Python Sets

Sets In Python

• A set is an unordered collection of data type that is mutable
• Each element in a set is unique
• It has an optimized method for checking whether a specific element is contained in set

• Sets have methods like union() & intersection() which are used to perform operations with other sets

Creating set using set() function

A set can be created by passing a list of values to the built-in set() function or by declaring a list of values inside curly braces {}

>>> set_from_list = set([4, 9, 5, 8])
>>> print(" Set : ",set_from_list)
 Set :  {8, 9, 6, 0}

The order of elements have been changed in the result because set is an unordered collection of data unlike list and tuple

Creating set using curly braces {}

>>> curly_braces_set = {3, 4.5}
>>> print(" Set : ", curly_braces_set)
 Set :  {3, 4.5}

Set With Mixed Data Types

A set can have any number of elements but the datatype for the elements should be hashable like integer, float, string etc

A list can have a set as an element but not vice versa

>>> mixed_set = {'string in a set', 50.05, 1000}
Set :  {1000, 50.05, 'string in a set'}

Creating Empty Set

For creating empty set, set( ) method has to be used without any argument

>>> empty_set = set()
>>> print("Set : ", empty_set)
Set :  set() 

Empty set can be created using curly braces { } because empty curly braces make empty dictionary in python

Set Removes Duplicate Values

Set also remove duplicate values

>>> set1 = set([1, 1, 3, 11, 6, 11, 1, 2])
>>> print("Set :", set1)
Set : {1, 2, 3, 6, 11}

Above example shows a set set1 is declared by passing a list which contains duplicate values

The resultant set doesn't contain any duplicate element

Manipulating Sets In Python

Adding values to a Set

To add more values to a set, add( ) function is used

>>> print("Initial set :", set1)
Initial set : {1, 3, 5}
>>> set1.add(7)
>>> print("Updated set :",set1)
Updated set : {1, 3, 5, 7} 

• To add multiple values, update() method can be used

Removing values from a Set

Methods remove() or discard() can be used to remove elements from a set

remove( ) method

>>> set1 = set([1,3,5])
>>> print("Initial set :", set1)
Initial set : {1, 3, 5}
>>> set1.remove(3)
>>> print("Updated set :",set1)
Updated set : {1, 5} 

discard() method

discard() method is also used to remove value from a set

The difference between discard() and remove() method is that –

• if remove() is used and element does not exist in set, an error(KeyError) is raised
• if discard() is used, then no error is raised

>>> print("Initial set :", set1)
Initial set : {1, 3, 5}
>>>
>>> set1.discard(7)
>>> set1.discard(3)
>>> print("Updated set", set1)
Updated set {1, 5} 

Updating a Set

A set can be updated by using update( ) method which allows add multiple values to a set at one time

>>> set1 = set([1, 3, 5])
>>> print("Initial set :", set1)
Initial set : {1, 3, 5}
>>>
>>> set1.update(["One", "Three", "Five"])
>>> print("Updated set :", set1)
Updated set : {1, 3, 5, 'Three', 'One', 'Five'} 

A set can also be updated with another set

>>> set1 = set([1, 3, 5.2])
>>> print("Initial set :",set1)
Initial set : {1, 3, 5.2}
>>> set2 = set([1, "Three", "Five"])
>>>
>>> set1.update(["Seven", "9", 3], set2)
>>> print("Updated set :", set1)
Updated set : {1, 3, 5.2, '9', 'Three', 'Seven', 'Five'} 

• Set gets updated with the second set along with the list of values, and the duplicate values are not added keeping the values unique

Python Set Operations

Following operations can be performed on sets

• Union
• Intersection
• Difference
• Symmetric difference

Set Union Operation

Union of two sets is a resultant set which contains all elements of both sets

Above figure shows union of two sets X and Y which is the set of all elements from both sets

Python set provides the method union() to perform union operation

>>> set1 = set([1, 3, 5, 7])
>>> set2 = set([2, 4, 5, 6, 7])
>>> print("set1:", set1)
set1: {1, 3, 5, 7}
>>> print("set2:", set2)
set2: {2, 4, 5, 6, 7}
>>>
>>> set3 = set1.union(set2)
>>> print("Union:", set3)
Union: {1, 2, 3, 4, 5, 6, 7} 

Set Intersection Operation

Intersection of two sets results in the set of common elements among the sets

In above figure, the intersection of sets X and Y is highlighted

>>> set1 = set([1, 3, 5, 7])
>>> set2 = set([2, 4, 5, 6, 7])
>>> print("set1:", set1)
set1: {1, 3, 5, 7}
>>> print("set2:", set2)
set2: {2, 4, 5, 6, 7}
>>>
>>> set3 = set1.intersection(set2)
>>> set3
{5, 7}

Set Difference Operation

Set difference is the set of elements which are present in one set but not in the other

So difference of set X from Y (X-Y) is the set of elements in set X which are not in set Y

Similarly (Y-X) is the set of elements that are in set Y but not in set X

In python, set difference operation can be performed using difference() method

>>> set1 = set([1, 3, 5, 7])
>>> set2 = set([2, 4, 5, 6, 7])
>>> print("set1:", set1)
set1: {1, 3, 5, 7}
>>> print("set2:", set2)
set2: {2, 4, 5, 6, 7}
>>>
>>> set1.difference(set2)
{1, 3}
>>> set2.difference(set1)
{2, 4, 6}