Python `set.union()` Method

set.union() combines items from one set with items from one or more other iterables.

It is useful when you want:

all unique values from multiple collections
a new set as the result
to keep the original set unchanged

Quick example

set1 = {1, 2, 3}
set2 = {3, 4, 5}

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

union() returns a new set containing all unique items from both sets.

What `set.union()` does

set.union():

combines items from one set and one or more other iterables
keeps only unique values
returns a new set
does not change the original set

If the same value appears more than once, it will appear only once in the result.

If you are new to sets, see Python sets explained.

Basic syntax

set1.union(set2, set3, ...)

Key points:

You can pass one or more arguments.
Each argument can be a set or another iterable.
The method returns a new set with all unique items.

Example with two sets:

a = {1, 2}
b = {2, 3}

result = a.union(b)
print(result)

What it returns

set.union() always returns a new set.

That returned set contains every unique item found in:

the original set
all arguments passed to union()

Example:

a = {1, 2}
b = {2, 3}

result = a.union(b)

print(result)  # {1, 2, 3}
print(a)       # {1, 2}

Notice that a did not change.

Also remember:

sets are unordered
printed output order is not guaranteed

So you may see the same values in a different order.

Simple example

Here is a basic example with overlapping values:

fruits1 = {"apple", "banana", "orange"}
fruits2 = {"banana", "grape", "orange"}

result = fruits1.union(fruits2)

print(result)
print(fruits1)

Possible output:

{'banana', 'orange', 'apple', 'grape'}
{'banana', 'orange', 'apple'}

What this shows:

"banana" and "orange" appear in both sets
they appear only once in the result
fruits1 stays unchanged

Using `union()` with multiple sets

You can combine three or more sets in one call.

set1 = {1, 2}
set2 = {2, 3}
set3 = {3, 4}
set4 = {4, 5}

result = set1.union(set2, set3, set4)

print(result)

Possible output:

{1, 2, 3, 4, 5}

This is useful when collecting unique values from many sources.

Using `union()` with other iterables

union() does not only work with sets. It also accepts other iterables such as lists and tuples.

With a list

numbers = {1, 2, 3}
more_numbers = [3, 4, 5, 5]

result = numbers.union(more_numbers)
print(result)

Possible output:

{1, 2, 3, 4, 5}

Even though 5 appears twice in the list, it appears only once in the result.

With a tuple

letters = {"a", "b"}
more_letters = ("b", "c", "d")

result = letters.union(more_letters)
print(result)

Possible output:

{'a', 'b', 'c', 'd'}

With a string

A string is treated as an iterable of individual characters.

chars = {"a", "b"}
result = chars.union("cat")

print(result)

Possible output:

{'a', 'b', 'c', 't'}

This happens because "cat" is processed as:

"c"
"a"
"t"

`union()` vs `update()`

union() and update() are similar, but they behave differently.

`union()`

returns a new set
does not change the original set

a = {1, 2}
b = {2, 3}

result = a.union(b)

print(result)  # {1, 2, 3}
print(a)       # {1, 2}

`update()`

changes the existing set in place
does not create a separate result set

a = {1, 2}
b = {2, 3}

a.update(b)

print(a)  # {1, 2, 3}

Use union() when you want to keep the original set unchanged.

For the in-place version, see Python set.update() method.

Common beginner mistakes

Expecting `union()` to change the original set

This is very common:

a = {1, 2}
b = {2, 3}

a.union(b)
print(a)

Output:

{1, 2}

Why? Because union() returned a new set, but you did not save it.

Fix it like this:

a = {1, 2}
b = {2, 3}

a = a.union(b)
print(a)

Assuming the result keeps insertion order

Sets are unordered. Do not rely on the printed order.

a = {3, 1}
b = {2, 4}

print(a.union(b))

The values may not appear in the order you expect.

Passing a string and forgetting it becomes characters

words = {"dog"}
result = words.union("cat")
print(result)

This adds "c", "a", and "t" as separate items, not "cat" as one item.

If you want the whole string as one item, put it inside another iterable:

words = {"dog"}
result = words.union(["cat"])
print(result)

Trying to use unhashable values like lists inside a set

This causes an error:

my_set = {1, 2, [3, 4]}

Lists cannot be set items because they are mutable and unhashable.

But a list can be used as an argument to union():

my_set = {1, 2}
result = my_set.union([3, 4])
print(result)

Common causes of confusion

Beginners often run into problems because they:

call union() without saving the returned set
confuse union() with update()
expect duplicates to remain in the result
use a list as a set item instead of as an iterable argument

If you need a different set operation, compare this method with Python set.intersection() method and Python set.difference() method.

Useful checks while debugging:

print(my_set)
print(type(my_set))
print(result)
help(set.union)

FAQ

Does `set.union()` change the original set?

No. It returns a new set and leaves the original set unchanged.

Can `union()` combine more than two sets?

Yes. You can pass multiple sets or iterables in one call.

Can I use `union()` with a list or tuple?

Yes. union() accepts other iterables, not just sets.

Why is the output order different from my input?

Sets are unordered, so their printed order is not guaranteed.

What is the difference between `union()` and `|`?

They both combine sets. The | operator is shorter, but union() can be clearer for beginners.

Python set.union() Method

Python `set.union()` Method