[Python-ideas] A comprehension scope issue in PEP 572

[Nick Coghlan]

On 7 May 2018 at 13:15, Tim Peters <tim.peters at gmail.com> wrote:

> [Tim]
> >> There's a difference, though:  if `y` "leaks", BFD.  Who cares? ;-)
> >> If `y` remains inaccessible, there's no way around that.
> > Part of it is just that people seem to be fighting for the sake of
> > fighting. I'm weary of it, and I'm not going to debate this point with
> > you. You want 'em to leak? No problem. Implement it that way and I'm
> > not going to argue it.
>
> I'm more interested in real-life use cases than in arguments.  My
> suggestion came from staring at my real-life use cases, where binding
> expressions in comprehensions would clearly be more useful if the
> names bound leaked.  Nearly (but not all) of the time,, they're quite
> happy with that for-target names don't leak.  Those are matters of
> observation rather than of argument.
>

The issue is that because name binding expressions are just ordinary
expressions, they can't be defined as "in comprehension scope they do X, in
other scopes they do Y" - they have to have consistent scoping semantics
regardless of where they appear.

However, it occurs to me that a nonlocal declaration clause could be
allowed in comprehension syntax, regardless of how any nested name bindings
are spelt:

    p = rem = None
    while any((rem := n % p) for p in small_primes nonlocal (p, rem)):
        # p and rem were declared as nonlocal in the nested scope, so our
rem and p point to the last bound value

I don't really like that though, since it doesn't read as nicely as being
able to put the nonlocal declaration inline.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20180507/52da183d/attachment.html>