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Python Membership Operators

Membership operators check whether something is inside a group, like looking to see if a toy is in the toy box. in and not in give you True or False.

In this page:

  1. What Are Membership Operators?
  2. in and not in
  3. Membership in Strings
  4. Membership in Dictionaries
  5. How in Connects to __contains__
Syntax
python 
item in collection
item not in collection

What Are Membership Operators?

Membership operators test whether a value exists inside a container like a list, tuple, string, set, or dictionary, returning a plain True or False without needing a manual loop.

They read almost like English, which is one of the reasons Python code checking for membership is so concise and readable.

Example: What Are Membership Operators?

python 
fruits = ["apple", "banana"]
print("apple" in fruits)

in and not in

The in operator returns True if the left-hand value is found inside the right-hand container, and not in is its exact opposite.

Both work across every built-in sequence and collection type, making them one of the most broadly reusable operators in the language.

Example: in and not in

python 
fruits = ["apple", "banana"]
print("apple" in fruits)  # True, value is present
print("kiwi" not in fruits)  # True, value is absent

Membership in Strings

For strings, in performs a substring search rather than checking individual characters only, so cat in concatenate returns True because that exact sequence of letters appears inside the larger string.

This makes it a quick way to check for substrings without calling a separate method.

Example: Membership in Strings

python 
print("cat" in "concatenate")

Membership in Dictionaries

When used on a dictionary, in checks against the dictionary's keys by default, not its values — name in my_dict tests whether name is a key, so checking for a value requires in my_dict.values() explicitly.

This trips up beginners who expect it to search values.

Example: Membership in Dictionaries

python 
user = {"name": "Alex", "age": 30}
print("name" in user)  # checks the dict's keys by default
print("Alex" in user.values())  # must check .values() explicitly for values

How in Connects to __contains__

Under the hood, in calls the container's __contains__ method if one is defined, which is why custom classes can support membership testing by implementing that dunder method themselves.

For objects without __contains__, Python falls back to iterating and comparing each element.

Example: How in Connects to __contains__

python 
class MyBox:
    def __init__(self, items):
        self.items = items
    def __contains__(self, item):  # lets "in" work on instances of this class
        return item in self.items

box = MyBox([1, 2, 3])
print(2 in box)  # calls MyBox.__contains__ under the hood
Related Topics

Python Identity Operators · Python Operators · Python Assignment Operators · Python Bitwise Operators

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Common Mistakes
  1. Using in on a dictionary and expecting it to search values; "a" in d checks keys only (use d.values()).
  2. Testing "ab" in ["abc"] on a list and expecting substring matching; on lists in needs an exact element match.
  3. Writing if x in a or b:, which tests x in a or the truthiness of b; write x in a or x in b.
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