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

Sept. 26, 2011

      On Mon, Sep 26, 2011 at 2:56 AM, Greg Ewing <greg.ewing@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@gmail.com   |   Brisbane, Australia