Python Set Methods

Table of Contents

In the last lesson, we learned about Python Set Difference. Now let’s use built-in set methods to add, remove, update, and compare items.

Let’s Talk About Tools for Sets

We know what a set is. It is an unordered collection of unique items.

We know how to create one. We know how to loop through it. We know about set operations like union, intersection, and difference.

Now let’s talk about set methods. These are built-in functions that belong to set objects. They let us add items, remove items, update the set, and more.

Let’s remember, sets are mutable. So most methods change the set in place. They do not return a new set. They modify the original.

Let’s go through the most common ones.

Adding Items to a Python Set

add()

Adds a single item to the set.

fruits = {"apple", "banana"}
fruits.add("orange")
print(fruits)

Output

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

If the item already exists, nothing happens. No error.

fruits.add("apple")
print(fruits)

Output

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

Still the same set. Because sets don’t allow duplicates.

update()

Adds multiple items to the set. We can pass another set, a list, a tuple, or any iterable.

fruits = {"apple", "banana"}
fruits.update(["orange", "grape"])
print(fruits)

Output

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

We can also pass multiple iterables.

fruits.update(["kiwi"], ["mango"])
print(fruits)

Output

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

Duplicates are ignored.

Removing Items from a Python Set

remove()

Removes a specific item. If the item is not found, it raises a KeyError.

fruits = {"apple", "banana", "orange"}
fruits.remove("banana")
print(fruits)

Output

{'apple', 'orange'}

fruits.remove("grape")   # Error!

Output

KeyError: 'grape'

So let’s only use remove() if we are sure the item exists.

discard()

Removes a specific item. If the item is not found, it does nothing. No error.

fruits = {"apple", "banana", "orange"}
fruits.discard("banana")
print(fruits)

Output

{'apple', 'orange'}

fruits.discard("grape")   # No error
print(fruits)

Output

{'apple', 'orange'}

discard() is safer. Let’s use it when we are not sure if the item is there.

pop()

Removes and returns an arbitrary item from the set. Because sets are unordered, we do not know which item will be removed.

fruits = {"apple", "banana", "orange"}
removed = fruits.pop()
print(removed)
print(fruits)

Output (may vary):

banana
{'apple', 'orange'}

If the set is empty, pop() raises a KeyError.

empty = set()
empty.pop()   # Error!

clear()

Removes all items from the set. The set becomes empty.

fruits = {"apple", "banana", "orange"}
fruits.clear()
print(fruits)

Output

set()

Copying a Python Set

copy()

Returns a shallow copy of the set.

original = {1, 2, 3}
copied = original.copy()
print(copied)

Output

{1, 2, 3}

Now copied is a separate set. Changing one does not affect the other.

Set Operations as Methods in Python

We already covered union, intersection, and difference in earlier articles. But they are also methods. Let’s list them here for completeness.

union()

Returns a new set with all items from both sets.

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

Output

{1, 2, 3}

intersection()

Returns a new set with only the common items.

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

Output

{2, 3}

difference()

Returns a new set with items in the first set but not the second.

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

Output

{1}

symmetric_difference()

Returns a new set with items in either set but not both.

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

Output

{1, 4}

In-Place Update Methods in Python

These methods modify the set in place. They do not return a new set.

update()

We already saw this. It adds items from another iterable.

intersection_update()

Keeps only the items that are in both sets.

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

Output

{3, 4}

difference_update()

Removes items that are in another set.

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

Output

{1, 2}

symmetric_difference_update()

Keeps only the items that are in one set or the other, but not both.

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

Output

{1, 2, 4, 5}

Checking Relationships in Python Sets

issubset()

Checks if all items of one set are in another.

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

Output

True

issuperset()

Checks if one set contains all items of another.

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

Output

True

isdisjoint()

Checks if two sets have no items in common.

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

Output

True

If they share at least one item, it returns False.

set1 = {1, 2}
set2 = {2, 3}
print(set1.isdisjoint(set2))

Output

False