Lists, Tuples, and Sets

Learning outcomes

Create lists and access their elements
Common list operations
Create tuples and access their elements
Understand the commonality between lists and strings
Learn about sets, their purpose, and methods of working with them

In this lesson, we will learn about the list, tuple, and set data types that allow us to store multiple items in a single variable. These data types are built into Python, and there is one more data type called a dictionary that we will learn in the next lesson.

Lists

Lists store a collection of items that can be changed, of different data types, have duplicated, and are ordered.

Initializing Lists

Lists are created using square brackets [] and items are separated by commas.

fruits = ["apple", "bannana", "peach"]
classes = ["Math", "History", "Math"]
numbers = [1, 2, "Three"]
print(fruits)
print(classes)
print(numbers)

Output:

['apple', 'bannana', 'peach']
['Math', 'History', 'Math']
[1, 2, 'Three']

As seen above, we can create a list using brackets [] and place our items inside that are separated by a comma. We can also create an empty list with empty brackets or use the list() method.

empty_list_1 = []
empty_list_2 = list()
print(empty_list_1)
print(empty_list_2)

Output:

[]
[]

Indexing and Slicing Lists

Just like we could with strings, we can index and slice strings to get their values.

Indexing:

nums = [1, 2, 3, 4, 5]
print(nums[0])
print(nums[-1])

Output:

1
5

Slicing:

nums = [1, 2, 3, 4, 5]
print(nums[:3])
print(nums[2:5])

Output:

[1, 2, 3]
[3, 4, 5]

slicing: courses[0:2]

We can find the index of an item in our list by using the list.index(item). If the item doesn't exist in the list, we will receive a ValueError.

nums = [1, 2, 3, 4, 5]
print(nums.index(3))

Output:

List Methods

Similar to strings, we can also use len() method to find the length of a string.

nums = [1, 2, 3, 4, 5]
print(len(nums))

Output:

We can add values to our list by using the list.append(item) method.

nums = [1, 2, 3, 4, 5]
nums.append(6)
print(nums)
nums.append(10)
print(nums)

Output:

[1, 2, 3, 4, 5, 6]
[1, 2, 3, 4, 5, 6, 10]

We can also use the list.insert(index, value) method where we can place an item in a specific position of a list.

nums = [1, 2, 3, 4, 5]
nums.insert(3, "Hello")
print(nums)
nums.insert(0, "World")
print(nums)

Output:

[1, 2, 3, 'Hello', 4, 5]
['World', 1, 2, 3, 'Hello', 4, 5]

We can also remove items by using the list.remove(item) method. To remove the last element on the list, you can call list.pop(), which returns the last item and also modifies the list.

nums = [1, 2, 3, 4, 5]
nums.remove(1)
print(nums)
print(nums.pop())
print(nums)

Output:

[2, 3, 4, 5]
5
[2, 3, 4]

We can merge two lists by using the list.extend(list) method.

nums = [1, 2, 3, 4, 5]
nums_2 = [6, 7 ,8 , 9, 10]
nums.extend(nums_2)
print(nums)
print(nums_2)

Output:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[6, 7, 8, 9, 10]

To check if an item is contained in the list, we can use the item in list conditional.

nums = [1, 2, 3, 4, 5]
print(1 in nums)
animals = ['bear', 'ram', 'cat']
print('cat' in animals)
print('rat' in animals)

Output:

True
True
False

Sorting and Reversing

To reverse a list, the list.reverse() method can be called on it.

nums = [1, 2, 3, 4, 5]
nums.reverse()
print(nums)

Output:

[5, 4, 3, 2, 1]

We can sort a list by using the list.sort() method. It will numerically sort numbers and sort strings by alphabetical order.

nums = [3, 5, 1, 2, 4]
nums.sort()
print(nums)
animals = ['bear', 'ram', 'cat']
animals.sort()
print(animals)

Output:

[1, 2, 3, 4, 5]
['bear', 'cat', 'ram']

We can also specify our sort method to sort in reverse order with list.sort(reverse=True).

nums = [3, 5, 1, 2, 4]
nums.sort(reverse=True)
print(nums)
animals = ['bear', 'ram', 'cat']
animals.sort(reverse=True)
print(animals)

Output:

[5, 4, 3, 2, 1]
['ram', 'cat', 'bear']

We can also use the sorted(list) function, which will copy our list so that we don't modify our original list.

nums = [3, 5, 1, 2, 4]
sorted_nums = sorted(nums)
print(nums)
print(sorted_nums)

Output:

[3, 5, 1, 2, 4]
[1, 2, 3, 4, 5]

We can also use the min(list) and max(list) to find the minimum and maximum of our list.

nums = [1, 2, 3, 4, 5]
print(min(nums))
print(max(nums))
animals = ['bear', 'ram', 'cat']
print(min(animals))
print(max(animals))

