Python __bool__

Summary: in this tutorial, you will learn how to implement the Python __bool__ method to return boolean values for objects of a custom class.

Introduction to the Python __bool__ method

An object of a custom class is associated with a boolean value. By default, it evaluates to True. For example:

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age


if __name__ == '__main__':
    person = Person('John', 25)Code language: Python (python)

In this example, we define the Person class, instantiate an object, and show its boolean value. As expected, the person object is True.

To override this default behavior, you implement the __bool__ special method. The __bool__ method must return a boolean value, True or False.

For example, suppose that you want the person object to evaluate False if the age of a person is under 18 or above 65:

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __bool__(self):
        if self.age < 18 or self.age > 65:
            return False
        return True


if __name__ == '__main__':
    person = Person('Jane', 16)
    print(bool(person))  # FalseCode language: Python (python)

In this example, the __bool__ method returns False if the age is less than 18 or greater than 65. Otherwise, it returns True. The person object has the age value of 16 therefore it returns False in this case.

The __len__ method

If a custom class doesn’t have the __bool__ method, Python will look for the __len__() method. If the __len__ is zero, the object is False. Otherwise, it’s True.

If a class doesn’t implement the __bool__ and __len__ methods, the objects of the class will evaluate to True.

The following defines a Payroll class that doesn’t implement __bool__ but the __len__ method:

class Payroll:
    def __init__(self, length):
        self.length = length

    def __len__(self):
        print('len was called...')
        return self.length


if __name__ == '__main__':
    payroll = Payroll(0)
    print(bool(payroll))  # False

    payroll.length = 10
    print(bool(payroll))  # TrueCode language: Python (python)

Since the Payroll class doesn’t override the __bool__ method, Python looks for the __len__ method when evaluating the Payroll’s objects to a boolean value.

In the following example payroll’s __len__ returns 0, which is False:

payroll = Payroll(0)
print(bool(payroll))  # FalseCode language: Python (python)

However, the following example __len__ returns 10 which is True:

payroll.length = 10
print(bool(payroll))  # TrueCode language: Python (python)

Summary

• All objects of custom classes return True by default.
• Implement the __bool__ method to override the default. The __bool__ method must return either True or False.
• If a class doesn’t implement the __bool__ method, Python will use the result of the __len__ method. If the class doesn’t implement both methods, the objects will be True by default.