Mutating Lists

Lists are mutable.

In programming, 'mutate' means ‘change’. Mutable means we can change the lists.

We can:

• assign a new value to a list index
• add and remove items from a list
• change the order of items

Assigning new values

We can change any element of a list by assigning at an index.

For example, to change the second item in the list above to the "Nigeria", we can assign to index 1, like this:

# The old list
countries = ["Kenya", "Ghana", "Ethiopia", "Zimbabwe"]
# Change the second item
countries[1] = 'Nigeria'
# The new list
print(countries) # ['Kenya', 'Nigeria', 'Ethiopia', 'Zimbabwe']

Adding an item to the end of a list

To add a new element to the end of the list, you use .append()

countries.append("Cameroon")
print(countries) # ['Kenya', 'Nigeria', 'Ethiopia', 'Zimbabwe', 'Cameroon']

Removing an item

To remove an item from a list, you use pop(). With no arguments, pop removes the last element.

countries = ['Kenya', 'Nigeria', 'Ethiopia', 'Zimbabwe', 'Cameroon']
print(countries) # ['Kenya', 'Nigeria', 'Ethiopia', 'Zimbabwe', 'Cameroon']
countries.pop() # removes the last element, "Cameroon"
print(countries) # ['Kenya','Nigeria', 'Ethiopia', 'Zimbabwe']

Or, you can specify an index to remove, and pop() will remove the element at that index.

print(countries) # ['Kenya','Nigeria', 'Ethiopia', 'Zimbabwe']
countries.pop(1) # Removes the element at index 1, "Nigeria"
print(countries) # ['Kenya', 'Ethiopia', 'Zimbabwe']

Check your understanding

Try this quiz to check how well you understand what list operations will do.

Practice

my_list = [10, 20, 30, 40, 50]
print(my_list)

# Assign the first list item the value 5
my_list[0] = 5
print(my_list)

# Assign the last list item the value 'dog'
my_list[4] = 'dog'
print(my_list)

# Remove the second item in the list
my_list.pop(1)
print(my_list)

# Add another item to the end of the list with value False
my_list.append(False)
print(my_list)

print("The number of items in the list is", len(my_list))