Output:

1
5
bear
ram

If we have a list of floats or integers, we can use the sum(list) method to find the summation of all items in the list.

nums = [1, 2, 3, 4, 5]
nums_and_floats = [1, 2.5, 6.01, -19.025]
print(sum(nums))
print(sum(nums_and_floats))

Output:

15
-9.514999999999999

Lists and Strings

We can convert lists to strings and strings to lists using the separator.join(list) and str.split(connector), where the separator is a string that will be placed between each item in the list and the connector is the string that will be removed from the string. For an integer or floating-point items in an array, we will need to pass each value as an integer using list comprehensions, which will be taught in the advanced python course.

nums = [1, 2, 3, 4, 5]
print("".join(str(item) for item in nums))
animals = ["cat", "bear", "ram"]
print("".join(animals))
print("MOO".join(animals))

Output:

12345
catbearram
catMOObearMOOram

nums_str = "1, 2, 3, 4, 5"
animals_str_1 = "catbearram"
animals_str_2 = "catMOObearMOOram"
print(nums_str.split(" ")) # converts to list items
print(animals_str_1.split(" "))
print(animals_str_2.split(" "))

Output:

['1,', '2,', '3,', '4,', '5']
['catbearram']
['catMOObearMOOram']

Tuples

As we have seen, we can add and remove elements from a list, sort the list, and extend it with another list. All of these operations change the original list meaning that the list data type is "mutable". The tuple data type is similar to the list, but it is "immutable" as we cannot modify it once it is made. We can create an empty tuple with parenthesis () or the tuple() method.

empty_tuple_1 = ()
empty_tuple_2 = tuple()
print(empty_tuple_1)
print(empty_tuple_2)

Output:

()
()

Tuples are used when you have a collection of items that will not change and you would like to have fast access (covered in the Data Structures and Algorithms Course) to knowing if elements are in your tuple with the item in tuple conditional.

animals = ['cat', 'racoon', 'chipmunk']
print('racoon' in animals)
print('ram' in animals)

Output:

True
False

Sets

Sets are also similar to lists except that they do not care about order and contain no duplicates. They have fast access to items the way tuples do. To initialize a set, we use the set() method. If we use empty curly braces {}, we will initialize a dictionary (we will learn about this in the next lesson).

empty_set = set()
print(empty_set)

Output:

set()

Since sets do not contain an order, the ordering when we print the elements of a set will change each time. As previously mentioned, sets have fast access to elements inside them with the item in set syntax.

nums = {1, 2, 3, 3}
print(nums) # will vary
print("2" in nums)
print(2 in nums)

Output:

{1, 2, 3}
False
True

We can add items to a set using the set.add(item) method as opposed to the lists list.append(item) method. Otherwise, we can remove and pop elements just like we could with lists.

nums = {1, 2, 3, 3}
nums.add(5)
nums.add(3)
print(nums)
print(nums.pop())
print(nums)
nums.remove(2)
print(nums)

Output:

{1, 2, 3, 5}
1
{2, 3, 5}
{3, 5}

Sets provide the capability for operations that are used in set theory. We can use the set.intersection(set) method to find values in boths sets.

nums_1 = {1, 2, 3, 4}
nums_2 = {3, 4, 5, 6}
print(nums_1.intersection(nums_2))

Output:

{3, 4}

We can use the set.difference(set) to find elements that are only present in the first set.

nums_1 = {1, 2, 3, 4}
nums_2 = {3, 4, 5, 6}
print(nums_1.difference(nums_2))

Output:

{1, 2}

We can use the set.union(set) to find all elements that are present in either of the sets.

nums_1 = {1, 2, 3, 4}
nums_2 = {3, 4, 5, 6}
print(nums_1.union(nums_2))

Output:

{1, 2, 3, 4, 5, 6}

Exercises

Lists

Create a list of 5 fruits.
Create another list of 3 elements that is a sublist of the first variable. Use negative indexing.
Add the second variable to the end of the first variable and store it as another variable.
Now do the same operation but add the second variable after the first element of the first variable.
Loop through the list and print each item on a new line.
Sort the list alphabetically.

Tuples

Create a tuple with three values: an integer, a string, and a Boolean.
Deconstruct that tuple into three variables.

Sets

Create a set from the list made in the Lists part of this exercise.
Create a set that contains 3 of the same fruits as the set in (1), and 2 different ones (or just 2 random strings).
Perform and print the results of three operations: intersection, difference, and union.

Knowledge Check

How to create lists? (Don't forget about string methods)
What is negative indexing?
How to add elements to a list?
How to remove elements from a list?
What are some of the ways to sort a list?
How to create empty tuples?
What's the difference between a list, a tuple, and a set?

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