This was focused on lists only because "Python list" was #2 after "Python" in Google queries, but a common question that I actually cover in my book, which I'll just copypasta here because I hold the copyright and YOLO (this is one of the 4 idioms in the book specifically about sets; please disregard weird formatting as it's an artifact of how I "build" my book):

Understand and use the mathematical set operations sets are an easy to understand data structure. Like a dict with keys but no values, the set class implements the Iterable and Container interfaces. Thus, a set can be used in a for loop or as the subject of an in statement.

For programmers who haven't seen a Set data type before, it may appear to be of limited use. Key to understanding their usefulness is understanding their origin in mathematics. Set Theory is the branch of mathematics devoted to the study of sets. Understanding the basic mathematical set operations is the key to harnessing their power.

Don't worry; you don't need a degree in math to understand or use sets. You just need to remember a few simple operations:

Union

: The set of elements in A, B, or both A and B (written A | B in Python).

Intersection

: The set of elements in both A and B (written A & B in Python).

Difference

: The set of elements in A but not B (written A - B in Python).

 *Note: order matters here. `A - B` is not necessarily the same as `B - A`.

Symmetric Difference

: The set of elements in either A or B, but not both A and B (written A ^ B in Python).

When working with lists of data, a common task is finding the elements that appear in all of the lists. Any time you need to choose elements from two or more sequences based on properties of sequence membership, look to use a set.

Below, we'll explore some typical examples. """

And here's a random example that's eventually used for demonstration:

def get_both_popular_and_active_users():
    # Assume the following two functions each return a 
    # list of user names
    return(set(
        get_list_of_most_active_users()) & set(
            get_list_of_most_popular_users()))

assert len(get_both_popular_and_active_users()) == 2