Delete many elements of list (python)

Question

I have a list L.

I can delete element i by doing:

del L[i]

But what if I have a set of non contiguous indexes to delete?

Answer 1

Eine Minuten bitte, Ich hap eine kleine Problemo avec diese Religione. -- Eddie Izzard (doing his impression of Martin Luther)

Deleting by reverse-iterating over a list to preserve the iterator is a common solution to this problem. But another solution is to change this into a different problem. Instead of deleting items from the list using some criteria (in your case, the index exists in a list of indexes to be deleted), create a new list that leaves out the offending items.

L[:] = [ item for i,item in enumerate(L) if i not in I ]

For that matter, where did you come up with the indexes in I in the first place? You could combine the logic of getting the indexes to be removed and building the new list. Assuming this is a list of objects and you only want to keep those that pass an isValid test:

L[:] = [ item for item in L if item.isValid() ]

This is much more straightforward than:

I = set()
for i in range(len(L)):
    if not L[i].isValid():
        I.add(i)

for i in sorted(I, reverse=True):
    del L[i]

For the most part, I turn any question about "how to delete from a list the items that I don't want" into "how to create a new list containing just the items I want".

EDITED: changed "L = ..." to "L[:] = ..." per Alex Martelli's answer to this question.