Membership Operators in Python (in, not in) with Examples

Membership operators check whether a value is present inside a collection such as a list, string, tuple, set, dictionary or range. Python has two of them, in and not in, and both return True or False.

They make code read almost like English: if "mango" in fruits: means exactly what it says.

Membership Operators in Python

Operator	Returns True when	Example
in	the value is found in the collection	"a" in "cat" gives True
not in	the value is not found in the collection	5 not in [1, 2, 3] gives True

With Lists

fruits = ["apple", "mango", "banana"]
print("mango" in fruits)
print("grapes" in fruits)
print("grapes" not in fruits)

Output:

True
False
True

With Strings

For strings, in checks for a substring, so it can find whole words or parts of words. The check is case-sensitive.

sentence = "Python is easy to learn"
print("easy" in sentence)
print("Easy" in sentence)
print("" in sentence)

Output:

True
False
True

An empty string is considered to be inside every string, which is why the last line is True. For a case-insensitive check, convert both sides: "Easy".lower() in sentence.lower().

With Tuples and Sets

vowels = ("a", "e", "i", "o", "u")
print("e" in vowels)

primes = {2, 3, 5, 7, 11}
print(9 in primes)

Output:

True
False

Sets are the fastest choice for membership checks. Python checks a set or a dictionary almost instantly, however large it is, while a list has to be scanned item by item. If you need to check membership many times in a big collection, convert the list to a set first.

With Dictionaries

With a dictionary, in checks the keys, not the values. Use .values() or .items() when you need those.

marks = {"Riya": 91, "Aman": 78}
print("Riya" in marks)
print(91 in marks)
print(91 in marks.values())
print(("Aman", 78) in marks.items())

Output:

True
False
True
True

With range()

print(50 in range(1, 101))
print(100 in range(1, 100))

Output:

True
False

range(1, 100) stops before 100, so 100 is not in it.

Practical Examples

1. Count the Vowels in a Word

word = "education"
count = 0
for ch in word:
    if ch in "aeiou":
        count += 1
print(count)

Output:

5

2. Validate a Menu Choice

choice = "3"
if choice in ("1", "2", "3"):
    print("Valid choice")
else:
    print("Invalid choice")

Output:

Valid choice

This is much cleaner than writing choice == "1" or choice == "2" or choice == "3".

3. Remove Duplicates While Keeping the Order

names = ["Riya", "Aman", "Riya", "Kabir", "Aman"]
unique = []
for n in names:
    if n not in unique:
        unique.append(n)
print(unique)

Output:

['Riya', 'Aman', 'Kabir']

Watch Out: Nested Lists

in only looks at the top level of a collection. It does not search inside inner lists.

matrix = [[1, 2], [3, 4]]
print(3 in matrix)
print([3, 4] in matrix)
print(any(3 in row for row in matrix))

Output:

False
True
True

The items of matrix are the two inner lists, so 3 in matrix is False. To search inside every row, use any() as in the last line.

Practice Questions

1. print("a" in "Banana")
2. print("ana" in "Banana")
3. print(2 in [[1, 2], [3]])

Answers: 1) True. “Banana” contains a small “a” at index 1. 2) True. “ana” appears as a substring. 3) False. The list holds two inner lists, not the number 2.

Frequently Asked Questions

What are membership operators in Python?

in and not in. They test whether a value is present in a sequence or collection.

Does in check keys or values in a dictionary?

Keys. Use value in my_dict.values() to check values.

Is the in operator case-sensitive for strings?

Yes. "Easy" in "Python is easy" is False. Convert both strings to lower case for a case-insensitive check.

What is the difference between in and ==?

== compares two whole values. in checks whether one value is contained inside another.

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