Incorrect scope of list comprehension variables

Lie Ryan lie.1296 at gmail.com
Tue Apr 6 18:34:04 CEST 2010


On 04/06/10 18:42, Alain Ketterlin wrote:
> Alain Ketterlin <alain at dpt-info.u-strasbg.fr> writes:
> 
>> d = dict()
>> for r in [1,2,3]:
>>     d[r] = [r for r in [4,5,6]]
>> print d
> 
> Thanks to Chris and Paul for the details (the list comp. r actually
> leaks). I should have found this by myself.
> 
> My background is more on functional programming languages, that's why I
> thought the list comprehension iterator should be purely local. And yes,
> I think a "classical" for-loop iterator should also be local to the
> loop, but I understand this may be too counter-intuitive to many :-)

Actually in other programming languages, loop counter is usually local:

for (int i = 0; i < something; i++) {
    ....
}
foo(i); // illegal

The reason why python's loop counter leaks is for implementation
simplicity because otherwise python will have to deal with multi-layered
local namespace. Currently in python, the local namespace is just sugar
for an array access (a bit of hand-waving here). In other languages, a
{} block is a namespace and nested {} block means nested namespace even
if they're still in a single function; in python there is only a flat
local namespace and the names resolver becomes a thousand times simpler
(and faster).



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