Python Sets

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.

Sets are written with curly brackets.

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

Note: Set items are unchangeable, but you can remove and add items.

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.

Once a set is created, you cannot change its items, but you can remove items and add new items.

Duplicates Not Allowed

Sets cannot have two items with the same value.

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

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

print("banana" in thisset)

Add Items

Once a set is created, you cannot change its items, but you can add new 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)

Note: If the item to remove does not exist, remove() will raise an error.

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

thisset.discard("banana")

print(thisset)

Note: If the item to remove does not exist, discard() will NOT raise an error.

You can also use the pop() method to remove an item, but this method will remove the last item. Remember that sets are unordered, so you will not know what item that gets removed.

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

x = thisset.pop()

print(x)
print(thisset)

The clear() method empties the set:

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

thisset.clear()

print(thisset)

The del keyword will delete the set completely:

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

del thisset

print(thisset)

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)

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

set1.update(set2)
print(set1)

Note: Both union() and update() will exclude any duplicate items.