How do you write a set in python?

The set() function creates a set in Python.

Example

list_numbers = [1, 2, 3, 4, 2, 5]

# create set from list
numbers_set = set(list_numbers)
print(numbers_set)

# Output: {1, 2, 3, 4, 5}

set() Syntax

The syntax of set() is:

set(iterable)

Recommended Reading: Python sets

set() Parameters

set() takes a single optional parameter:

• iterable (optional) - a sequence (string, tuple, etc.) or collection (set, dictionary, etc.) or an iterator object to be converted into a set.

set() Return Value

set() returns:

• an empty set if no parameters are passed
• a set constructed from the given iterable parameter

Example 1: Create sets from string, tuple, list, and range

# empty set
print(set())

# from string
print(set('Python'))

# from tuple
print(set(('a', 'e', 'i', 'o', 'u')))

# from list
print(set(['a', 'e', 'i', 'o', 'u']))

# from range
print(set(range(5)))

Output

set()
{'P', 'o', 't', 'n', 'y', 'h'}
{'a', 'o', 'e', 'u', 'i'}
{'a', 'o', 'e', 'u', 'i'}
{0, 1, 2, 3, 4}

Note: We cannot create empty sets using { } syntax as it creates an empty dictionary. To create an empty set, we use set().

Example 2: Create sets from another set, dictionary and frozen set

# from set
print(set({'a', 'e', 'i', 'o', 'u'}))

# from dictionary
print(set({'a':1, 'e': 2, 'i':3, 'o':4, 'u':5}))

# from frozen set
frozen_set = frozenset(('a', 'e', 'i', 'o', 'u'))
print(set(frozen_set))

Output

{'a', 'o', 'i', 'e', 'u'}
{'a', 'o', 'i', 'e', 'u'}
{'a', 'o', 'e', 'u', 'i'}

Example 3: Create set() for a custom iterable object

class PrintNumber:
    def __init__(self, max):
        self.max = max

    def __iter__(self):
        self.num = 0
        return self

    def __next__(self):
        if(self.num >= self.max):
            raise StopIteration
        self.num += 1
        return self.num

# print_num is an iterable
print_num = PrintNumber(5)

# creating a set
print(set(print_num))

Output

{1, 2, 3, 4, 5}

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

Set

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.

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

Sets are written with curly brackets.

Example

Create a Set:

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

Try it Yourself »

Note: Sets are unordered, so you cannot be sure in which order the items will appear.

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.

Example

Duplicate values will be ignored:

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

print(thisset)

Try it Yourself »

Get the Length of a Set

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

Example

Get the number of items in a set:

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

print(len(thisset))

Try it Yourself »

Set Items - Data Types

Set items can be of any data type:

Example

String, int and boolean data types:

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

Try it Yourself »

A set can contain different data types:

Example

A set with strings, integers and boolean values:

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

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type()

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

Example

What is the data type of a set?

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

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The set() Constructor

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

Example

Using the set() constructor to make a set:

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

Try it Yourself »

Python Collections (Arrays)

There are four collection data types in the Python programming language:

• List is a collection which is ordered and changeable. Allows duplicate members.
• Tuple is a collection which is ordered and unchangeable. Allows duplicate members.
• Set is a collection which is unordered, unchangeable*, and unindexed. No duplicate members.
• Dictionary is a collection which is ordered** and changeable. No duplicate members.

*Set items are unchangeable, but you can remove items and add new items.

**As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.

When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.

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