Question

I was wondering, if there is way in Python to modify collections without creating new ones. E.g.:

lst = [1, 2, 3, 4, 5, 6]
new_lst = [i for i in lst if i > 3]

Works just fine, but a new collection is created. Is there a reason, that Python collections lack a filter() method (or similar) that would modify the collection object in place?

Answer 1

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])