Python iterator when underlying object changes

Question

I would like to know what\'s the general behaviour of an iterator if the underlaying object changes during the iteration.

Using a simple mutable list, it seems obvi

Answer 1

There are three typical behaviours of changing an object while iterating over it:

• the new data will be returned
• the new data will be ignored
• old data will be skipped

In other words: the actual behaviour is undefined.

Changing objects while iterating over them was such a common problem that in Python 3 the types set and dict (and possibly others) were changed to immediately raise an error if additions or removals were detected during iteration.