Python Sets

Sets are used to store multiple items in a single variable.

Set is one of 4 built-in data types in Python used to store collections of data, the other 3 are List, Tuple, and Dictionary, all with different qualities and usage.

A set is a collection which is unordered, unchangeable*, and unindexed.

thisset = {"apple", "banana", "cherry"}
print(thisset)

Set Items

Set items are unordered, unchangeable, and do not allow duplicate values.

Unordered

Unordered means that the items in a set do not have a defined order.

Set items can appear in a different order every time you use them, and cannot be referred to by index or key.

Unchangeable

Set items are unchangeable, meaning that we cannot change the items after the set has been created.

Duplicates Not Allowed

Sets cannot have two items with the same value.

thisset = {"apple", "banana", "cherry", "apple"}

print(thisset)

thisset = {"apple", "banana", "cherry", True, 1, 2}

print(thisset)

thisset = {"apple", "banana", "cherry", False, True, 0}

print(thisset)

Get the Length of a Set

To determine how many items a set has, use the len() function.

thisset = {"apple", "banana", "cherry"}

print(len(thisset))

Set Items - Data Types

Set items can be of any data type:

set1 = {"apple", "banana", "cherry"}
set2 = {1, 5, 7, 9, 3}
set3 = {True, False, False}

A set can contain different data types:

set1 = {"abc", 34, True, 40, "male"}

type()

From Python's perspective, sets are defined as objects with the data type 'set':

myset = {"apple", "banana", "cherry"}
print(type(myset))

The set() Constructor

It is also possible to use the set() constructor to make a set.

thisset = set(("apple", "banana", "cherry")) # note the double round-brackets
print(thisset)

Access Items

You cannot access items in a set by referring to an index or a key.

But you can loop through the set items using a for loop, or ask if a specified value is present in a set, by using the in keyword.

thisset = {"apple", "banana", "cherry"}

for x in thisset:
  print(x)

Check if "banana" is present in the set:

thisset = {"apple", "banana", "cherry"}

print("banana" in thisset)

Add Items

To add one item to a set use the add() method.

thisset = {"apple", "banana", "cherry"}

thisset.add("orange")

print(thisset)

Add Sets

To add items from another set into the current set, use the update() method.

thisset = {"apple", "banana", "cherry"}
tropical = {"pineapple", "mango", "papaya"}

thisset.update(tropical)

print(thisset)

Add Any Iterable

The object in the update() method does not have to be a set, it can be any iterable object (tuples, lists, dictionaries etc.).

thisset = {"apple", "banana", "cherry"}
mylist = ["kiwi", "orange"]

thisset.update(mylist)

print(thisset)

Remove Item

To remove an item in a set, use the remove(), or the discard() method.

thisset = {"apple", "banana", "cherry"}

thisset.remove("banana")

print(thisset)

thisset = {"apple", "banana", "cherry"}

thisset.discard("banana")

print(thisset)

Loop Items

You can loop through the set items by using a for loop:

thisset = {"apple", "banana", "cherry"}

for x in thisset:
  print(x)

Join Two Sets

There are several ways to join two or more sets in Python.

You can use the union() method that returns a new set containing all items from both sets, or the update() method that inserts all the items from one set into another:

set1 = {"a", "b" , "c"}
set2 = {1, 2, 3}

set3 = set1.union(set2)
print(set3)

Keep ONLY the Duplicates

The intersection_update() method will keep only the items that are present in both sets.

x = {"apple", "banana", "cherry"}
y = {"google", "microsoft", "apple"}

x.intersection_update(y)

print(x)

The intersection() method will return a new set, that only contains the items that are present in both sets.

x = {"apple", "banana", "cherry"}
y = {"google", "microsoft", "apple"}

z = x.intersection(y)

print(z)

Keep All, But NOT the Duplicates

The symmetric_difference_update() method will keep only the elements that are NOT present in both sets.

x = {"apple", "banana", "cherry"}
y = {"google", "microsoft", "apple"}

x.symmetric_difference_update(y)

print(x)

The symmetric_difference() method will return a new set, that contains only the elements that are NOT present in both sets.

x = {"apple", "banana", "cherry"}
y = {"google", "microsoft", "apple"}

z = x.symmetric_difference(y)

print(z)

x = {"apple", "banana", "cherry", True}
y = {"google", 1, "apple", 2}

z = x.symmetric_difference(y)

print(z)

Set Methods

Python has a set of built-in methods that you can use on sets.