What is __contains__() in Python?

In Python, the __contains__() method is a special method that allows objects to define their own behavior when used with the in operator. It is called by the in operator to check if a given value is present in the object.

The __contains__() method takes two parameters: self (representing the object itself) and item (representing the value being checked). It should return True if the item is found in the object, and False otherwise.

Here's an example to illustrate the usage of __contains__() method:

class MyList:
def __init__(self, items):
self.items = items

def __contains__(self, item):
return item in self.items

my_list = MyList([1, 2, 3, 4, 5])

print(3 in my_list) # Output: True
print(6 in my_list) # Output: False

In this example, we define a custom class MyList that wraps a list of items. The __contains__() method is implemented to check if an item is present in the list by using the in operator on the self.items attribute.

By implementing the __contains__() method, objects of the MyList class can now be used with the in operator to check for membership of a specific value in the list.

Note that if the __contains__() method is not defined for an object, Python falls back to other methods to determine the containment. For example, it checks if the object has an __iter__() method and uses it to iterate over the elements, or it checks if the object has a __getitem__() method to access the elements by index.

__contains__() is a special method in Python that allows an object to define its own behavior when it is checked for containment in another object using the in keyword. The method should return True if the object contains the given value, and False otherwise.

Here's an example that demonstrates how to use __contains__():

class MyList:
    def __init__(self, values):
        self.values = values

    def __contains__(self, value):
        return value in self.values

my_list = MyList([1, 2, 3, 4, 5])

print(3 in my_list) # output: True
print(6 in my_list) # output: False

In this example, the MyList class defines its own behavior for the in keyword by implementing the __contains__() method. The method simply checks whether the given value is in the values list and returns True or False accordingly.

Note that __contains__() can be implemented for any custom object type in Python, and is often used for container types such as lists, sets, and dictionaries.