__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
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.
bool(x) function converts value
x to a Boolean value
False. It uses implicit Boolean conversion on the input argument
x. Any Python object has an associated truth value.
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
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
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
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
class Data: def __bool__(self): return 42 x = Data() res = bool(x) print(res)
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
Where to Go From Here?
Enough theory. Let’s get some practice!
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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
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His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.