Sets

A set is an unordered collection of unique items. It can be defined in two different manners:

Taking all the elements from an iterable such as a string, list or tuple using the built-in set() constructor:

python

    > > > set('hello')
    > > > {'e', 'h', 'l', 'o'}
    > > > set([1, 2, 2, 3])
    > > > {1, 2, 3}

Placing a comma-separated list of elements between curly braces.

python

    > > > {2, 2, 3, 5, 7, 7, 7, 11}
    > > > {2, 3, 5, 7, 11}

As a sidenote, the empty set must be built with set() since Python interprets {} as an empty dictionary.

Operators and methods

Sets support the already familiar x in set, len(set), and for x in set. Since sets are unordered, there is no way to record the position of an element and thus indexing or slicing is not possible.

The set type is mutable in such a way that its contents can be changed using methods like add() and remove().

The Python translation of fundamental operations is pretty undisguised. Let's consider the following sets,

python

s1 = {1, 2, 3, 4}
s2 = {3, 4, 5, 6}

and we'll take a look at some of the most usual. For a complete outlook refer to the Python documentation. In all cases we have an operator and a method available which return a new set and are in essence equivalent.

Union: Items appearing in either

python

    s1 | s2
    s1.union(s2)

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

Intersection: Items appearing in both

python

    s1 & s2
    s1.intersection(s2)

# {3, 4}

Difference: Items appearing in s1 but not in s2

python

    s1 - s2
    s1.difference(s2)

# {1, 2}

Symmetric difference: Items appearing in only one set

python

    s1 ^ s2
    s1.symmetric_difference(s2)

# {1, 2, 5, 6}

Sets ​

Operators and methods ​

Sets

Operators and methods