Overloading operators using __getattr__ in Python

Question

I am trying to overload several operators at once using the __getattr__ function. In my code, if I call foo.__add__(other) it works as expected, but when I try foo + bar, it does not. Here is a minimal example:

class Foo():
    
    
    def add(self, other):
        return 1 + other
    
    def sub(self, other):
        return 1 - other
    
    def __getattr__(self, name):
                
        stripped = name.strip('_')
        if stripped in {'sub', 'add'}:
            return getattr(self, stripped)
        else:
            return
    
if __name__=='__main__':
    
    bar = Foo()
    
    print(bar.__add__(1)) # works
    print(bar + 1) # doesn't work

I realize that it would be easier in this example to just define __add__ and __sub__, but that is not an option in my case.

Also, as a small side question, if I replace the line:

if stripped in {'sub', 'add'}:

with

if hasattr(self, name):

the code works, but then my iPython kernel crashes. Why does this happen and how could I prevent it?

Answer 1

This is happening because python operators use an optimization to look up the function implementing the operator. The following lines are roughly equivalent:

foo + 1
type(foo).__add__(foo, 1)

Operators are found specifically on the class object only, never on the instance.

bar.__add__(1) calls __getattr__ to find the missing attribute on bar. This works because it bypasses normal operator lookup procedures.

bar + 1 calls Foo.__add__(bar, 1) followed by int.__radd(1, bar). The first attribute lookup fails, and the second option raises TypeError.