Python Set update() Method

Q: What is the difference between add() and update() in a Python set?

`add()` inserts one element, while `update()` inserts every item of one or more iterables. Passing a string to `update()` adds its characters.

Q: Does set.update() create a new set?

No. It modifies the set in place and returns `None`. Use [union()](/python-set-union/) for a new set.

By Yogesh Sharma (LinkedIn) · Every code example is run and checked on Python 3.13 (how we test) · Last updated 21 Oct 2026

Short answer

set.update(*iterables) adds every item of each iterable into the set in place and returns None. Duplicates are ignored.

Use update() when you already have a collection of values to add: new tags from a list, extra cities from another set, or the characters of a string. It is the multi-item version of add().

It changes the set you call it on. If you want to keep the original and get a new set instead, use union(). Remember that strings are iterables, so update("abc") adds the single characters, which surprises many beginners.

Quick facts
Syntax	`my_set.update(*iterables)` or `my_set \|= other_set`
Parameters	Zero or more iterables
Returns	`None` (the set is changed in place)
Time complexity	O(total number of items added)

How set.update() works

Python loops through each iterable and adds every item, exactly like calling add() repeatedly. Items already present are skipped.

Python

tags = {"python"}
tags.update(["sets", "python"], ("loops",), {"dicts"})
print(sorted(tags))

update() with strings and other iterables

Because every iterable is unpacked, a string contributes its characters, and a dictionary contributes its keys. To add a whole word, wrap it in a list or use add().

Python

letters = set()
letters.update("cab")
print(sorted(letters))
words = set()
words.update(["cab"])
print(words)

update() vs add() vs union() vs |=

Tool	Adds	Changes the set?	Returns
`add(x)`	One element, kept whole	Yes	`None`
`update(it)`	Every item of the iterable	Yes	`None`
`union(it)`	Every item, in a copy	No	New set
`s \|= other`	Items of another set	Yes	The same set
`s.update(a, b)`	Items of several iterables	Yes	`None`

Examples you can run

Press Run, then edit the code and run it again.

Merge new tags into a post

The set grows only by tags it did not have yet.

Merge new tags into a post

post_tags = {"python", "sets"}
new_tags = ["sets", "tips", "tutorial"]
post_tags.update(new_tags)
print(sorted(post_tags))
print(len(post_tags))

Expected output['python', 'sets', 'tips', 'tutorial']
4

Characters of a string

Collect the unique letters of a word.

Characters of a string

seen = set()
seen.update("banana")
print(sorted(seen))

Expected output['a', 'b', 'n']

Common mistakes

Passing a single number

✗ Wrong

s = {1, 2}
s.update(3)

TypeError: 'int' object is not iterable

✓ Right

s = {1, 2}
s.add(3)
print(sorted(s))

update() needs an iterable. Use add(3) for one element or update([3]).

Using the return value

✗ Wrong

s = {1}
s = s.update([2, 3])
print(s)

None

✓ Right

s = {1}
s.update([2, 3])
print(sorted(s))

update() returns None. Call it on its own line.

Python Set update() Method: questions people ask

What is the difference between add() and update() in a Python set?

add() inserts one element, while update() inserts every item of one or more iterables. Passing a string to update() adds its characters.

Does set.update() create a new set?

No. It modifies the set in place and returns None. Use union() for a new set.

Can update() take a list or a dictionary?

Yes, any iterable works. A list adds its items and a dictionary adds its keys.

Keep learning

Step-by-step lessons:
Python Sets Python Set Operations

More in set methods:
Python Set add() Method Python Set remove() Method Python Set discard() Method Python Set pop() Method Python Set union() Method Python Set intersection() Method Python Set difference() Method Python Set symmetric_difference() Method See all →