The __Contains__ Method in Python

Hemank Mehtani Oct 26, 2023
  1. Understanding Membership Testing
  2. the __contains__ Method in Python
  3. Customize the __contains__ Method in Python
  4. Use the __contains__ Method in Strings
  5. Overload the __contains__ Method in Classes
  6. Conclusion
The __Contains__ Method in Python

One of the fundamental operations in programming is checking whether an element is present in a collection like a list, tuple, or dictionary. Python provides a convenient way to do this using the in keyword, which utilizes the __contains__ method under the hood.

In this article, we will discuss the __contains__ method in Python and discover how it simplifies membership testing.

Understanding Membership Testing

Membership testing is a common task in programming. It involves checking whether a given value exists within a collection (e.g., list, tuple, set, dictionary).

Python makes this operation straightforward and efficient through the in keyword.

Consider the following code snippet:

my_list = [1, 2, 3, 4, 5]
if 3 in my_list:
    print("3 is in the list")


3 is in the list

In this example, we use the in keyword to check if the value 3 exists in the list my_list. If the value is present, the message 3 is in the list will be printed.

the __contains__ Method in Python

The __contains__ method is defined in Python as follows:

class YourClass:
    def __contains__(self, item):
        # Custom logic to check if 'item' is present in 'self'

When you use the in operator to check for the presence of an item in an object of YourClass, Python will call the __contains__ method with the self and item as its arguments.

Essentially, this means that the in keyword utilizes the __contains__ method to perform membership testing. When you use in, Python effectively calls the __contains__ method of the collection on which you’re performing the test.

It returns True if the item is found and False otherwise.

The __contains__ method is a magic method, denoted by double underscores, that allows you to customize the behavior of membership testing for your classes.

Here’s a simple example of how you can define the __contains__ method in a custom class:

class MyContainer:
    def __init__(self, data): = data

    def __contains__(self, item):
        return item in

container = MyContainer([1, 2, 3, 4, 5])

if 3 in container:
    print("3 is in the custom container")

When you run this code, it will output:

3 is in the custom container

In this code, the MyContainer class takes a list [1, 2, 3, 4, 5] as its data attribute. The __contains__ method checks if an item is present in the data list.

When you use the in operator with the container instance and the value 3, it returns True, and the message 3 is in the custom container is printed.

Customize the __contains__ Method in Python

The __contains__ method is a powerful tool for customizing how membership testing works for your objects. You can define the behavior of in for your classes by implementing this method.

For example, you might want to check for membership based on specific criteria or filters.

Here’s an example of a custom class that implements a unique membership test:

class EvenNumbers:
    def __contains__(self, item):
        if isinstance(item, int) and item % 2 == 0:
            return True
        return False

even_numbers = EvenNumbers()

if 6 in even_numbers:
    print("6 is an even number")

When you run this code, it will output:

6 is an even number

In this example, the EvenNumbers class defines a custom membership test that checks if a number is even. When you use the in operator with an instance of this class and the number 6, it returns True because 6 is indeed an even number.

This showcases how you can create specialized membership tests to suit your needs.

Use the __contains__ Method in Strings

Python’s built-in collection types, like lists and sets, are optimized for membership testing using the in keyword.

However, the __contains__ method is not exclusive to user-defined classes. It is also an integral part of Python’s built-in classes, such as strings.

For example, you can use the __contains__ method in a string to check for the existence of a substring:

str1 = "Delftstack"
str2 = "Del"

val = str1.__contains__(str2)



Here, the __contains__ method is employed to check if the substring Del is present in the string Delftstack.

Overload the __contains__ Method in Classes

Magic methods, like __contains__, are invoked internally in classes. You can also use the __contains__ method in a class and overload it to customize its behavior.

In the example below, we overload the __contains__ method within a class to always return False:

class abc(object):

    name = "Delftstack"

    def __contains__(self, m):
            return False

b = abc()
print("stack" in b)



In the above example, the __contains__ method returns False if a substring is in a string. When the in operator is used, the final output is False, even if the string stack is present in Delftstack.


The __contains__ method in Python empowers developers to customize membership testing for their classes and even extends its magic to built-in classes like strings.

Whether you are building custom containers, data structures, or specialized filtering mechanisms, the __contains__ method offers you the ability to tailor membership testing to your specific needs, ultimately enhancing the expressiveness and versatility of Python.

Additionally, this article has shed light on the presence of __contains__ in built-in classes, allowing you to explore its possibilities in various contexts, including strings and other Python objects.