Why does Python skip elements when I modify a list while iterating over it?

Question

I'm currently developing a program in python and I just noticed that something was wrong with the foreach loop in the language, or maybe the list structure. I'll just give a generic example of my problem to simplify, since I get the same erroneous behavior on both my program and my generic example:

x = [1,2,2,2,2]

for i in x:
    x.remove(i)

print x        

Well, the problem here is simple, I though that this code was supposed to remove all elements from a list. Well, the problem is that after it's execution, I always get 2 remaining elements in the list.

What am I doing wrong? Thanks for all the help in advance.

Edit: I don't want to empty a list, this is just an example...

Answer 1

This is a well-documented behaviour in Python, that you aren't supposed to modify the list being iterated through. Try this instead:

for i in x[:]:
    x.remove(i)

The [:] returns a "slice" of x, which happens to contain all its elements, and is thus effectively a copy of x.