kcrisman wrote:
> Dear support,
>
> I'm trying to resolve #7315 and have discovered something that
> disturbs me, but probably is reasonable to someone who really
> understands Python lists. Namely:
>
> {{{
>>>> L=[1,2,3,4]
>>>> for x in L:
> .... L.remove(x)
> .... x
> .... L
> ....
> 1
> [2, 3, 4]
> 3
> [2, 4]
>>>> L
> [2, 4]
> }}}
>
> Somehow it is going by the index of the list, not the actual
> elements. Assuming this is intended behavior, what is the right
> workaround? Any link to the official Python documentation would be
> wonderful as well.
http://docs.python.org/reference/compound_stmts.html#the-for-statement
"There is a subtlety when the sequence is being modified by the loop
(this can only occur for mutable sequences, i.e. lists). An internal
counter is used to keep track of which item is used next, and this is
incremented on each iteration. When this counter has reached the length
of the sequence the loop terminates. This means that if the suite
deletes the current (or a previous) item from the sequence, the next
item will be skipped (since it gets the index of the current item which
has already been treated). Likewise, if the suite inserts an item in the
sequence before the current item, the current item will be treated again
the next time through the loop. This can lead to nasty bugs that can be
avoided by making a temporary copy using a slice of the whole sequence,
e.g.,
for x in a[:]:
if x < 0: a.remove(x)
Thanks,
Jason
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