Removing from a list while iterating over it

Question

The following code:

a = list(range(10))
remove = False
for b in a:
    if remove:
        a.remove(b)
    remove = not remove
print(a)

Outp

Answer 1

Of course it is not safe to modify an array as you are iterating over it. The spec says it is a bad idea and the behavior is undefined:

http://docs.python.org/tutorial/controlflow.html#for-statements

So, the next question is what exactly is happening under the hood here? If I had to guess, I would say that it is doing something like this:

for(int i=0; i

If you suppose that this is indeed what is going on, then it explains the observed behavior completely. When you remove an element at or before the current pointer, you shift the whole list by 1 to the left. The first time, you remove a 1 -- like usual -- but now the list shifts backwards. The next iteration instead of hitting a 2, you hit a 3. Then you remove a 4, and the list shifts backwards. Next iteration 7, and so on.