Python Sets (Set)

In this article, you will learn everything about Python sets; how to create them, add or remove elements from them, and all the operations that can be performed on sets in Python.

What is a set in Python?

Sets are an unordered collection of items. Each element is unique (no duplicates), and must be immutable (cannot be changed).

However, the set itself is mutable. We can add or remove items from it.

Sets can be used to perform mathematical set operations, such as union, intersection, and symmetric difference, etc.

How to create a set?

By placing all items (elements) within curly braces {} and separating them with commas or using built-in functions to create a set set().

It can have any number of items, and they can have different types (integers, floats, tuples, strings, etc.). However, sets cannot have mutable elements (such aslist, set ordictionary) as its elements.

#Integer set
my_set = {1,　2,　3}
print(my_set)
#Mixed data type set
my_set = {1.0, "Hello", (1,　2,　3)}
print(my_set)

Try the following example as well.

#No duplicate set
# Output: {1,　2,　3,　4}
my_set = {1,2,3,4,3,2}
print(my_set)
#Set cannot contain mutable items
#Here [3,4] is a mutable list
#If you uncomment line12line,
#This will cause an error.
#TypeError: unhashable type: 'list'
#my_set = {1,　2, [3,　4])
#We can generate a set from a list
# Output: {1,　2,　3}
my_set = set([1,2,3,2])
print(my_set)

Creating an empty set is a bit special.

Empty parentheses {} create an empty dictionary in Python. To create an empty set with no elements, we use the set() function with no parameters.

#Initialize using {}
a = {}
#Check the data type of a
#Output: <class 'dict'>
print(type(a))
#Initialize using set()
a = set()
#Check the data type of a
#Output: <class 'set'>
print(type(a))

How to modify a set in Python?

Sets are mutable. However, since they are unordered, the index has no meaning.

We cannot use indexing or slicing to access or change the elements of a set. Sets do not support it.

We can use the add() method to add a single element, and use the update() method to add multiple elements. The update() method can taketuple,stringor other set as its parameter. In all cases, avoid repetition.

# Initialize my_set
my_set = {1,3}
print(my_set)
#If you uncomment line9line,
#You will get an error
#TypeError: 'set' object does not support indexing
#my_set[0]
Add an element
# Output: {1,　2,　3}
my_set.add(2)
print(my_set)
Add multiple elements
# Output: {1,　2,　3,　4}
my_set.update([2,3,4])
print(my_set)
Add list and set
# Output: {1,　2,　3,　4,　5,　6,　8}
my_set.update([4,5], {1,6,8)
print(my_set)

When running the program, the output is:

{1,　3}
{1,　2,　3}
{1,　2,　3,　4}
{1,　2,　3,　4,　5,　6,　8}

How to remove elements from a set?

The discard() and remove() methods can be used to delete specific items from the set.

The only difference between the two is that if discard() is used and the item does not exist in the set, it remains unchanged. However, remove() will raise an error in this case.

The following example will illustrate this.

# Initialize my_set
my_set = {1,　3,　4,　5,　6}
print(my_set)
# Discard an element
# Output: {1,　3,　5,　6}
my_set.discard(4)
print(my_set)
Remove an element
# Output: {1,　3,　5}
my_set.remove(6)
print(my_set)
# Discard an element
# The item is not present in my_set
# Output: {1,　3,　5}
my_set.discard(2)
print(my_set)
# Remove an element
# The item is not present in my_set
# If there is no comment #my_set.remove(2),
# An error will be raised.
# Output: KeyError:　2
#my_set.remove(2)

Similarly, we can use the pop() method to delete and return an item.

Sets are unordered, so the item popped cannot be determined. This is completely arbitrary.

We can also use the clear() method to delete all items in the set.

# Initialize my_set
# Output: Unique element set
my_set = set("HelloWorld")
print(my_set)
# Pop an element
# Output: Random element
print(my_set.pop())
# Pop an arbitrary element
# Output: Random element
my_set.pop()
print(my_set)
# Clear my_set
# Output:　set()
my_set.clear()
print(my_set)

Python Set Operations

Sets can be used to perform mathematical set operations, such as union, intersection, difference, and symmetric difference. We can achieve this through operators or methods.....

Let's consider the following two groups used for the following operations.

