Remove items from a list while iterating without using extra memory in Python

Question

My problem is simple: I have a long list of elements that I want to iterate through and check every element against a condition. Depending on the outcome of the condition I

Answer 1

If you perform a reverse iteration, you can remove elements on the fly without affecting the next indices you'll visit:

numbers = range(20)

# remove all numbers that are multiples of 3
l = len(numbers)
for i, n in enumerate(reversed(numbers)):
    if n % 3 == 0:
        del numbers[l - i - 1]

print numbers

The enumerate(reversed(numbers)) is just a stylistic choice. You may use a range if that's more legible to you:

l = len(numbers)
for i in range(l-1, -1, -1):
    n = numbers[i]
    if n % 3 == 0:
        del numbers[i]

If you need to travel the list in order, you can reverse it in place with .reverse() before and after the reversed iteration. This won't duplicate your list either.

Answer 2

How about this?

[x for x in list_of_g if not subtle_condition(x)]

its return the new list with exception from subtle_condition