Change Items

Assign to a position

A list is mutable, so you can assign straight to an index:

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

['Ada', 'Alan', 'Katherine']

The list is changed in place. Every name pointing at it sees the new value, which is the behaviour that makes lists useful and occasionally surprising.

a = [1, 2, 3]
b = a
a[0] = 99
print(b)

[99, 2, 3]

The position has to exist

Assigning past the end is an error, not an extension:

names = ["Ada"]
try:
    names[5] = "Grace"
except IndexError as problem:
    print("IndexError:", problem)

IndexError: list assignment index out of range

To make the list longer, use append() or insert(), which the next chapter covers.

Assign to a slice

Assigning to a slice replaces that whole stretch. The replacement does not have to be the same length:

numbers = [1, 2, 3, 4, 5]
numbers[1:3] = ["a", "b", "c"]
print(numbers)

[1, 'a', 'b', 'c', 4, 5]

Two items came out and three went in, so the list grew.

Deleting with a slice

An empty replacement removes the stretch:

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

[1, 4, 5]

Replacing everything

[:] is the whole list, so assigning to it swaps the contents while keeping the same object:

a = [1, 2, 3]
b = a
a[:] = [7, 8]
print(a)
print(b)

[7, 8]
[7, 8]

Compare that with a = [7, 8], which points a at a new list and leaves b looking at the old one:

a = [1, 2, 3]
b = a
a = [7, 8]
print(a)
print(b)

[7, 8]
[1, 2, 3]

Changing while looping

Removing items from a list while looping over it skips things, because the positions shift underneath you:

numbers = [1, 2, 3, 4]
for n in numbers:
    if n % 2 == 0:
        numbers.remove(n)
print(numbers)

[1, 3]

That one happens to look right. Change the data and it stops being right:

numbers = [2, 4, 6]
for n in numbers:
    if n % 2 == 0:
        numbers.remove(n)
print(numbers)

[4]

Build a new list instead, which the comprehension chapter makes short work of:

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

[]