{"id":2595,"date":"2021-10-20T01:55:48","date_gmt":"2021-10-20T01:55:48","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=2595"},"modified":"2025-03-31T09:01:19","modified_gmt":"2025-03-31T09:01:19","slug":"python-__bool__","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-oop\/python-__bool__\/","title":{"rendered":"Python __bool__"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you will learn how to implement the Python __bool__<\/code> method to return boolean values for objects of a custom class.<\/p>\n\n\n\n

Introduction to the Python __bool__ method #<\/a><\/h2>\n\n\n\n

An object of a custom class<\/a> is associated with a boolean value. By default, it evaluates to True<\/code>. For example:<\/p>\n\n\n

class<\/span> Person<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, name, age)<\/span>:<\/span>\n        self.name = name\n        self.age = age\n\n\nperson = Person('John'<\/span>, 25<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, we define the Person<\/code> class, instantiate an object, and show its boolean value. As expected, the person<\/code> object is True<\/code>.<\/p>\n\n\n\n
To override this default behavior, you implement the __bool__<\/code> special method. The __bool__<\/code> method must return a boolean value, True<\/code> or False<\/code>.<\/p>\n\n\n\n
For example, suppose that you want the person<\/code> object to evaluate  False<\/code> if the age of a person is under 18 or above 65:<\/p>\n\n\n
class<\/span> Person<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, name, age)<\/span>:<\/span>\n        self.name = name\n        self.age = age\n\n    def<\/span> __bool__<\/span>(self)<\/span>:<\/span>\n        if<\/span> self.age < 18<\/span> or<\/span> self.age > 65<\/span>:\n            return<\/span> False<\/span>\n        return<\/span> True<\/span>\n\n\nperson = Person('Jane'<\/span>, 16<\/span>)\nprint(bool(person))  # False<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
False<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, the __bool__<\/code> method returns False<\/code> if the age is less than 18 or greater than 65.  Otherwise, it returns True<\/code>. The person object has the age value of 16 therefore it returns False in this case.<\/p>\n\n\n\n
The __len__ method #<\/a><\/h2>\n\n\n\n
If a custom class doesn’t have the __bool__<\/code> method, Python will look for the __len__()<\/code> method. If the __len__<\/code> is zero, the object is False<\/code>. Otherwise, it’s True<\/code>.<\/p>\n\n\n\n
If a class doesn’t implement the __bool__<\/code> and __len__<\/code> methods, the objects of the class will evaluate to True<\/code>.<\/p>\n\n\n\n
The following defines a Payroll<\/code> class that doesn’t implement __bool__<\/code> but the __len__<\/code> method:<\/p>\n\n\n
class<\/span> Payroll<\/span>:<\/span>\n    def<\/span> __init__<\/span>(self, length)<\/span>:<\/span>\n        self.length = length\n\n    def<\/span> __len__<\/span>(self)<\/span>:<\/span>\n        print('len was called...'<\/span>)\n        return<\/span> self.length\n\n\n\npayroll = Payroll(0<\/span>)\nprint(bool(payroll))  # False<\/span>\n\npayroll.length = 10<\/span>\nprint(bool(payroll))  # True<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Since the Payroll<\/code> class doesn’t override the __bool__<\/code> method, Python looks for the __len__<\/code> method when evaluating the Payroll’s objects to a boolean value.<\/p>\n\n\n\n
In the following example payroll’s __len__<\/code> returns 0, which is False<\/code>:<\/p>\n\n\n
payroll = Payroll(0<\/span>)\nprint(bool(payroll))  # False<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
However, the following example __len__<\/code> returns 10 which is True<\/code>:<\/p>\n\n\n
payroll.length = 10<\/span>\nprint(bool(payroll))  # True<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n