List iterator thread safety

Peter Otten __peter__ at
Thu Aug 27 14:41:07 CEST 2009

Emanuele D'Arrigo wrote:

> On Aug 27, 2:01 am, a... at (Aahz) wrote:
>> Well, I'm not sure about exceptions, but you almost certainly won't get
>> the results you want.
> What I'd like in this context is to iterate through the items in the
> list without processing the same item twice and without skipping item
> that are in front of the current iterator position. Somehow I can't
> quite prove to myself if this is possible or not over multiple
> threads. I.e. a dictionary will throw an exception about the object
> changing size while iterating through it. A list doesn't, hence the
> question.

This is not even possible in a single thread:

>>> items = [1, 2, 3]
>>> for i in items:
...     print i
...     if 1 in items: items.remove(1)

Basically a list iterator is just a list reference and an index into that 
list. No effort is made to keep track of added or removed items.


More information about the Python-list mailing list