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

[Phillip J. Eby]

At 10:21 AM 7/5/2006 +0200, Guido van Rossum wrote:
>Thanks for bringing this up. I'm not sure what I think of it yet. One
>problem I see is that there might end up being two ways to reference
>variables in outer scopes: .num if you plan to assign to it, or just
>num if you only reference it. I find that the most disurbing issue so
>far; modified global declarations or outer declarations don't have
>this problem.

Well, you could make it mandatory in Py3K I suppose, though I'm not sure I 
like it being mandatory, due to the frequent need to reference top-level 
names that would cause an awful lot of dots to start popping up.

But for existing Python, the optional nature of the '.' allows existing 
code to run unchanged.  And for versions that support the new syntax, using 
a leading '.' for all non-global, non-locals should be considered "good 
style" since it highlights the dependency and practically shouts "tricky 
stuff here, pay attention."

Ironically, having *only* one way to refer to outer variables makes it 
impossible to communicate this distinction in the common read-only case.


>Would this also use ..num to refer to num in an outer scope two levels 
>removed?

I think that's unnecessary; it would be much better to use variables with 
distinct names.

By the way, an interesting thought for Py3K is that you could maybe use 
this syntax to do away with explicit 'self', if you consider the class' 
namespace to be part of a function's closure.

E.g.:

      class Foo:

          whee = 42

          def bar(baz):
              print .whee

Consider this: if Foo were a function rather than a class, each invocation 
of Foo would yield a new namespace in which 'whee' is defined.  However, 
each invocation of a *class* also yields a new namespace.  So there's a 
definite symmetry in using .whee to refer to an instance attribute of Foo.

The big problem that comes to mind with that idea is that it makes it 
impossible to have argument names that are the same as attribute names, 
unless the 'whee'/'.whee' prohibition were relaxed.  :(  But it's an 
intriguing thought, nonetheless.