Sets in Python

A Set is an unordered collection data type that is iterable, mutable and has no duplicate elements. Python’s set class represents the mathematical notion of a set. The major advantage of using a set, as opposed to a list, is that it has a highly optimized method for checking whether a specific element is contained in the set. This is based on a data structure known as a hash table.

Frozen Sets Frozen sets are immutable objects that only support methods and operators that produce a result without affecting the frozen set or sets to which they are applied.





# Python program to demonstrate differences
# between normal and frozen set
# Same as {"a", "b","c"}
normal_set = set(["a", "b","c"])
# Adding an element to normal set is fine
print("Normal Set")
# A frozen set
frozen_set = frozenset(["e", "f", "g"])
print("Frozen Set")
# Uncommenting below line would cause error as
# we are trying to add element to a frozen set
# frozen_set.add("h")



Normal Set
set(['a', 'c', 'b', 'd'])
Frozen Set
frozenset(['e', 'g', 'f'])


 Methods for Sets

1. add(x) Method: Adds the item x to set if it is not already present in the set.

people = {"Jay", "Idrish", "Archil"}

-> This will add Daxit in people set.

2. union(s) Method: Returns a union of two set.Using the ‘|’ operator between 2 sets is the same as writing set1.union(set2)

people = {"Jay", "Idrish", "Archil"}
vampires = {"Karan", "Arjun"}
population = people.union(vampires)


population = people|vampires

-> Set population set will have components of both people and vampire

3. intersect(s) Method: Returns an intersection of two sets.The ‘&’ operator comes can also be used in this case.

victims = people.intersection(vampires)

-> Set victims will contain the common element of people and vampire

4. difference(s) Method: Returns a set containing all the elements of invoking set but not of the second set. We can use ‘-‘ operator here.

safe = people.difference(vampires)


safe = people – vampires

-> Set safe  will have all the elements that are in people but not vampire
5. clear() Method: Empties the whole set.


-> Clears victim set

However there are two major pitfalls in Python sets:

  1. The set doesn’t maintain elements in any particular order.
  2. Only instances of immutable types can be added to a Python set.

Operators for Sets

Sets and frozen sets support the following operators:

key in s       # containment check

key not in s   # non-containment check

s1 == s2       # s1 is equivalent to s2

s1 != s2       # s1 is not equivalent to s2

s1 <= s2    # s1is subset of s2 s1 < s2     # s1 is proper subset of s2 s1 >= s2    # s1is superset of s2

s1 > s2     # s1 is proper superset of s2

s1 | s2        # the union of s1 and s2

s1 & s2 # the intersection of s1 and s2

s1 – s2        # the set of elements in s1 but not s2

s1 ˆ s2        # the set of elements in precisely one of s1 or s2

Code Snippet to illustrate all Set operations in Python





# Python program to demonstrate working# of
# Set in Python
# Creating two sets
set1 = set()
set2 = set()
# Adding elements to set1
for i in range(1, 6):
# Adding elements to set2
for i in range(3, 8):
print("Set1 = ", set1)
print("Set2 = ", set2)
# Union of set1 and set2
set3 = set1 | set2# set1.union(set2)
print("Union of Set1 & Set2: Set3 = ", set3)
# Intersection of set1 and set2
set4 = set1 & set2# set1.intersection(set2)
print("Intersection of Set1 & Set2: Set4 = ", set4)
# Checking relation between set3 and set4
if set3 > set4: # set3.issuperset(set4)
    print("Set3 is superset of Set4")
elif set3 < set4: # set3.issubset(set4)
    print("Set3 is subset of Set4")
else : # set3 == set4
    print("Set3 is same as Set4")
# displaying relation between set4 and set3
if set4 < set3: # set4.issubset(set3)
    print("Set4 is subset of Set3")
# difference between set3 and set4
set5 = set3 - set4
print("Elements in Set3 and not in Set4: Set5 = ", set5)
# checkv if set4 and set5 are disjoint sets
if set4.isdisjoint(set5):
    print("Set4 and Set5 have nothing in common\n")
# Removing all the values of set5
print("After applying clear on sets Set5: ")
print("Set5 = ", set5)



('Set1 = ', set([1, 2, 3, 4, 5]))
('Set2 = ', set([3, 4, 5, 6, 7]))

('Union of Set1 & Set2: Set3 = ', set([1, 2, 3, 4, 5, 6, 7]))
('Intersection of Set1 & Set2: Set4 = ', set([3, 4, 5]))

Set3 is superset of Set4
Set4 is subset of Set3

('Elements in Set3 and not in Set4: Set5 = ', set([1, 2, 6, 7]))

Set4 and Set5 have nothing in common

After applying clear on sets Set5: 
('Set5 = ', set([]))

Recent articles on Python Set.

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