Argument passing to __add__ magic method

Question

Hello dear coders,

Could you please advice how the value of a is passed to the “+” operator (actually __add__ method) in the following code:

class malicious(int):
def __init__(self, value):
pass
def __add__(self, x):
res = self // x
return res

a = malicious(5)
print(3+a)
#Output: 8

I know that the add method of the class isn’t working here as I stated 3+a not a+3, but it is unclear how the value of the a is seen and passed by to the usual + operator.

Answer 1

malicious is a subclass of int, so the instance "a" still represents a number.
When you use the operation 3+a then first python is checking if addition is defined for the left operand (3) as this is just a normal number, the default addition is used.
You would get a different result if you calculate a+3 because malicious class has overwritten this operator.

Additionally if the __add__ magic method is not defined for the object on the left side of the + sign, then Python still checks if you have __radd__ (right addition) defined for the object on the right side. If this doesn't work either, you get an error.

Answer 2

Tibor Santa, thanks a lot, it is very comperehensive answer!
Could you please explain how the 5 is passed to the __add__ method of int class?
I even didn’t stated that self.value = value, but it is directly passed to the function.
This part is the last dark point for me.