Incorrect scope of list comprehension variables

Chris Rebert clp2 at
Sat Apr 3 06:53:57 EDT 2010

On Sat, Apr 3, 2010 at 3:30 AM, Alain Ketterlin
<alain at> wrote:
> I've just spent a few hours debugging code similar to this:
> d = dict()
> for r in [1,2,3]:
>    d[r] = [r for r in [4,5,6]]
> print d
> THe problem is that the "r" in d[r] somehow captures the value of the
> "r" in the list comprehension, and somehow kills the loop interator. The
> (unexpected) result is {6: [4, 5, 6]}. Changing r to s inside the list
> leads to the correct (imo) result.
> Is this expected? Is this a known problem? Is it solved in newer
> versions?

Quoting :

[1]	In Python 2.3 and later releases, a list comprehension “leaks” the
control variables of each 'for' it contains into the containing scope.
However, this behavior is deprecated, and relying on it will not work
in Python 3.0


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