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[Python-Dev] PEP for Better Control of Nested Lexical Scopes

Greg Ewing greg.ewing at canterbury.ac.nz
Fri Feb 24 07:54:14 CET 2006 
Thomas Wouters wrote:
> On Thu, Feb 23, 2006 at 05:25:30PM +1300, Greg Ewing wrote:
>
>>As an aside, is there any chance that this could be
>>changed in 3.0? I.e. have the for-loop create a new
>>binding for the loop variable on each iteration.
> 
> You can't do that without introducing a whole new scope 

for the body of the
> 'for' loop,

There's no need for that. The new scope need only
include the loop variable -- everything else could
still refer to the function's main scope.

There's even a rather elegant way of implementing
this in the current CPython. If a nested scope
references the loop variable, then it will be in
a cell. So you just create a new cell each time
round the loop, instead of changing the existing
one.

This would even still let you use the value after
the loop finished, if that were considered a good
idea. But it might be better not to allow that,
since it could make alternative implementations
difficult.

--
Greg
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