[Python-ideas] Tweaking closures and lexical scoping to include the function being defined

[Nick Coghlan]

On Mon, Sep 26, 2011 at 2:56 AM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> - A 'nonlocal' declaration makes it refer to some namespace in
>  between local and module-level (there may be more than one of
>  these, so I wouldn't say that there are only 4 namespaces).
>
> Now, while the *value* of your proposed new kind of variable
> would be stored as part of the function's closure, its *name*
> would be part of the function's *local* namespace. Consider
> this:
>
>    i = 88
>
>    def f():
>        nonlocal i = 17
>        print i
>
>    def g():
>        nonlocal i = 42
>        print i
>
>    f()
>    g()
>    print i
>
> I'm assuming you intend that the two i's here would have nothing
> to do with each other, or with the i in the enclosing scope, so
> that the output from this would be
>
>    17
>    42
>    88
>
> rather than
>
>    42
>    42
>    42
>
> So, your proposed use of 'nonlocal' would actually be declaring
> a name to be *local*. That strikes me as weird and perverse.

Ah, but it *wouldn't* be local, that's the point - it would be stored
on the function rather than on the frame, and hence be shared across
invocations.

Change your example function to this so it actually modifies the name
binding, and it becomes clear that this is *not* a local variable
declaration:

    i = 88

    def f():
        nonlocal i from 17
        print(i)
        i += 1

>>> f()
17
>>> f()
18

It would work exactly as if we had introduced a containing scope as a closure:

  def outer():
    i = 17
    def f():
        nonlocal i
        print(i)
        i += 1
    return f

>>> f = outer()
>>> f()
17
>>> f()
18

The *only* thing that would change is the way the closure reference
would be initialised - it would happen as part of the function
definition (just like default arguments) rather than needing the
developer to write the outer scope explicitly just to initialise the
closure references.

Cheers,
Nick.

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