Python __invert__() Magic Method

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Syntax

object.__invert__(self)

The Python __invert__() method implements the unary arithmetic operation bitwise NOT ~. So, when you cal ~x, Python will internally call x.__invert__() to obtain the inverted object. If the method is not implemented, Python will raise 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 Bitwise NOT ~

Python’s bitwise NOT operator ~x inverts each bit from the binary representation of integer x so that 0 becomes 1 and 1 becomes 0. This is semantically the same as calculating ~x == -x-1. For example, the bitwise NOT expression ~0 becomes -1, ~9 becomes -10, and ~32 becomes -33.

Example Custom __invert__()

In the following minimal example, you create a custom class Data and overwrite the __invert__() magic method so that it returns a dummy string when trying to calculate the bitwise NOT operation.

class Data:
        
    def __invert__(self):
        return 'finxter'


x = Data()
print(~x)
# finxter

If you hadn’t defined the __invert__() method, Python would’ve raised a TypeError.

TypeError: bad operand type for unary ~: ‘…’

Consider the following code snippet where you try to calculate the bitwise NOT operation on custom objects without defining the dunder method __invert__():

class Data:
    pass


x = Data()
print(~x)

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>
    print(~x)
TypeError: bad operand type for unary ~: 'Data'

The reason for this error is that the __invert__() method has never been defined—and it is not defined for a custom object by default. So, to resolve the TypeError: bad operand type for unary ~, you need to provide the __invert__(self) method in your class definition as shown previously:

class Data:
        
    def __invert__(self):
        return 'finxter'


x = Data()
print(~x)
# finxter

References:

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