List Methods#

Python lists come with a rich set of methods for adding, removing, searching, and reordering elements.

Adding Elements#

append(x) adds x to the end — the most common way to grow a list.
insert(i, x) inserts x before index i.
extend(other) appends all elements of other to the list.

Try it live and add elements of your own:

>>> words = ["up", "down"]
>>> words.append("over")
>>> print(words)
['up', 'down', 'over']
>>> words.insert(1, "under")
>>> print(words)
['up', 'under', 'down', 'over']
>>> words.extend(["in", "out"])
>>> print(words)
['up', 'under', 'down', 'over', 'in', 'out']

Removing Elements#

remove(x) deletes the first occurrence of x; raises ValueError if not found.
pop(i) removes and returns the element at index i (default: last element).
del lst[i] removes the element at index i without returning it.

Try it live and watch how each call changes the list:

>>> nums = [3, 1, 4, 1, 5]
>>> nums.remove(1)     # removes the first 1
>>> print(nums)
[3, 4, 1, 5]
>>> last = nums.pop()  # removes and returns 5
>>> print(last, nums)
5 [3, 4, 1]

Searching#

index(x) returns the index of the first occurrence of x; raises ValueError if not found.
count(x) returns how many times x appears.

Try it live and search for other values:

>>> nums = [3, 1, 4, 1, 5]
>>> print(nums.index(4))   # position of the first 4
2
>>> print(nums.count(1))   # how many 1s
2

Sorting and Reversing#

sort() sorts the list in place (ascending by default).
reverse() reverses the list in place.

Try it live and see how sort and reverse change the list:

>>> nums = [3, 1, 4, 1, 5, 9]
>>> nums.sort()
>>> print(nums)
[1, 1, 3, 4, 5, 9]
>>> nums.reverse()
>>> print(nums)
[9, 5, 4, 3, 1, 1]

List methods mutate the list in place. Run this, then add your own append, insert, pop, and sort calls to see the effect:

>>> words = ["up", "down"]
>>> words.append("over")
>>> words.insert(1, "under")
>>> words.extend(["in", "out"])
>>> print(words)
['up', 'under', 'down', 'over', 'in', 'out']
>>> nums = [3, 1, 4, 1, 5]
>>> print("first index of 1:", nums.index(1))
first index of 1: 1
>>> print("count of 1:", nums.count(1))
count of 1: 2
>>> nums.sort()
>>> print("sorted:", nums)
sorted: [1, 1, 3, 4, 5]

Built-in Functions#

These built-in functions work on any iterable and do not modify the original list:

sorted(lst) returns a new sorted list.
reversed(lst) returns an iterator over the list in reverse order.
min(lst), max(lst) return the smallest and largest element.
sum(lst) returns the sum of all elements (numbers only).

Try it live and confirm the original list is unchanged:

>>> nums = [3, 1, 4, 1, 5]
>>> print(sorted(nums))    # new list; nums unchanged
[1, 1, 3, 4, 5]
>>> print(min(nums), max(nums), sum(nums))
1 5 14

Concatenation and Repetition#

The + operator concatenates two lists into a new one; * repeats a list. Try it live and edit the operands:

>>> a = [1, 2]
>>> b = [3, 4]
>>> print(a + b)
[1, 2, 3, 4]
>>> print(a * 3)
[1, 2, 1, 2, 1, 2]

Note that + is different from extend: + produces a new list and leaves both operands unchanged, while extend mutates the list it is called on.

List Methods

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