List copy

When you want to empty a list in Python, you probably do lst = []. In fact, you just create a new empty list and assign it to lst, while all others owners of the same list still have the same content:

In : lst = [1, 2, 3]
In : lst2 = lst
In : lst = []
In : lst2
Out: [1, 2, 3]

While this may seem pretty obvious, the correct solution wasn't straightforward until lst.clear() was introduced in Python 3.3.

Before that, you should do del lst[:] or lst[:] = []. It works since slice syntax allows you to modify part of the list, and that part is the whole list in case of [:].