:link-with-uscores1: https://docs.python.org/3/reference/datamodel.html?highlight=__exit__%20special%20methods#emulating-numeric-types
:link-with-uscores2: https://docs.python.org/3/reference/datamodel.html?highlight=__exit__%20special%20methods#object.__lt__

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: {link-with-uscores1}[arithmetic and bitwise operators], {link-with-uscores2}[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()
----


:link-with-uscores1: https://docs.python.org/3/reference/datamodel.html?highlight=__exit__%20special%20methods#object.__lt__
:link-with-uscores2: https://docs.python.org/3/reference/datamodel.html?highlight=__exit__%20special%20methods#emulating-numeric-types

== See

* Python documentation - {link-with-uscores1}[Rich comparison methods]
* Python documentation - {link-with-uscores2}[Emulating numeric types]

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== Implementation Specification
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endif::env-github,rspecator-view[]