[Python-Dev] Explicit Lexical Scoping (pre-PEP?)

[Guido van Rossum]

+1 on nonlocal.

I think that the := operator is also in case (b), but as I don't like
it I'm find with not mentioning it. :-)

Could someone write a PEP for this? Doesn't have to be very long but
I'd like it to summarize the main options proposed and discuss them,
like I did for the switch PEP. It's a p3yk PEP. (We really need to
move this to the py3k list...)

--Guido

On 7/6/06, Ka-Ping Yee <python-dev at zesty.ca> wrote:
> On Wed, 5 Jul 2006, Guido van Rossum wrote:
> > On 7/5/06, Phillip J. Eby <pje at telecommunity.com> wrote:
> > > Using the classic nonsense example:
> > >
> > >      def counter(num):
> > >          def inc():
> > >              .num += 1
> > >              return .num
> > >          return inc
> > >
> > Would this also use ..num to refer to num in an outer scope two
> > levels removed?
>
> I don't think there's any need for that.  I see '.num' as just another
> way of saying "num, but don't make a new binding".
>
> I agree with Guido that the best proposals so far are converging on
> the idea that it's more Pythonic to say "don't make a new binding"
> when a variable is used, than to declare "this is the scope for this
> binding" ahead of time.
>
> Of those there are two kinds:
>
>     (a) State once (anywhere in a scope, but preferably at the
>         beginning) that a variable is non-local.  This is like
>         the "global" keyword works now, and this category includes:
>
>           - Change the meaning of 'global'.
>           - Add a new keyword 'outer' or 'nonlocal', etc.
>
>     (b) Indicate, when mentioning a variable, that the variable
>         is non-local.  This category includes:
>
>           - Say 'global.x' or 'outer.x' instead of 'x'.
>           - Say '.x' instead of 'x'.
>
> My favourite so far is to use a new keyword -- i think changing the
> meaning of 'global' would be misleading.  '.x' is probably my next
> favourite, though i share Guido's concern about allowing both 'x'
> and '.x' to refer to the same thing.
>
> I see that 'outer' is used as an identifier in hmac.py in the
> standard library and also as a variable in test_set*.py.  On the
> other hand 'nonlocal' does not appear anywhere in the standard
> library.  Thus, 'nonlocal' is the best option that i've seen so far;
> it's less likely to break anything and it says exactly what it means.
> I can't think of a more accurate keyword.
>
>
> -- ?!ng
>


-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)