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 = ageif __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)) # False
Code 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)) # True
Code 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)) # False
Code language: Python (python)
However, the following example __len__
returns 10 which is True
:
payroll.length = 10
print(bool(payroll)) # True
Code 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 eitherTrue
orFalse
. - 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 beTrue
by default.