Working with Lists

Unlike strings, lists are mutable. This means we can change the values of the elements inside the list, add new elements to the list, remove items from the list, or change the order of the elements.

Changing One Element

To update a single item in a list, use the syntax:

list_name[index] = new_value

list_name[index] identifies the element in the list that we want to change. The = operator assigns new_value to that index location.

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grocery_list = ["granola bars", "veggies", "TP", "other healthy stuff"]
print(grocery_list)

grocery_list[0] = "pears"        # Set the value at index 0 to be "pears"
grocery_list[-1] = "chocolate"   # Set the value at index -1 to be "chocolate"
print(grocery_list)

['granola bars', 'veggies', 'TP', 'other healthy stuff']
['pears', 'veggies', 'TP', 'chocolate']

Removing Elements

To remove one or more elements from a list, we can use the del function. Here, del stands for delete.

The general syntax is:

del list_name[index]       # Removes the element at index.
del list_name[start:end]   # Removes the elements from the 'start' index up to but
                           # NOT including the 'end' index.

Note that del does NOT use parentheses ().

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num_strings = ['one', 'two', 'three']
del num_strings[1]      # Removes the element at index 1.
print(num_strings)

letter_list = ['a', 'b', 'c', 'x', 'y', 'z']
del letter_list[1:5]    # Removes each element from index 1 - 4.
print(letter_list)

['one', 'three']
['a', 'z']

The Slice Operator

Used on the right hand side of the = operator, a slice returns a smaller portion of a list.

new_list = old_list[start : end]

Used on the left hand side of the = operator, a slice will either insert, replace, or remove elements.

The general syntax is:

list_name[start : end] = [new values...]

Note that the new values must be inside brackets [] and separated from each other by commas.

Inserting New Elements

Make the start and end values the same. This inserts all of the new values into the list, starting at the chosen index. Existing elements get pushed to later positions in the list.

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my_list = [3, 6, 9, 12]
print(my_list)

my_list[2:2] = ['a', 'b', 'cde'] # Inserts 3 new elements starting at index 2.
print(my_list)

[3, 6, 9, 12]
[3, 6, 'a', 'b', 'cde', 9, 12]

Replacing Elements

Make the start and end values different. The elements from index start to end (NOT including end) get replaced with the new values.

Note that the number of old and new values can be different.

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my_list = [10, 20, 30, 40, 50, 60, 70, 80]
print(my_list)

my_list[1:6] = [-1, -3] # Replaces the elements from indexes 1 - 5 with two new values.
print(my_list)

[10, 20, 30, 40, 50, 60, 70, 80]
[10, -1, -3, 70, 80]

Check Your Understanding

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a_list = [4, 2, 8, 6, 5, 4]
a_list[2] = True
print(a_list)

b_list = ['barber', 'baby', 'bubbles', 'bumblebee']
b_list[start_index : end_index] = ['B', 'b']