Syntax and Definition
__contains__() magic method implements the membership operation, i.e., the
in keyword. Semantically, the method returns
True if the argument object exists in the sequence on which it is called, and
False otherwise. For example,
3 in [1, 2, 3] returns
True as defined by the list method
[1, 2, 3].__contains__(3).
We call this a “Dunder Method” for “Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.
Basic Example Overriding Membership
You can overload the “
in” operator by overriding the
__contains__(self, item) method and return a Boolean value
False whether the item exists in the custom class object or not.
Here’s a generalized example:
class MyClass: def __init__(self, my_collection): self.my_collection = my_collection def __contains__(self, item): return item in self.my_collection my = MyClass('hello world') print('hello' in my) # True
The custom class
MyClass wouldn’t generally support membership. But by defining the
__contains__() “dunder” method, you can reduce membership of an object in the class to the problem of checking membership of an object in a collection using the “
in” operator. Now, you can check, for example, if a string is a member of a custom class object.
Python Membership Background
Membership Without __contains__()
But what if the
__contains__() dunder method is not defined?
Then, you attempt to check membership on this custom object—it works!
See the highlighted code part:
class Data: def __init__(self, data): self.data = data # an iterable def __iter__(self): self.current_index = 0 return self def __next__(self): if self.current_index < len(self.data): x = self.data[self.current_index] self.current_index += 1 return x raise StopIteration my_sequence = Data(['Alice', 'Bob', 'Carl']) print('Chris' in my_sequence) # False print('Alice' in my_sequence) # True
To improve your basic Python understanding, feel free to check out this related video on the
iter() built-in function:
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