[Tutor] Re: References in loops

Andrei project5 at redrival.net
Sat Feb 12 01:38:18 CET 2005

Matt Dimmic wrote:
> In Python, one bug that often bites me is this:
> (example A)
> aList = [1,2,3]
> for i in aList:
>     i += 1
> print aList
> --> [1,2,3]

Numbers are immutable, so the element 1 can't change into a 2 inside the 
list. If 1 was not immutable, e.g. a list you could modify it and then 
it would be "updated" in the original list too.

> This goes against my intuition, which is that aList == [2,3,4], probably
> because so much in Python is passed by reference and not by value. Of
> course I can always use range() or enumerate():

I tend to use list comprehensions for this:

     aList = [elem+1 for elem in aList]

but it's barely shorter than the explicit loop, so not necessarily an 
improvement from that point of view. But at least it prevents the bug 
from biting :).



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