Truthiness of Python Objects

When trying to evaluate the truthiness of an object x, basically the following is done in order:

• x.__bool__() is called. If __bool__ is not implemented, then
• x.__len__() is called. If __len__() is not implemented, then
• return True. As a result, all user defined objects are truthy unless __bool__ or __len__ are implemented. Also, we can use if iterable w/o the need to use if len(iterable).

class A:
    ...

class B:
    def __bool__(self):
        print("In __bool__")
        return False

class C:
    def __len__(self):
        print("In __len__")
        return False

class D:
    def __bool__(self):
        print("In __bool__")
        return False

    def __len__(self):
        print("In __len__")
        return False

if A(): print("Truthy")     #=> "Truthy"
if B(): print("Truthy")     #=> "In __bool__"
if C(): print("Truthy")     #=> "In __len__"
if D(): print("Truthy")     #=> "In __len__"