python - Why is list.remove only removing every second item?

Question

In my Python 2.7.2 IDLE interpreter:

>>> mylist = [1, 2, 3, 4, 5]
>>> for item in mylist:
        mylist.remove(item)

>>> mylist
[2, 4]

Why?

Answer 1

It's because when you iterate over a list, python keeps track of the index in the list. Consider the following code instead:

for i in range(len(mylist)):
    if i >= len(mylist):
       break
    item = mylist[i]
    mylist.remove(item)

If we track this (which is essentially what python is doing in your code), then we see that when we remove an item in the list, the number to the right shifts one position to the left to fill the void left when we removed the item. The right item is now at index i and so it will never actually get seen in the iteration because the next thing that happens is we increment i for the next iteration of the for loop.

---

Now for something a little clever. If instead we iterate over the list backward, we'll clear out the list:

for item in reversed(mylist):
    mylist.remove(item)

The reason here is that we're taking an item off the end of the list at each iteration of the for loop. Since we're always taking items off the end, nothing needs to shift (assuming uniqueness in the list -- If the list isn't unique, the result is the same, but the argument gets a bit more complicated).

Of course, If you're looking to remove all the items from a list, you can do that really easily:

del mylist[:]

or even with slice assignment:

mylist[:] = []

(I mention the latter because it can be useful to replace segments of a list with other items which don't even need to be the same length).