98 – Custom containment checks

The dunder method __contains__ can be used to implement custom containment checks in your classes. This will allow instances of your classes to be used with the keywords in and not in.

As an example, consider the class ClosedInterval shown below, that represents all numbers between the left and right boundaries, inclusive:

class ClosedInterval:
    def __init__(self, left, right):
        self.left = left
        self.right = right

    def __contains__(self, value):
        return self.left <= value <= self.right

By defining the dunder method __contains__, instances of ClosedInterval can be used with in and not in:

interval = ClosedInterval(2.7, 4.5)

print(3 in interval)  # True
print(7 in interval)  # False
print(7 not in interval)  # True

The dunder method __contains__ accepts the value that is being checked as its only argument and must return a Boolean value that must be True if the argument is contained in the instance and False if not.