Remove Items

remove, pop, del and clear, and which one to reach for.

remove takes a value

Python

names = ["Ada", "Grace", "Katherine"]
names.remove("Grace")
print(names)

Output

['Ada', 'Katherine']

It removes the first match only, and raises ValueError when there is none:

Python

names = ["Ada", "Grace", "Ada"]
names.remove("Ada")
print(names)

try:
    names.remove("Alan")
except ValueError as problem:
    print("ValueError:", problem)

Output

['Grace', 'Ada']
ValueError: list.remove(x): x not in list

pop takes a position and hands the item back

Python

names = ["Ada", "Grace", "Katherine"]
last = names.pop()
first = names.pop(0)
print(last, first)
print(names)

Output

Katherine Ada
['Grace']

With no argument pop() takes the last item, which makes a list a perfectly good stack. pop() is the one removal method that returns something useful.

del removes by position

Python

names = ["Ada", "Grace", "Katherine"]
del names[1]
print(names)

Output

['Ada', 'Katherine']

del also takes a slice, which removes a whole stretch:

Python

numbers = [0, 1, 2, 3, 4, 5]
del numbers[1:4]
print(numbers)

Output

[0, 4, 5]

And del on the name itself removes the name:

Python

numbers = [1, 2]
del numbers
print("the name is gone")

Output

the name is gone

clear empties the list

Python

names = ["Ada", "Grace"]
names.clear()
print(names, len(names))

Output

[] 0

clear() keeps the same list object, so other names pointing at it see it emptied. names = [] would give you a new empty list and leave theirs alone.

Python

a = [1, 2]
b = a
a.clear()
print(b)

Output

[]

Removing several

Removing while looping goes wrong, as the previous chapter showed. Build the list you want instead:

Python

numbers = [1, 2, 3, 4, 5, 6]
numbers = [n for n in numbers if n % 2 != 0]
print(numbers)

Output

[1, 3, 5]

To empty in place while other names watch, assign to a full slice:

Python

a = [1, 2, 3, 4]
b = a
a[:] = [n for n in a if n > 2]
print(b)

Output

[3, 4]

Which to use

You know	Use
the value	`remove(value)`
the position, and want the item	`pop(index)`
the position, and do not want it	`del items[index]`
you want it all gone	`clear()`

Test yourself

2 questions

What does names.remove("Ada") do when Ada appears twice?

Show the answer

Removes the first one only — remove drops the first match. It raises ValueError when there is none at all.

Which removal method hands the item back to you?

Show the answer

pop() — That is what makes append and pop together a perfectly good stack.

Next chapter

Loop Lists

Walk a list properly, with the index when you need it and without when you do not.