Python List Methods

In the previous lesson you created lists and changed items by index. Real lists grow and shrink all the time, though: you add a task, finish another, reorder a playlist. Python list methods are the built-in tools for exactly that. In this lesson you'll learn every list method, see when to use each one, and learn the important difference between sort() and sorted().

What is a list method?

A method is a function that belongs to a particular type of value. You call it by writing the value (or variable), a dot, the method name and parentheses: my_list.method(). You already used string methods like "hi".upper() in Python string methods; list methods work the same way.

There's one big difference, though. Strings can't change, so string methods return a new string. Lists are mutable, so most list methods change the list itself (we say they work in place) and return None.

How do you add items to a list?

append(): add one item to the end

todo = ["Buy milk", "Call mom"]
todo.append("Book dentist")
print(todo)

['Buy milk', 'Call mom', 'Book dentist']

insert(): add one item at a specific position

insert(index, item) puts the item before the given index. Everything after it shifts one place to the right.

todo = ["Buy milk", "Call mom", "Book dentist"]
todo.insert(0, "Pay rent")       # make it the very first task
todo.insert(2, "Water plants")
print(todo)

['Pay rent', 'Buy milk', 'Water plants', 'Call mom', 'Book dentist']

extend(): add every item from another list

queue = ["Song A", "Song B"]
album = ["Track 1", "Track 2", "Track 3"]

queue.extend(album)
print(queue)

['Song A', 'Song B', 'Track 1', 'Track 2', 'Track 3']

a = [1, 2]
a.append([3, 4])
print(a)

b = [1, 2]
b.extend([3, 4])
print(b)

[1, 2, [3, 4]]
[1, 2, 3, 4]

How do you remove items from a list?

remove(): delete by value

remove(value) deletes the first matching item. If the value isn't there, you get an error.

cart = ["socks", "hat", "socks", "scarf"]
cart.remove("socks")
print(cart)

cart.remove("gloves")

['hat', 'socks', 'scarf']
ValueError: list.remove(x): x not in list

pop(): remove by position and get the item back

pop() removes the last item and returns it, so you can store it in a variable. Give it an index to pop a different item.

orders = ["latte", "espresso", "mocha", "tea"]

served = orders.pop()       # last item
print(served)

first = orders.pop(0)       # item at index 0
print(first)
print(orders)

tea
latte
['espresso', 'mocha']

clear(): empty the whole list

notifications = ["New like", "New follower"]
notifications.clear()
print(notifications)

[]

How do you search a list?

index(value) returns the position of the first match, and count(value) returns how many times a value appears.

votes = ["pizza", "tacos", "pizza", "sushi", "pizza"]

print(votes.index("tacos"))
print(votes.count("pizza"))
print(votes.count("burgers"))

1
3
0

Like remove(), index() raises a ValueError if the value is missing. Check with in first if you're not sure.

How do you sort a list in Python?

sort() puts the items in order, smallest to largest (or A to Z for strings), and changes the list in place.

prices = [19.99, 4.5, 12.0, 7.25]
prices.sort()
print(prices)

prices.sort(reverse=True)     # largest first
print(prices)

[4.5, 7.25, 12.0, 19.99]
[19.99, 12.0, 7.25, 4.5]

Sorting with key

The key argument tells Python what to compare. You pass the name of a function (without parentheses), and Python sorts by that function's result for each item.

names = ["bob", "Alice", "dave", "Carol"]

names.sort()
print(names)            # capital letters come before lowercase

names.sort(key=str.lower)
print(names)            # case-insensitive A to Z

names.sort(key=len)
print(names)            # shortest name first

['Alice', 'Carol', 'bob', 'dave']
['Alice', 'bob', 'Carol', 'dave']
['bob', 'dave', 'Alice', 'Carol']

You'll write your own small key functions later with lambda.

reverse(): flip the order

reverse() doesn't sort; it just flips the current order back to front.

steps = ["wake up", "shower", "coffee"]
steps.reverse()
print(steps)

['coffee', 'shower', 'wake up']

sort() vs sorted(): what's the difference?

sorted() is a built-in function, not a method. It leaves the original list alone and returns a new sorted list. It also works on any iterable, like a string or a range.

scores = [72, 95, 88]

ranked = sorted(scores, reverse=True)
print(ranked)    # new list
print(scores)    # original is unchanged

print(sorted("python"))

[95, 88, 72]
[72, 95, 88]
['h', 'n', 'o', 'p', 't', 'y']

copy(): make an independent copy

As you saw in the last lesson, b = a doesn't copy a list. copy() does:

original = ["red", "green"]
backup = original.copy()

backup.append("blue")
print(original)
print(backup)

['red', 'green']
['red', 'green', 'blue']

copy() makes a shallow copy: nested lists inside are still shared. You'll learn the details in shallow vs deep copy.

Python list methods reference table

Real-world example: managing a to-do list

tasks = ["Reply to emails", "Gym", "Groceries"]

tasks.append("Plan weekend trip")
tasks.insert(0, "Submit report")      # urgent, goes first

done = tasks.pop(0)
print(f"Finished: {done}")

tasks.remove("Gym")                  # skipping it today
tasks.sort()

print(f"{len(tasks)} tasks left:")
for number, task in enumerate(tasks, start=1):
    print(f"  {number}. {task}")

Finished: Submit report
3 tasks left:
  1. Groceries
  2. Plan weekend trip
  3. Reply to emails

Common mistakes with list methods

1. Saving the result of an in-place method

numbers = [3, 1, 2]
numbers = numbers.sort()
print(numbers)

None

sort() returns None, so this line threw your list away. Either call numbers.sort() on its own line, or write numbers = sorted(numbers). The same goes for append(), reverse() and friends.

2. Sorting a list with mixed types

mixed = [10, "apple", 3]
mixed.sort()

TypeError: '<' not supported between instances of 'str' and 'int'

Python can't decide whether "apple" is bigger or smaller than 10. Keep one type per list if you plan to sort it.

3. Removing something that isn't there

Both remove() and index() crash on missing values. Guard them with in:

guests = ["Ana", "Ben"]
if "Cleo" in guests:
    guests.remove("Cleo")
print(guests)

['Ana', 'Ben']

Practice exercise

playlist = ["Levitate", "Golden", "Blue Sky"]

playlist.append("Night Drive")
playlist.insert(0, "Opening")
playlist.remove("Golden")

now_playing = playlist.pop()
print(f"Now playing: {now_playing}")

print(sorted(playlist))
print(playlist)

Now playing: Night Drive
['Blue Sky', 'Levitate', 'Opening']
['Opening', 'Levitate', 'Blue Sky']

What's next?

You can now add, remove, search and sort list items. Next, learn Python list comprehension, a short and powerful way to build a new list from an existing one in a single line.