10.5 Removing Elements

remove()

Removes the first occurrence of a given value (not a position) — raises ValueError if the value isn’t present.

>>> l = [1, 2, 3, 2, 1]
>>> l.remove(2)     # removes the first '2' only
>>> l
[1, 3, 2, 1]

pop()

Removes and returns the element at a given index (default: the last one) — the only removal method that gives the removed value back.

>>> popped = l.pop()     # removes and returns the last element
>>> popped, l
(1, [1, 3, 2])
>>> l.pop(0)              # remove and return by specific index
1

clear()

Empties the list completely, in place — the list object still exists (same id()), just with zero elements.

>>> l2 = [1, 2, 3]
>>> l2.clear()
>>> l2
[]

del

A statement (not a method) that removes an item by index — or, combined with a slice, an entire range at once.

>>> l3 = [1, 2, 3, 4, 5]
>>> del l3[0]
>>> l3
[2, 3, 4, 5]
>>> del l3[1:3]     # delete by slice
>>> l3
[2, 5]

Quick Interview Answer

“Four tools cover removal, and the interview-relevant distinction is what each one identifies and returns: remove(value) finds by value and raises ValueError if absent; pop(index) finds by position, defaults to the last element, and is the only one that hands back what it removed; clear() empties the list entirely while keeping the same object identity; and del is a statement, not a method, that can remove a single index or — combined with a slice — an entire range in one step.”

Common Mistakes

• Calling l.remove(x) expecting it to remove by index — it removes by value; l.remove(2) removes the value 2, not the element at index 2.
• Not catching ValueError when the value passed to remove() might not be present — check if x in l first, or wrap in try/except.
• Forgetting pop() with no argument removes the last element, not the first — pop(0) is needed to remove and return the first one, and that call is O(n) (see 10.13 Performance).