Python set add() Method

set.add(element) adds one hashable element to a Python set and returns None. Because a set is mutable, the original set is changed in place. If the value is already present, the set remains unchanged.

Use update() when you want to add the individual elements from one or more iterables. Use add() when the value itself should become one set element.

my_set={'Alex','Ronald','John'}
my_set.add('Lewis')
print(my_set)

{'Ronald', 'John', 'Alex', 'Lewis'}

my_set={'a','b','c','x'}
my_set.add('c')
print(my_set)

{'a', 'x', 'c', 'b'}

unique_items = set()
for item in ['apple', 'banana', 'apple', 'orange']:
    unique_items.add(item)
print(unique_items)  # Output: {'banana', 'apple', 'orange'}

Comparison of add() vs update() in Python Sets

add(): Adds a single element to the set.

my_set = {1, 2}
my_set.add(3)
print(my_set)  # Output: {1, 2, 3}

update(): Adds multiple elements from an iterable (e.g., list, tuple).

my_set = {1, 2}
my_set.update([3, 4, 5])  # Adds multiple elements
print(my_set)  # Output: {1, 2, 3, 4, 5}

The add() method adds one element at a time, while update() can add several elements in one go.

Example 1: Adding Items Conditionally

my_set = {1, 2, 3}

# Add item only if it is greater than 10
item = 15
if item > 10:
    my_set.add(item)

print(my_set)

{1, 2, 3, 15}

Example 2: Using a Set to Track Unique Items

Sets are great for tracking unique items in a collection. For instance, let’s collect unique words from a list.

words = ["apple", "banana", "apple", "cherry", "banana", "date"]
unique_words = set()

for word in words:
    unique_words.add(word)

print(unique_words)

{'apple', 'banana', 'cherry', 'date'}

Example 3: Combining Sets with Union

We can use the union method or the | operator to combine sets.

set_a = {1, 2, 3}
set_b = {3, 4, 5}

# Combine sets
combined_set = set_a | set_b

print(combined_set)

{1, 2, 3, 4, 5}

Use Case: Tracking Unique User Visits

Sets can be used to track unique users visiting a website or application:

visits = ["user1", "user2", "user1", "user3", "user2"]
unique_visits = set(visits)

print("Unique Users:", unique_visits)

Unique Users: {'user1', 'user2', 'user3'}

Use Case: Removing Duplicates from a List

Sets can remove duplicates from a list. Use sorted() when the example also needs predictable output order.

numbers = [1, 2, 2, 3, 4, 4, 5]
unique_numbers = sorted(set(numbers))

print(unique_numbers)

[1, 2, 3, 4, 5]

Example 4: Adding Nested Sets (frozenset)

Python sets cannot directly include other sets because they are mutable. However, we can use frozenset, which is an immutable version of a set, for this purpose:

inner_set = frozenset({1, 2, 3})
outer_set = {4, 5}

# Adding frozenset
outer_set.add(inner_set)

print(outer_set)

{frozenset({1, 2, 3}), 4, 5}

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

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