Membership Operators¶

Membership operators test whether a value exists in a sequence (string, list, tuple, set, dict).

Operator	Description	Example	Result
`in`	Value found	`'a' in 'cat'`	`True`
`not in`	Value not found	`'z' not in 'cat'`	`True`

String Membership¶

Check if a substring exists:

print('a' in 'cat')       # True
print('at' in 'cat')      # True
print('dog' in 'cat')     # False
print('z' not in 'cat')   # True

List Membership¶

Check if an element exists:

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

print('apple' in fruits)    # True
print('grape' in fruits)    # False
print('grape' not in fruits) # True

Boolean/Integer Gotcha¶

print(True in [1])    # True (because True == 1)
print(False in [0])   # True (because False == 0)
print(True in [2])    # False

This happens because True == 1 and False == 0 in Python.

Tuple Membership¶

Same as lists:

colors = ('red', 'green', 'blue')

print('red' in colors)     # True
print('yellow' in colors)  # False

Set Membership¶

Sets are optimized for membership testing (O(1)):

numbers = {1, 2, 3, 4, 5}

print(3 in numbers)   # True
print(10 in numbers)  # False

Dictionary Membership¶

By default, in checks keys:

person = {'name': 'Alice', 'age': 30}

print('name' in person)     # True (key exists)
print('Alice' in person)    # False (not a key)
print('city' not in person) # True

Check Values¶

print('Alice' in person.values())  # True

Check Key-Value Pairs¶

print(('name', 'Alice') in person.items())  # True

Performance Considerations¶

Container	Time Complexity
list	O(n)
tuple	O(n)
str	O(n)
set	O(1) average
dict	O(1) average

For large collections with frequent lookups, use set or dict:

# Slow for large lists
if item in large_list:  # O(n)
    ...

# Fast for sets
if item in large_set:   # O(1)
    ...

Practical Examples¶

Input Validation¶

valid_options = {'yes', 'no', 'maybe'}

user_input = input("Enter choice: ").lower()
if user_input in valid_options:
    print("Valid choice")
else:
    print("Invalid choice")

Filtering¶

vowels = 'aeiou'
text = "Hello World"

# Filter vowels
result = [c for c in text.lower() if c in vowels]
print(result)  # ['e', 'o', 'o']

Finding Missing Items¶

required = {'name', 'email', 'phone'}
provided = {'name', 'email'}

missing = [field for field in required if field not in provided]
print(missing)  # ['phone']

Range Membership¶

print(5 in range(10))     # True
print(10 in range(10))    # False (exclusive end)
print(15 in range(0, 20, 5))  # True (0, 5, 10, 15)

Custom Objects¶

Classes can define custom in behavior with __contains__:

class EvenNumbers:
    def __contains__(self, n):
        return n % 2 == 0

evens = EvenNumbers()
print(4 in evens)   # True
print(5 in evens)   # False

Summary¶

in checks membership, not in checks absence
Works with strings, lists, tuples, sets, dicts
Dict membership checks keys by default
Sets have O(1) lookup — use for frequent checks
Use __contains__ for custom membership logic