Why is it not safe to modify sequence being iterated on?

Question

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

Answer 1

When you modify a collection that you are iterating over the iterator can behave unexpectedly, for example missing out items or returning the same item twice.

This code loops indefinitely when I run it:

>>> a = [ 'foo', 'bar', 'baz' ]
>>> for x in a:
...    if x == 'bar': a.insert(0, 'oops')

This is because the iterator uses the index to keep track of where it is in the list. Adding an item at the beginning of the list results in the item 'bar' being returned again instead of the iterator advancing to the next item.