Short Answer:
To override the logical not
operator for a custom Python class My_Class
, redefine the dunder method My_Class.__bool__()
to return your custom Boolean value. This ensures that bool(x)
on a My_Class
object x
returns either True
or False
.
The operation not x
will then return the inverse Boolean value, i.e,
not x
evaluates toTrue
ifbool(x)
evaluates toFalse
, andnot x
evaluates toFalse
ifbool(x)
evaluates toTrue
.
Here’s how this works in a Gif:
Problem Formulation
Say, you want to override the class of a custom object x
so that you can define the return value of not x
. How to override the logical not
operator in Python?
Background Python “not” Operator
Python’s not
operator returns True
if the single operand evaluates to False
, and returns False if it evaluates to True
. Thus, it logically negates the implicit or explicit Boolean value of the operand.
Overriding the Python “not” Operator
Interestingly, you can apply the logical NOT operator on arbitrary Python objects. The base idea is the “truthiness” of Python objects, i.e., every Python object has an associated Boolean value as determined by the __bool__()
magic method.
💡 Note: 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.
In the following code, you create a custom class called My_Obj
and use the “not
” operator on instances of this class. You explicitly define the __bool__()
method to customize the behavior of the not operator on your custom classes.
class My_Obj: def __init__(self, number): self.number = number def __bool__(self): return bool(self.number) x = My_Obj(0) y = My_Obj(1) z = My_Obj(99) print('not', x.number, '=', not x) print('not', y.number, '=', not y) print('not', z.number, '=', not z)
Output:
not 0 = True not 1 = False not 99 = False
If you hadn’t overridden the __bool__
magic method, Python would assume all objects of the custom objects to be True
, so the results would be False
for all three objects.
References:
Where to Go From Here?
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