An unordered collection of elements without any duplicates.

A frozen set is immutable.

Resources

Python docs:

Create

Using keyword

# empty 
x = set()

# list
x = set(["abc", 123, True])

# Pass a single string inside a list.
x = set(["abbc"])
# { "abbc" }

# Warning - this will split a string.
x = set("abbc")
# { "a", "b", "c" }

Some examples with frozenset. Any methods covered later on this page which modify the frozen set will given an error.

x = frozenset()

x = frozenset(["abc", 123, True])

Using braces

Note that {} would create a dict, so you need the set keyword as above.

Set with values.

my_set = {VALUE, VALUE, VALUE}

e.g.

x = {"abc", "abc", 123, True, False, False}
# {"abc", 123, True, False}

x = {"abc"}
# {"abc"}

Membership testing

The values in a set are hashed, like dictionary keys, so it is very efficient to check if a value is in a set - O(1) time. While checking membership for a value in a set is O(n) time. So you should consider storing a list as a set if order doesn’t matter and lookup time needs to be fast.

"abc" in x
# True

"def" in x
# False

Update

Add element

my_set.add(VALUE)

Add elements

Pass an iterable, such an list or another set.

my_set |= ITERABLE

# or
my_set.update(ITERABLE)

e.g.

x = {"apple", "banana", "cherry"}
y = {"google", "microsoft", "apple"}

x.update(y)

Remove elements

From elements in set y from set x.

x -= y
# i.e.
x = x - y

Operations

Do a join to see what a set has in common or different from another.

Use bitwise operators or methods. The former only works with two sets while the latter works with iterables.

Intersection

OR

Returns a new set with elements common to the set and the other set.

The order does not matter.

x | y

# or
x.intersection(y)

Union

AND

Returns a new set with elements from the set and the other set.

x & y

# or
x.union(y)

Difference

NOT

Returns a new set with elements in the set that are not in the other set.

Note the order matters, as you remove elements of the 2nd from the 1st.

x - y

# or
x.difference(y)

Symmetric difference

XOR

Returns a new set with elements in either the set or the other set but not both.

x ^ y

# or
x.symmetric_difference(y)

Set comprehension

Similarly to list comprehensions, set comprehensions are also supported:

a = {x for x in 'abracadabra' if x not in 'abc'}
# {'r', 'd'}