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Lists and tuples are useful for keeping together collections of objects, but they make no guarantees regarding the uniqueness of the elements. Uniqueness can be useful, for example, if you want to count the number of unique customers that have entered your store, given that the same customer can enter and exit your store multiple times on any given day.

A set is the class for collections of unique elements. Unlike tuple, set is mutable, meaning we can add and remove objects from it. However, this mutability property has additional significance that we’ll revisit later. In addition to mutability, set cannot be indexed. That is, the bracket indexing syntax seen before will not work with set.

To create a set, we use curly braces instead of brackets or parentheses to surround our comma-delimited collection of Python objects:

s = {1, 2, 3, 2}

This will output:

{1, 2, 3}

Note that we deliberately passed in two 2 objects, but Python was able to deduce that they were exactly same and omit the last one.

To add an element to our set, we just use the add method:

s = {1, 2, 3}

This will output:

{1, 2, 3, 'cat'}

Note that if we had tried to add an object that already existed in the set, it would remain unchanged. To remove an element from our set, we just use the remove method:

s = {1, 2, 3}

This will output:

{2, 3}

What happens if we attempt to remove an element that already does not exist?

s = {1, 2, 3}

As with list, Python will raise an error:

KeyError: 4

Before concluding, we should revisit the mutable property of list, tuple, set, and Python objects in general. One requirement of set is that it can only contain Python objects that are immutable. Numbers, strings, and tuple are immutable. list and set are not, unfortunately.

What happens if we try to create a set with a mutable object?

s = {[1, 2], 3}

Not surprisingly, Python will raise an error:

TypeError: unhashable type: 'list'

With that caveat explained, let’s practice what we have learned with sets:

  • Create a set with the following elements: 1, "dog", "cat", and 1. How many elements are in our set? (solution)

  • With your newly-created set, add the following elements: 3, "house", and "cat". Then remove the element 1. How many elements are in our set now? (solution)

If you feel comfortable with sets, feel free to move on to Lesson 9!