Python Sets

Python comes equipped with several built-in data types to help us organize our data. These structures include lists, dictionaries, tuples and sets.

Read Python Sets: What, Why and How for a more in-deep reference.

Initializing a set

There are two ways to create sets: using curly braces {} and the built-in function set()

# Create set using curly braces or set() function
s = {1, 2, 3}  # Using curly braces
s = set([1, 2, 3])  # Using set() constructor

# Warning: empty {} creates a dictionary, not a set
s = {}  # this will create a dictionary instead of a set
type(s)  # Returns <class 'dict'>

<class 'dict'>

Unordered collections of unique elements

A set automatically removes all the duplicate values.

# Sets automatically remove duplicates
s = {1, 2, 3, 2, 3, 4}  # Duplicates are removed
s  # Returns {1, 2, 3, 4}

{1, 2, 3, 4}

And as an unordered data type, they can’t be indexed.

s = {1, 2, 3}
s[0]

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'set' object does not support indexing

set add and update

Using the add() method we can add a single element to the set.

# add() method: add a single element to the set
s = {1, 2, 3}
s.add(4)  # Add element 4
s

{1, 2, 3, 4}

And with update(), multiple ones:

# update() method: add multiple elements from an iterable
s = {1, 2, 3}
s.update([2, 3, 4, 5, 6])  # Add multiple elements (duplicates ignored)
s

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

set remove and discard

Both methods will remove an element from the set, but remove() will raise a key error if the value doesn’t exist.

# remove() method: remove element, raises KeyError if not found
s = {1, 2, 3}
s.remove(3)  # Remove element 3
s

{1, 2}

s.remove(3)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 3

discard() won’t raise any errors.

# discard() method: remove element, no error if not found
s = {1, 2, 3}
s.discard(3)  # Remove element 3 (safe, no error if missing)
s

{1, 2}

s.discard(3)

set union

union() or | will create a new set with all the elements from the sets provided.

# union(): combine all elements from multiple sets (no duplicates)
s1 = {1, 2, 3}
s2 = {3, 4, 5}
s1.union(s2)  # or 's1 | s2' - returns {1, 2, 3, 4, 5}

{1, 2, 3, 4, 5}

set intersection

intersection() or & will return a set with only the elements that are common to all of them.

# intersection(): return elements common to all sets
s1 = {1, 2, 3}
s2 = {2, 3, 4}
s3 = {3, 4, 5}
s1.intersection(s2, s3)  # or 's1 & s2 & s3' - returns {3}

{3}

set difference

difference() or - will return only the elements that are unique to the first set (invoked set).

# difference(): return elements in first set but not in others
s1 = {1, 2, 3}
s2 = {2, 3, 4}

s1.difference(s2)  # or 's1 - s2' - returns {1}

{1}

s2.difference(s1) # or 's2 - s1'

{4}

set symmetric_difference

symmetric_difference() or ^ will return all the elements that are not common between them.

# symmetric_difference(): return elements in either set, but not both
s1 = {1, 2, 3}
s2 = {2, 3, 4}
s1.symmetric_difference(s2)  # or 's1 ^ s2' - returns {1, 4}

{1, 4}

Relevant links

• Python Data Types Blog Post
• Python Sets: What, Why and How
• Python Dictionaries
• Python Lists and Tuples