>>> A = {1,　2,　3,　4,　5}
>>> B = {4,　5,　6,　7,　8}

Set union

The union of A and B is a set of all elements from these two sets.

The union is executed using the | operator. It can also be completed using the union() method.

# Initialize A and B
A = {1,　2,　3,　4,　5}
B = {4,　5,　6,　7,　8}
# Use the | operator
# Output: {1,　2,　3,　4,　5,　6,　7,　8}
print(A | B)

Try the following examples in the Python shell.

# Use the union function
>>> A.union(B)
{1,　2,　3,　4,　5,　6,　7,　8}
# Use the union function on B
>>> B.union(A)
{1,　2,　3,　4,　5,　6,　7,　8}

set intersection

>>> BandB'sIntersection is a set of elements that are common to both sets.

Intersection is executed using the & operator. The intersection() method can also achieve the same operation.

# Initialize A and B
A = {1,　2,　3,　4,　5}
B = {4,　5,　6,　7,　8}
# Use the & operator
# Output:　{4,　5}
print(A & B)

Try the following examples in the Python shell.

# Use the intersection function on A
>>> A.intersection(B)
{4,　5}
# Use the intersection function on B
>>> B.intersection(A)
{4,　5}

Set difference

The difference between A and B (A-B) is a set of elements that are only in A but not in B. Similarly, B-A is a set of elements that are in B but not in A.

Difference is using -operator is executed. Using the difference() method can achieve the same operation.

# Initialize A and B
A = {1,　2,　3,　4,　5}
B = {4,　5,　6,　7,　8}
# Use the difference function on A　-　operator
# Output:　{1,　2,　3}
print(A　-　B)

Try the following examples in the Python shell.

# Use the difference function on A
>>> A.difference(B)
{1,　2,　3}
# Use on B-运算符符
The operator ^ executes the symmetric difference. The same operation can be performed using the symmetric_difference() method.　-　>>> B
{8,　6,　7}
A
# Using the difference function on B
{8,　6,　7}

>>> B.difference(A)

Set Symmetric Difference

The symmetric difference of A and B is a set of elements that are in either A or B, but not in both.

# Initialize A and B
A = {1,　2,　3,　4,　5}
B = {4,　5,　6,　7,　8}
# Using the ^ operator
# Output: {1,　2,　3,　6,　7,　8}
print(A ^ B)

Try the following examples in the Python shell.

# Using the symmetric_difference function on A
>>> A.symmetric_difference(B)
{1,　2,　3,　6,　7,　8}
# Using the symmetric_difference function on B
>>> B.symmetric_difference(A)
{1,　2,　3,　6,　7,　8}

Different Python set methods

There are many set methods in Python, some of which have been used above. This is a list of all the methods available for the set object.

Other Set Operations

Set Membership Testing

We can use the in keyword to test if an item exists in the set.

# Initialize my_set
my_set = set("apple")
# Check if 'a' exists
# Output: True
print('a' in my_set)
# Check if 'p' exists
# Output: False
print('p' not in my_set)

Traverse the set

Using a for loop, we can iterate over each item in the set.

>>> for letter in set("apple"):
...        print(letter)
...　　　　
a
p
e
l

Set and Built-in Functions

Built-in functions such as all(), any(), enumerate(), len(), max(), min(), sort(), sum(), and others are commonly used with set to perform different tasks.

Python Frozenset

Frozenset is a new class with set features, but once allocated, its elements cannot be changed. Tuples are immutable lists, while frozensets are immutable sets.

Mutable sets are not hashable and cannot be used as dictionary keys. On the other hand, frozenset is hashable and can be used as a dictionary key.

You can use the functionFrozenset()CreationFrozensets.

This data type supports methods such as copy(), difference(), intersection(), isdisjoint(), issubset(), issuperset(), symmetric_difference(), and union(). Since it is immutable, there are no methods to add or delete elements.

# Initialize A and B
A = frozenset([1,　2,　3,　4])
B = frozenset([3,　4,　5,　6])

Try these examples in the Python shell.

>>> A.isdisjoint(B)
False
>>> A.difference(B)
frozenset({1,　2)
>>> A | B
frozenset({1,　2,　3,　4,　5,　6)
>>> A.add(3)
...
AttributeError: 'frozenset' object has no attribute 'add'