[Python-talk] Stability of dict.iteritems() (etc) under deletions

[Kent Johnson]

Bill Freeman wrote:
> Assume that "d" is a dictionary below.  Is the following safe?
> 
>   for k, v in d.iteritems():
>     if testFunction(v):
>       del d[k]
> 
> or, equivalently:
> 
>   for k in d.iterkeys():
>     if testFunction(d[k]):
>       del d[k]

No, it is not safe. Commentary here:
http://mail.python.org/pipermail/python-list/2002-February/125989.html

> (I presume that the last one is equivalent to "for k in d:...".)

Yes, plus-or-minus an attribute lookup and a function call...
> 
> That is, does the deletion within the loop invalidate the
> iterator, or at least can prevent it from correctly returning
> the remainder of the items (e.g.; some might not be visited, some
> already visited might be visited again)?

I don't think any Python iterators are safe this way.
> 
> On the other hand, I presume that the following is safe:
> 
>   for k, v in d.items():
>     if testFunction(d[k]):
>       del d[k]
> 
>  - because items has returned a list object that is no longer
> tied to the dictionary by the time the first iteration runs.

Correct.

Kent