Membership operators (in, not in) in Python

Introduction

Membership operators help you check if something is inside a group or not. They make it easy to find out if a value exists in a list, string, or other collections.

Checking if a name is in a list of invited guests before allowing entry.

Finding out if a word contains a certain letter.

Verifying if a key exists in a dictionary before using it.

Seeing if a number is part of a set of allowed values.

Syntax

Python

value in collection
value not in collection

in returns True if the value is found inside the collection.

not in returns True if the value is NOT found inside the collection.

Examples

Checks if 5 is in the list. Result is True.

Python

5 in [1, 2, 3, 4, 5]

Checks if letter 'a' is NOT in the string 'hello'. Result is False.

Python

'a' not in 'hello'

Checks if 'key' is a key in the dictionary. Result is True.

Python

'key' in {'key': 10, 'other': 20}

Sample Program

This program checks if 'banana' is in the fruits list and prints a message. Then it checks if 'orange' is not in the list and prints another message.

Python

fruits = ['apple', 'banana', 'cherry']

# Check if 'banana' is in the list
if 'banana' in fruits:
    print('Banana is in the list!')

# Check if 'orange' is not in the list
if 'orange' not in fruits:
    print('Orange is not in the list!')

Important Notes

Membership operators work with many types like lists, strings, tuples, sets, and dictionaries (checks keys).

They return True or False, so you can use them directly in conditions.

Summary

in checks if a value is inside a collection.

not in checks if a value is NOT inside a collection.

Use them to write clear and simple checks for membership.