Lists — Mutation Methods

Lists are mutable

In Python, strings are "immutable", meaning once you create "Python", you cannot change the 'P' to a 'C'. You have to create a brand new string.

Lists, however, are mutable. You can easily add, remove, and change items inside a list after it has been created.

Changing an existing item

To change an item, you access it via its index and simply assign a new value to it with =.

python
inventory = ["Sword", "Shield", "Potion"]

# Swap the Shield for a Bow
inventory[1] = "Bow"

print(inventory)
# Output: ['Sword', 'Bow', 'Potion']

Adding items

.append()

The most common way to add an item is to slap it onto the very end of the list using the .append() method.

python
shopping_cart = ["Milk", "Eggs"]
shopping_cart.append("Bread")

print(shopping_cart)
# Output: ['Milk', 'Eggs', 'Bread']

.insert()

If you need to put an item in a specific spot, use .insert(). You provide the index, then the item.

python
# Insert "Apples" at index 1
shopping_cart.insert(1, "Apples")

print(shopping_cart)
# Output: ['Milk', 'Apples', 'Eggs', 'Bread']

Removing items

.remove()

If you know the value of the item you want to remove, use .remove(). Python scans the list from left to right and deletes the first match it finds.

python
colors = ["Red", "Green", "Blue", "Green"]
colors.remove("Green")

print(colors)
# Output: ['Red', 'Blue', 'Green']  (Notice only the first 'Green' was removed!)

If the item isn't in the list, Python will throw a ValueError.

.pop()

If you know the index of the item, use .pop(). It deletes the item at that spot.

If you leave the parentheses empty (), .pop() automatically removes and returns the very last item in the list.

python
deck = ["Ace", "King", "Queen"]

# Remove and grab the last item
drawn_card = deck.pop()
print(f"You drew the {drawn_card}!")
print(f"Remaining deck: {deck}")

What this lesson should give you

After this lesson, you should understand how to:

• overwrite an existing item using its index (my_list[0] = "New")
• add an item to the end of a list using .append()
• add an item at a specific location using .insert()
• delete an item by its value using .remove()
• delete an item by its index, or grab the last item, using .pop()