisA
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