Python Union

Let’s Talk About Union in Detail

We already saw a quick example of union in the last article. Now let’s go deeper.

Union is one of the most common set operations. It combines two sets into one. All items from both sets. No duplicates. That’s it.

Think of it like two guest lists for a party. One list has our friends. The other list has our cousin’s friends. The union is everyone who is invited. If someone is on both lists, they only appear once.

The Union Operator | in Python

The simplest way to take the union of two sets is with the | operator.

set1 = {1, 2, 3}
set2 = {3, 4, 5}
result = set1 | set2
print(result)

{1, 2, 3, 4, 5}

Notice that 3 appears in both sets, but in the result it appears only once. That’s because sets don’t allow duplicates.

We can chain multiple unions.

set1 = {1, 2}
set2 = {2, 3}
set3 = {3, 4}
result = set1 | set2 | set3
print(result)

{1, 2, 3, 4}

It works from left to right. So it’s like (set1 | set2) | set3. But the result is the same.

The union() Method in Python

We can also use the .union() method. It does the same thing.

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

{1, 2, 3, 4, 5}

The union() method can take multiple arguments.

set1 = {1, 2}
set2 = {2, 3}
set3 = {3, 4}
result = set1.union(set2, set3)
print(result)

{1, 2, 3, 4}

So we can pass as many sets as we want.

Union with Other Iterables in Python

Here’s something nice about the union() method. It can take any iterable, not just sets. We can pass a list, a tuple, or even a string.

set1 = {1, 2, 3}
list1 = [3, 4, 5]
result = set1.union(list1)
print(result)

{1, 2, 3, 4, 5}

The list is converted to a set internally, and then the union is performed.

But the | operator only works with sets. We cannot use it with a list.

set1 = {1, 2, 3}
list1 = [3, 4, 5]
result = set1 | list1   # Error!

TypeError: unsupported operand type(s) for |: 'set' and 'list'

So if we have a list and want to union it with a set, we use the .union() method.

In-Place Union with update() in Python

The | operator and .union() method create a new set. The original sets are unchanged.

If we want to modify a set in place, we use the .update() method.

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

{1, 2, 3, 4, 5}

Now set1 has been changed. It contains all items from both sets. The original set1 is gone.

We can also update with multiple sets.

set1 = {1, 2}
set2 = {2, 3}
set3 = {3, 4}
set1.update(set2, set3)
print(set1)

{1, 2, 3, 4}

And update() also accepts any iterable.

set1 = {1, 2}
set1.update([3, 4])
print(set1)

{1, 2, 3, 4}

Python Union: A Real-Life Example

Let’s say we have two lists of email addresses. One from our newsletter signup. One from a giveaway. We want to send an email to everyone. But we don’t want to send duplicates.

newsletter = {"alex@example.com", "sam@example.com", "john@example.com"}
giveaway = {"sam@example.com", "emma@example.com", "david@example.com"}

all_emails = newsletter | giveaway
print(all_emails)

{'alex@example.com', 'sam@example.com', 'john@example.com', 'emma@example.com', 'david@example.com'}

Now we can send one email to each address. No duplicates.

Another Example — Combining Tags

post1_tags = {"python", "coding", "beginner"}
post2_tags = {"python", "advanced", "tips"}

all_tags = post1_tags.union(post2_tags)
print(all_tags)

{'python', 'coding', 'beginner', 'advanced', 'tips'}

That’s useful for finding all tags used across multiple posts.

Union with an Empty Set

Union with an empty set does nothing. It just returns a copy of the other set.

set1 = {1, 2, 3}
empty = set()
result = set1 | empty
print(result)

{1, 2, 3}

That’s expected.

Common Mistakes with Python Union

set1 = {1, 2}
list1 = [2, 3]
result = set1 | list1   # Error!

result = set1.union(list1)

set1 = {1, 2}
set2 = {2, 3}
set1 | set2
print(set1)   # {1, 2}

set1 = {1, 2, 2, 3}
set2 = {3, 4, 4, 5}
result = set1 | set2
print(result)

{1, 2, 3, 4, 5}

result = [1, 2] | [2, 3]   # Error!

result = set([1, 2]) | set([2, 3])
print(result)

{1, 2, 3}