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This rule raises an issue when a special method raises a NotImplementedError instead of returning NotImplemented.
Why is this an issue?
In Python, special methods corresponding to numeric operators and rich comparison operators should return NotImplemented when the
operation is not supported.
For example A + B is equivalent to calling A.__add__(B). If this binary operation is not supported by class A,
A.__add__(B) should return NotImplemented. The interpreter will then try the reverse operation, i.e.
B.__radd__(A). If these special methods were to raise NotImplementedError, the callers would not catch the exception and the
reverse operation would not be called.
Below is the list of special methods this rule applies to:

   __lt__(self, other) 
   __le__(self, other) 
   __eq__(self, other) 
   __ne__(self, other) 
   __gt__(self, other) 
   __ge__(self, other) 
   __add__(self, other) 
   __sub__(self, other) 
   __mul__(self, other) 
   __matmul__(self, other) 
   __truediv__(self, other) 
   __floordiv__(self, other) 
   __mod__(self, other) 
   __divmod__(self, other) 
   __pow__(self, other[, modulo]) 
   __lshift__(self, other) 
   __rshift__(self, other) 
   __and__(self, other) 
   __xor__(self, other) 
   __or__(self, other) 
   __radd__(self, other) 
   __rsub__(self, other) 
   __rmul__(self, other) 
   __rmatmul__(self, other) 
   __rtruediv__(self, other) 
   __rfloordiv__(self, other) 
   __rmod__(self, other) 
   __rdivmod__(self, other) 
   __rpow__(self, other[, modulo]) 
   __rlshift__(self, other) 
   __rrshift__(self, other) 
   __rand__(self, other) 
   __rxor__(self, other) 
   __ror__(self, other) 
   __iadd__(self, other) 
   __isub__(self, other) 
   __imul__(self, other) 
   __imatmul__(self, other) 
   __itruediv__(self, other) 
   __ifloordiv__(self, other) 
   __imod__(self, other) 
   __ipow__(self, other[, modulo]) 
   __ilshift__(self, other) 
   __irshift__(self, other) 
   __iand__(self, other) 
   __ixor__(self, other) 
   __ior__(self, other) 
   __length_hint__(self) 

How to fix it
Make sure special methods return NotImplemented instead of raising a NotImplementedError.
Code examples
Noncompliant code example
class MyClass:
    def __add__(self, other):
        raise NotImplementedError()  # Noncompliant: the exception will be propagated
    def __radd__(self, other):
        raise NotImplementedError()  # Noncompliant: the exception will be propagated

class MyOtherClass:
    def __add__(self, other):
        return 42
    def __radd__(self, other):
        return 42

MyClass() + MyOtherClass()  # This will raise NotImplementedError

Compliant solution
class MyClass:
    def __add__(self, other):
        return NotImplemented
    def __radd__(self, other):
        return NotImplemented

class MyOtherClass:
    def __add__(self, other):
        return 42
    def __radd__(self, other):
        return 42

MyClass() + MyOtherClass()  # This returns 42

Pitfalls
The __length_hint__ special method also requires to return a NotImplemented. Its behavior differs from the other methods,
because when it returns NotImplemented, a default value will be returned instead. See PEP
424 for more information.
Resources
Documentation

   Python documentation - Built-in Constants - NotImplemented 
   Python documentation - Implementing the
  arithmetic operations 

Standards

   PEP 424 - A method exposing __length_hint__