[Python-ideas] For-loop variable scope: simultaneous possession and ingestion of cake

[George Sakkis]

On Fri, Oct 3, 2008 at 10:56 PM, Greg Ewing <greg.ewing at canterbury.ac.nz>wrote:

> Guido van Rossum wrote:
>
>  This leads me to reject claims that "the for-loop is broken" and in
>> particular clamoring for fixing the for-loop without allowing us to
>> fix this example.
>>
>
> Yeah, I never said the for-loop was the *only*
> thing that's broken. :-)
>
> Perhaps "broken" is too strong a word. What I really
> mean is that it's designed in a way that interacts
> badly with nested functions.
>
> More generally, Python's inability to distinguish
> clearly between creating new bindings and changing
> existing bindings interacts badly with nested
> functions.
>
> I agree that the wider problem needs to be addressed
> somehow, and perhaps that should be the starting
> point. Depending on the solution adopted, we can
> then look at whether a change to the for-loop is
> still needed.


Since this idea didn't get much steam, a more modest proposal would be to
relax the restriction on cells: allow the creation of new cells and the
rebinding of func_closure in pure Python. Then one could explicitly create a
new scope without any other change in the language through a 'localize'
decorator that would create a new cell for every free variable (i.e. global
or value from an enclosing scope) of the function:

lst = []
for i in range(10):
   @localize
   def f(): print i
   lst.append(f)
   lst.append(localize(lambda: i**2))

I'd love to be proven wrong but I don't think localize() can be implemented
in current Python.

George
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