Calling an operator in python is equivalent to calling a special method (except for the identity operator is). Python provides a set of built-in operations. It is for example possible to add two integers: 1 + 2. It is however not possible to add a string and an integer: 1 + "2" and such an operation will raise a TypeError.

It is possible to define how an operator will behave with a custom class by defining the corresponding special method. See python documentation for a complete list of operators and their methods: arithmetic and bitwise operators, comparison operators.

For symmetrical binary operators you need to define two methods so that the order of operands doesn't matter, ex: __add__ and __radd__.

This rule raises an issue when an operator is used on incompatible types. Types are considered incompatible if no built-in operations between those types exist and none of the operands has implemented the corresponding special methods.

Noncompliant Code Example

class Empty:
    pass

class Add:
    def __add__(self, other):
        return 42

1 + 2
1 + "2"  # Noncompliant
Empty() + 1  # Noncompliant
Add() + 1
1 + Add()  # Noncompliant
Add() + Empty()
Empty() + Add()  # Noncompliant

Compliant Solution

class Empty:
    pass

class Add:
    def __add__(self, other):
        return 42

    def __radd__(self, other):
        return 42

Add() + 1
1 + Add()
Add() + Empty()
Empty() + Add()

See