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.

From the Python 3 documentation A set is an unordered collection with no duplicate elements. Basic uses include membership testing and eliminating duplicate entries.

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()

Empty Sets When creating set, be sure to not use empty curly braces {} or you will get an empty dictionary instead. 
>>> s = {1, 2, 3}
>>> s = set([1, 2, 3])

>>> s = {}  # this will create a dictionary instead of a set
>>> type(s)
# <class 'dict'>

Unordered collections of unique elements

A set automatically remove all the duplicate values.

>>> s = {1, 2, 3, 2, 3, 4}
>>> s
# {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.

>>> s = {1, 2, 3}
>>> s.add(4)
>>> s
# {1, 2, 3, 4}

And with update(), multiple ones:

>>> s = {1, 2, 3}
>>> s.update([2, 3, 4, 5, 6])
>>> 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.

>>> s = {1, 2, 3}
>>> s.remove(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.

>>> s = {1, 2, 3}
>>> s.discard(3)
>>> s
# {1, 2}
>>> s.discard(3)

set union()

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

>>> s1 = {1, 2, 3}
>>> s2 = {3, 4, 5}
>>> s1.union(s2)  # or 's1 | s2'
# {1, 2, 3, 4, 5}

set intersection

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

>>> s1 = {1, 2, 3}
>>> s2 = {2, 3, 4}
>>> s3 = {3, 4, 5}
>>> s1.intersection(s2, s3)  # or 's1 & s2 & s3'
# {3}

set difference

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

>>> s1 = {1, 2, 3}
>>> s2 = {2, 3, 4}

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

>>> s2.difference(s1) # or 's2 - s1'
# {4}

set symetric_difference

symetric_difference or ^ will return all the elements that are not common between them.

>>> s1 = {1, 2, 3}
>>> s2 = {2, 3, 4}
>>> s1.symmetric_difference(s2)  # or 's1 ^ s2'
# {1, 4}

Python abs() built-in function Python aiter() built-in function Python all() built-in function Python any() built-in function Python ascii() built-in function Python bin() built-in function Python bool() built-in function Python breakpoint() built-in function Python bytearray() built-in function Python bytes() built-in function Python callable() built-in function Python chr() built-in function Python classmethod() built-in function Python compile() built-in function Python complex() built-in function Python delattr() built-in function Python dict() built-in function Python dir() built-in function Python divmod() built-in function Python enumerate() built-in function Python eval() built-in function Python exec() built-in function Python filter() built-in function Python float() built-in function Python format() built-in function Python frozenset() built-in function Python getattr() built-in function Python globals() built-in function Python hasattr() built-in function Python hash() built-in function Python help() built-in function Python hex() built-in function Python id() built-in function Python __import__() built-in function Python input() built-in function Python int() built-in function Python isinstance() built-in function Python issubclass() built-in function Python iter() built-in function Python len() built-in function Python list() built-in function Python locals() built-in function Python map() built-in function Python max() built-in function Python memoryview() built-in function Python min() built-in function Python next() built-in function Python object() built-in function Python oct() built-in function Python open() built-in function Python ord() built-in function Python pow() built-in function Python print() built-in function Python property() built-in function Python range() built-in function Python repr() built-in function Python reversed() built-in function Python round() built-in function Python set() built-in function Python setattr() built-in function Python slice() built-in function Python sorted() built-in function Python staticmethod() built-in function Python str() built-in function Python sum() built-in function Python super() built-in function Python tuple() built-in function Python type() built-in function Python vars() built-in function Python zip() built-in function