Why does str not have a __radd__ method?

[Steven D'Aprano]

I just tried to override str.__radd__:

class Special(str):
    def __radd__(self, other):
        print("I'm special!")
        return super().__radd__(self, other)

My __radd__ method was called correctly by the + operator, but to my
surprise, the super().__radd__ call failed with:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in __radd__
AttributeError: 'super' object has no attribute '__radd__'

Sure enough, in both Python 3.3 and 2.7:

py> str.__radd__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'str' has no attribute '__radd__'


This is especially astonishing, since int and float both have __radd__
methods, and yet numeric addition is commutative (x+y == y+x) whereas the
same is not true for string concatenation.

What is the rationale for str not having __radd__ method?


-- 
Steven