Python __bool__() Magic Method

Syntax

object.__bool__(self)

The Python __bool__() method implements the built-in bool() function. So, when you cal bool(x), Python attempts to call x.__bool__(). If the return value is not a Boolean, Python raises a TypeError.

We call this a “Dunder Method” for Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.

Background bool()

Python’s built-in bool(x) function converts value x to a Boolean value True or False. It uses implicit Boolean conversion on the input argument x. Any Python object has an associated truth value.

The bool(x) function takes only one argument, the object for which a Boolean value is desired.

To understand this operation in detail, feel free to read over our tutorial or watch the following video:

Example Custom __bool__()

In the following example, you create a custom class Data and overwrite the __bool__() magic method so that it returns a Boolean value when trying to call bool(x) on a custom Data object.

class Data:
    def __bool__(self):
        return False


x = Data()
res = bool(x)

print(res)
# ... False ...

If you hadn’t defined the __bool__() method, Python would’ve used the default one that always returns True:

class Data:
    pass


x = Data()
res = bool(x)

print(res)
# ... True ...

TypeError: __bool__ should return bool, returned …

Consider the following code snippet where you try to return a string value in the dunder method __bool__():

class Data:
    def __bool__(self):
        return "42"


x = Data()
res = bool(x)

print(res)

Running this leads to the following error message on my computer:

Traceback (most recent call last):
  File "C:\Users\xcent\Desktop\code.py", line 7, in <module>
    res = bool(x)
TypeError: __bool__ should return bool, returned str

The reason for this error is that the __bool__() method must return a Boolean value. To resolve the has never been defined—and it is not defined for a custom object by default. So, to resolve the TypeError: __bool__ should return bool, returned str , you need to return a Boolean value in your method definition as shown previously:

class Data:
    def __bool__(self):
        return True


x = Data()
res = bool(x)

print(res)
# True

Note that a similar error occurs when you return other non-Boolean data types from __bool__():

class Data:
    def __bool__(self):
        return 42


x = Data()
res = bool(x)

print(res)

This yields:

Traceback (most recent call last):
  File "C:\Users\xcent\Desktop\code.py", line 7, in <module>
    res = bool(x)
TypeError: __bool__ should return bool, returned int

References:

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