Python Set `difference()` Method

set.difference() returns the values that are in one set but not in another.

This is useful when you want to find items that exist in the first set and remove anything that also appears in one or more other sets.

Quick answer

a = {1, 2, 3, 4}
b = {3, 4, 5}
result = a.difference(b)
print(result)

Output:

{1, 2}

difference() returns a new set with items from a that are not in b.

What `difference()` does

The difference() method compares sets and keeps only the values from the first set that do not appear in the other set.

Key points:

difference() returns items that are in the first set but not in the other set
It creates a new set
It does not change the original set
It is useful for finding removed, missing, or unique values

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

Basic syntax

Use this form:

set1.difference(set2)

You can also compare against more than one set:

set1.difference(set2, set3)

In that case, Python removes any value found in any of the other sets.

Example:

a = {1, 2, 3, 4, 5}
b = {2, 3}
c = {5}

result = a.difference(b, c)
print(result)

Output:

{1, 4}

Here, Python starts with a and removes anything found in b or c.

Return value

difference() always returns a new set.

Important details:

The return value is a new set
The result may be an empty set if all items are removed
The result is still a set, even if nothing remains

Example:

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

result = a.difference(b)

print(result)
print(type(result))

Output:

set()
<class 'set'>

An empty set is shown as set(), not {}. In Python, {} means an empty dictionary.

Simple example

Here is a basic example with two sets:

fruits_a = {"apple", "banana", "orange"}
fruits_b = {"banana", "orange", "grape"}

result = fruits_a.difference(fruits_b)

print(result)
print(fruits_a)
print(fruits_b)

Possible output:

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

What happens here:

Start with fruits_a
Remove any values also found in fruits_b
Only "apple" remains
The original sets stay unchanged

Notice that the printed order may look different. Sets do not keep items in a fixed order.

Using `difference()` with multiple sets

You can pass two or more sets into difference().

Python removes any value from the first set if that value appears in any of the other sets.

numbers = {1, 2, 3, 4, 5, 6}
group_a = {2, 4}
group_b = {5}
group_c = {6, 7}

result = numbers.difference(group_a, group_b, group_c)
print(result)

Output:

{1, 3}

This is helpful when you want to compare one main set against several groups at once.

`difference()` vs `difference_update()`

These two methods are similar, but they work differently.

`difference()`

Returns a new set
Does not change the original set

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

result = a.difference(b)

print(result)
print(a)

Output:

{1, 2}
{1, 2, 3, 4}

`difference_update()`

Changes the original set
Does not return a new set

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

a.difference_update(b)

print(a)

Output:

{1, 2}

Use difference() when you want to keep the original data.

For more on the in-place version, see Python set difference_update() method.

`difference()` vs the `-` operator

The - operator does the same kind of comparison as difference() for two sets.

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

print(a.difference(b))
print(a - b)

Output:

{1, 2}
{1, 2}

Both lines give the same result.

Why use difference()?

It is often clearer for beginners
The method name explains what the code is doing
It easily supports multiple sets

Example with multiple sets:

a = {1, 2, 3, 4, 5}
b = {2}
c = {4}

print(a.difference(b, c))

Output:

{1, 3, 5}

You cannot write that in the same simple way with a single - operator expression.

Common beginner mistakes

Here are some common problems when using difference().

Expecting the original set to change

This does not change a:

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

a.difference(b)
print(a)

Output:

{1, 2, 3}

If you want to keep the result, save it:

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

a = a.difference(b)
print(a)

Output:

{1, 2}

Expecting items from the second set instead of the first set

a.difference(b) means:

keep items from a
remove anything also in b

It does not mean “show me what is unique in b”.

Example:

a = {1, 2, 3}
b = {3, 4, 5}

print(a.difference(b))
print(b.difference(a))

Output:

{1, 2}
{4, 5}

The order of the sets matters.

Comparing lists instead of sets

difference() is a set method, not a list method.

This will fail:

a = [1, 2, 3]
b = [3, 4]

# a.difference(b)

Convert lists to sets first:

a = [1, 2, 3]
b = [3, 4]

result = set(a).difference(set(b))
print(result)

Output:

{1, 2}

Thinking the output order will match the input order

Sets are unordered, so printed output may appear in a different order.

a = {"cat", "dog", "bird"}
b = {"dog"}

print(a.difference(b))

You should focus on which values are present, not their position.

Trying to use duplicate values in a set example

Sets automatically remove duplicates.

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

print(a)
print(a.difference(b))

Output:

{1, 2, 3}
{1, 2}

The duplicates are already gone before difference() runs.

Quick debugging checks

If your result is not what you expect, print the values you are working with:

a = {1, 2, 3, 4}
b = {3, 4, 5}

print(a)
print(b)
print(a.difference(b))
print(type(a.difference(b)))
print(len(a.difference(b)))

These checks help you confirm:

what is in each set
what result Python creates
that the result is a set
how many items remain

FAQ

Does `difference()` change the original set?

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

What is the difference between `difference()` and `difference_update()`?

difference() returns a new set. difference_update() modifies the original set.

Can I use `difference()` with more than two sets?

Yes. You can pass multiple sets, and Python removes items found in any of them.

Why is my output in a different order?

Sets are unordered collections, so item order is not guaranteed.

Python Set difference() Method

Python Set `difference()` Method