[Tutor] inconsistent destruction

[Steven D'Aprano]

On Wed, Aug 07, 2013 at 08:54:26PM -0700, Jim Mooney wrote:
> This bugs me for some reason. The final variable is saved in a for
> loop but not in a list comprehension. It just seems to me they should
> both be destroyed to avoid confusion.

The variable in a for-loop is deemed to be in the same scope as the rest 
of the function, the same as any other local variable. This is often 
very useful:


for item in objects:
    if condition(item):
        break
print(item)


If for-loops introduced their own scope, as they do in a few other 
languages, you would need to do something like this:

found = None
for item in objects:
    if condition(item):
        found = item
        break
print(found)


On the other hand, list comprehensions (since Python 3) and generator 
expressions (always) are deemed to exist in their own scope. That makes 
conceptual sense, since a generator expression is its own chunk of 
executable code, like a function, and in fact are implemented much the 
same way that functions are implemented internally. List comps look the 
same, and since Python 3 are now treated the same. That way you can use 
a list comp or generator expression without worrying that its loop 
variable will clobber an existing local variable of the same name.


[...]
> Is there a form of for loop that would destroy the loop variable? I
> could always do del cnt right after the for, but that seems
> artificial.

Why do you care? The typical function is chock full of local variables 
that are used a few times, then hang around doing nothing until the 
function exits. This doesn't cause any problems in practice.



-- 
Steven