variable scope in list comprehensions

Steve Holden steve at holdenweb.com
Sat Apr 5 14:44:13 CEST 2008


Duncan Booth wrote:
> Steve Holden <steve at holdenweb.com> wrote:
> 
>>> For a moment I thought that maybe list comprehension has its own
>>> scope, but it doesn't seem to be so:
>>> print [[y for y in range(8)] for y in range(8)]
>>> print y
>>>
>>> Does anybody understand it?
>>>
>>>
>> This isn't _a_ list comprehension, it's *two* list comprehensions. The
>> interpreter computes the value if the inner list comprehension and
>> then duplicates eight references to it (as you will see if you change
>> an element).
>>
> Do you want to reconsider that statement? The interpreter recomputes the 
> inner list comprehension eight times, there are no duplicated 
> references.
> 
[...]
You are correct. I wrote that before testing, and then after testing 
wrote """The outer loop's control variable is never used in the inner 
loop, so there is no chance of conflict: each time the inner loop 
terminates the outer loop assigns the next value from its range - it 
isn't "adding one" to the variable, but merely calling an iterator's 
next() method."""

So I didn't do enough editing after testing to (in)validate my 
preconceptions.

regards
  Steve
-- 
Steve Holden        +1 571 484 6266   +1 800 494 3119
Holden Web LLC              http://www.holdenweb.com/




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