Message7350

When special methods (e.g. __add__ or __sub__) return NotImplemented, then operations which internally use these methods should raise appropriate TypeError exceptions.

In CPython:

>>> class A:
...   def __add__(self, other):
...     return NotImplemented
...   def __sub__(self, other):
...     return NotImplemented
... 
>>> A() + A()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'instance' and 'instance'
>>> A() - A()
Traceback (most recent call last):                                                                                                                           
  File "<stdin>", line 1, in <module>                                                                                                                        
TypeError: unsupported operand type(s) for -: 'instance' and 'instance'                                                                                      
>>>

In Jython:

>>> class A:
...   def __add__(self, other):
...     return NotImplemented
...   def __sub__(self, other):
...     return NotImplemented
... 
>>> A() + A()
NotImplemented
>>> A() - A()
NotImplemented
>>>