[Python-ideas] Sublocal scoping at its simplest

[Nick Coghlan]

On 29 April 2018 at 13:14, 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.
>

The challenge with doing this implicitly is that there's no indication
whatsoever that the two "e"'s are different, especially given the
longstanding precedent that the try/except level one will overwrite any
existing reference in the local namespace.

By contrast, if the sublocal marker could be put on the *name itself*, then:

1. Sublocal names are kept clearly distinct from ordinary names
2. Appropriate sublocal semantics can be defined for any name binding
operation, not just exception handlers
3. When looking up a sublocal for code compiled in exec or eval mode,
missing names can be identified and reported at compile time (just as they
can be for nonlocal declarations) (Such a check likely wouldn't be possible
for code compiled in "single" mode, although working out a suitable
relationship between sublocal scoping and the interactive prompt is likely
to prove tricky no matter what)

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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