How to modify python collections by filtering in-place?

The other answers are correct; if you want all the names pointing to the old list to point to the new list you can use slice assignment.

However, that's not truly in-place creation; the new list is first created elsewhere. The link in Sven's answer is good.

The reason there isn't one that truly operates in-place is that while making a new list like that is O(n), each truly in-place item removal would be O(k) by itself, where k is the length of the list from the removal point on. The only way to avoid that with Python lists is to use some temporary storage, which is what you're doing by using slice assignment.

An example of an in-place O(n) filter on a collections.deque, in case you don't need to store your data in a list:

from collections import deque

def dequefilter(deck, condition):
    for _ in xrange(len(deck)):
        item = deck.popleft()
        if condition(item):
            deck.append(item)

deck = deque((1, 2, 3, 4, 5))
dequefilter(deck, lambda x: x > 2) # or operator.gt(2)
print deck
# deque([3, 4, 5])

Correcting @larsmans original solution, you could either do

    i = 0
    while i < len(lst):
        if lst[i] <= 3:
            del lst[i]
        else:
            i += 1

or

    i = len(lst)
    while i > 0:
        if lst[i-1] <= 3:
            del lst[i-1]
        i -= 1

The reason is the "index shift" which happens with the del. If I del at a certain index, that index needs to be re-examined because it now holds a different value.

If you want to do this in place, just use

lst[:] = [i for i in lst if i > 3]

This won't be faster or save any memory, but it changes the object in place, if this is the semantics you need.