Working With a Python Set: Methods, Operators, and Tips

Defining a set

You can define a set by using the Python set() function. To use it, you’ll need to provide the elements you want to include in the set in the form of an iterable object such as a string.

There are two types of sets in Python: set and frozenset. The difference between the two is that a set is mutable, which means that you can change its contents, while a frozenset is not. Because a frozenset can’t be changed once created, you can only use operations that don’t modify set contents on a frozenset.

Note: in Python, strings, lists and tuples are all iterable.

yourSet = set(['bmw', 'toyota', 'volvo'])
print(yourSet)

Alternatively, you can define a set explicitly using curly braces {}. When you define a set in this manner, each element you add is treated as a distinct element of the set:

yourSet = {'bmw', 'toyota', 'volvo', 'chrysler'}

Note: Python sets can be modified at any time after creation.

Creating an empty set

You can define an empty set using the set function:

yourSet = set()
print(yourSet)

You cannot, however, define an empty set using curly brackets ({}) since Python interprets a set of empty curly brackets as a dictionary:

yourSet = {}
print(type(yourSet))

Set elements

Python sets can include objects of different types, as long as they cannot be modified. That means you can’t include data types such as dictionaries:

yourSet = {122, 'toyota', 2.66, (3, 2, 1), True}
print(yourSet)

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Getting information about the set

In Python, you can retrieve the length of a set as follows:

yourSet = {'bmw', 'toyota', 'volvo'}
print(len(yourSet))

It’s also easy to determine if an element is a member of a set or not:

yourSet = {'bmw', 'volvo', 'toyota'}
print('bmw' in yourSet)
print('chrysler' in yourSet)

Methods and operators for sets

Below is a list of available Python set operations. If you have two sets (one called set1 and another called set2), you can perform joint operations on both sets by either using an operator or calling a method function. Any exceptions are noted in the following table.

Modifying sets

If you need to modify individual sets, you can also do so by performing set operations. Python allows you to:

• Add an element to a set: yourSet.add(<element>)
• Remove an element from a set: yourSet.remove(<element>)
• Remove an element from a set if present; otherwise, nothing occurs: yourSet.discard(<element>)
• Remove a random element from a set: yourSet.pop(<element>)
• Clear a set by removing all elements: yourSet.clear()