confusion with __add__ and __radd__ [SOLVED]

[Walter Moreira]

On Tue, Dec 03, 2002 at 05:43:18PM -0300, Walter Moreira wrote:
> Hello. I'm confused about the behavior of the 'reverse' methods
> __radd__, __rmul__, etc, and Google didn't help me.
> 
> 
>     Python 2.2.2 (#1, Nov 21 2002, 08:18:14)
>     [GCC 2.95.4 20011002 (Debian prerelease)] on linux2
>     Type "help", "copyright", "credits" or "license" for more information.
>     >>>
>     >>> class Foo(object):
>     ...     def __add__(self, other):
>     ...             print 'add'
>     ...             return NotImplemented
>     ...     def __radd__(self, other):
>     ...             print 'radd'
>     ...             return 1
>     ...
>     >>>
>     >>>
>     >>> a=Foo()
>     >>> b=Foo()
>     >>> a+b
>     add
>     add
>     Traceback (most recent call last):
>       File "<stdin>", line 1, in ?
>       TypeError: unsupported operand types for +: 'Foo' and 'Foo'
>     >>>
>     
> Why isn't the __radd__ method called, since __add__ returns
> 'NotImplemented'? If 'Foo' is a classic class it works ok.  Am I
> overlooking something obvious?

Just for the record.  I read the sources and I found that when the type
of new style objects is the same, then only the slots of the left operand
are tried.  I don't fully understand the reason (but that's another
question).

So, if I define a 'Bar' class in the same way as 'Foo' above, then 'a+b'
correctly calls Foo.__add__ and then Bar.__radd__.

Walter.