9.3. 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.

9.3.1. 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.

Example

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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)

Console Output

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

9.3.2. Removing Elements (Part 1)

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 ().

Example

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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)

Console Output

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

There are multiple ways to add or remove elements from a list. We will examine these below and on the List Methods page.

9.3.3. 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.

9.3.3.1. 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.

Example

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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)

Console Output

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

9.3.3.2. 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.

Example

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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)

Console Output

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

9.3.3.3. Removing Elements (Part 2)

The syntax is the same as above, but use the empty list instead of specific values.

Example

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

my_list[1:6] = [] # Removes the elements from indexes 1 - 5.
print(my_list)

Console Output

[10, 20, 30, 40, 50, 60, 70, 80]
[10, 70, 80]

9.3.4. Try It!

Example

In the Replit below:

  1. Change a single value inside conjunctions.

  2. Use del to remove a single value from conjunctions.

  3. Use del to remove multiple values from conjunctions.

  4. Practice using the slice operator to insert, remove, or replace one or more elements from conjunctions.

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# Follow the listed steps (pun intended) to change one or more elements:
conjunctions = ['and', 'nor', 'yet', 'for', 'or', 'but', 'so']

repl.it

9.3.5. Check Your Understanding

Question

What is printed by the following code?

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

  1. [4, 2, True, 8, 6, 5, 4]

  2. [4, True, 2, 8, 6, 5, 4]

  3. [4, 2, True, 6, 5, 4]

  4. [4, True, 8, 6, 5, 4]

Question

In the following code, we want to add 'B' and 'b' to the beginning of b_list without losing any of the other items.

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

What values should we use for start_index and end_index?

  1. [0 : 0]

  2. [0 : 1]

  3. [1 : 1]

  4. [0 : ]

  5. [ : 1]