Add Set Items

Python sets are unordered collections of unique elements. Once a set is created you can grow it at any time using two dedicated methods: add() for inserting a single element and update() for inserting many elements at once from any iterable. This chapter covers both methods in depth, explains when to use each one, and highlights common gotchas.

If you are new to sets, read the Python Sets chapter first. To learn how to remove items, see Remove Set Items.

The add() Method

add() inserts one element into the set. If the element already exists, the set is not changed — no error is raised and no duplicate is created.

Syntax

set.add(element)

• element — the value to insert. It must be hashable (strings, numbers, tuples of hashable values, etc.). Lists and dicts cannot be added.
• Return value: None. The set is modified in place.

Adding a new element

Add a string to a set

Adding a duplicate is safe

Calling add() with an element that already exists does nothing — the set stays the same size:

Add a duplicate element to a set

fruits = {'apple', 'banana', 'cherry'}
fruits.add('apple')   # 'apple' is already present
print(fruits)
# {'apple', 'banana', 'cherry'}   (unchanged)

This is one of the most useful properties of sets: you never have to check whether an element exists before adding it.

add() only accepts one argument

add() takes exactly one argument. To insert several elements at once, use update() instead (see the next section).

# This raises TypeError: set.add() takes exactly one argument (2 given)
# fruits.add('kiwi', 'mango')

add() requires a hashable element

Only hashable types can be stored in a set. Lists and dictionaries are not hashable, so passing them to add() raises a TypeError:

s = {1, 2, 3}
# s.add([4, 5])  # TypeError: unhashable type: 'list'
# Use a tuple instead:
s.add((4, 5))
print(s)
# {1, 2, 3, (4, 5)}

The update() Method

update() adds all elements from one or more iterables to the set in a single call. It accepts any iterable — another set, a list, a tuple, or even a string.

Syntax

set.update(iterable1, iterable2, ...)

• One or more iterables may be passed.
• Duplicates between the existing set and the new elements are silently ignored.
• Return value: None. The set is modified in place.

Adding elements from a list

Update a set from a list

fruits = {'apple', 'banana'}
fruits.update(['cherry', 'date', 'elderberry'])
print(fruits)
# {'apple', 'banana', 'cherry', 'date', 'elderberry'}

Adding elements from another set

Merge two sets with update()

set1 = {1, 2, 3}
set2 = {3, 4, 5}
set1.update(set2)
print(set1)
# {1, 2, 3, 4, 5}

Note that update() modifies set1 in place, whereas union() returns a new set without touching either original.

Adding from multiple iterables at once

You can pass several iterables to a single update() call:

Update a set from multiple iterables

numbers = {1, 2, 3}
numbers.update([4, 5], {5, 6, 7})
print(numbers)
# {1, 2, 3, 4, 5, 6, 7}

Passing a string to update()

Because a string is iterable, update() breaks it into individual characters. This is usually not what you want, so be careful:

Update with a string — adds individual characters

s = {'hello'}
s.update('abc')
print(s)
# {'hello', 'a', 'b', 'c'}   — each character becomes a separate element

To add a whole string as one element, use add() instead.

add() vs update() — Quick Comparison

Practical Examples

Building a tag set dynamically

A common use case is collecting unique tags or identifiers as a program runs:

Collect unique tags with a set

tags = set()

# add() for one tag at a time
tags.add('python')
tags.add('tutorial')
tags.add('python')   # duplicate — ignored

print(tags)
# {'python', 'tutorial'}

# update() to add several tags at once
tags.update(['coding', 'beginner', 'tutorial'])
print(tags)
# {'python', 'tutorial', 'coding', 'beginner'}

Removing duplicates while accumulating data

Because add() silently ignores duplicates, you can use a set as an accumulator to deduplicate items from multiple sources:

Deduplicate items from two lists using a set

batch1 = [101, 102, 103, 102]
batch2 = [103, 104, 105]

seen = set()
seen.update(batch1)
seen.update(batch2)

print(sorted(seen))
# [101, 102, 103, 104, 105]

Key Points

• Sets store only unique elements; add() and update() never create duplicates.
• Sets are unordered — the insertion order is not preserved and may vary between runs.
• add() inserts exactly one hashable element; update() inserts all elements from any number of iterables.
• Both methods return None and mutate the set in place.
• To combine two sets into a new set without modifying either, use union() (see Set Methods).

Practice