Delaney, Timothy (Tim) wrote:
> Dave Hansen wrote:
>
>
>>Again, iterating over an item that is mutating seems like a Bad
>>Idea(tm) to me.
>
>
> Absolutely. It can be safe to do it, but only if the iterator in
> question supports it, and all modifications occur through the iterator
> (this is how Java does it). In a Python for loop, the actual iterator is
> never exposed in any case, so this is not possible - it's just like
> using the new Java foreach syntax: for (element : iterable), but more
> flexible.
>
>
>>But I was curious: is this the intended behavior, or
>>does this fall under what C programmers would call 'undefined
>>behavior.'
>
>
> It's intended. From the tutorial (section 4.2 - for statement):
> http://docs.python.org/tut/node6.html#SECTION006200000000000000000
>
> It is not safe to modify the sequence being iterated over in the
> loop (this can only happen for mutable sequence types, such as
> lists). If you need to modify the list you are iterating over
> (for example, to duplicate selected items) you must iterate over
> a copy. The slice notation makes this particularly convenient:
>
> >>> for x in a[:]: # make a slice copy of the entire list
> ... if len(x) > 6: a.insert(0, x)
> ...
> >>> a
> ['defenestrate', 'cat', 'window', 'defenestrate']
>
>
> Tim Delaney
I should have thought nowadays
for x in tuple(a):
...
would have been slightly more acceptable, but I'm not the ultimate
arbiter of style.
regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC http://www.holdenweb.com/
--
http://mail.python.org/mailman/listinfo/python-list
