[ python-Bugs-1584028 ] remove() during iteration causes items to be skipped

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Bugs item #1584028, was opened at 2006-10-24 16:44
Message generated for change (Comment added) made by rhettinger
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Category: Python Interpreter Core
Group: Python 2.4
>Status: Closed
>Resolution: Invalid
Priority: 5
Private: No
Submitted By: Kevin Rabsatt (krabsa)
Assigned to: Nobody/Anonymous (nobody)
Summary: remove() during iteration causes items to be skipped

Initial Comment:
If, when iterating over the contents of a list, the
current item is removed, the next item is skipped.

#Code:
if __name__ == '__main__':
  items = [0,1,2,3,4,5]
  for i in items:
    print i
    items.remove(i)

#End Code

Outputs:
0
2
4

I believe the behavior is undesirable.  An argument can
be made to not fix, but the issue is worth noting.



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>Comment By: Raymond Hettinger (rhettinger)
Date: 2006-10-24 20:06

Message:
Logged In: YES 
user_id=80475

Sorry, this isn't a bug -- it is a natural side-effect of
mutating a object while iterating over it.  The various
approaches to dealing with this include:
* don't allow mutation while iterating -- dict.iterkeys()
uses this approach
* iterate over a copy of the object -- dict.keys() uses this
approach
* iterate over consecutive indices and ignore mutation --
lists use this approach

Programmers can avoid dealing with this issue by:

* precopying the list:
     for i in items[:]:
         print i
         remove(i)

* building a new list during iteration:
      items[:]  = [x for x in items if f(x)]



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