[Python-Dev] nonstandard behavior of reflected functions

[Darren Dale]

On Sun, Oct 18, 2009 at 10:50 AM, Darren Dale <dsdale24 at gmail.com> wrote:
> According to http://docs.python.org/reference/datamodel.html , the
> reflected operands functions like __radd__ "are only called if the
> left operand does not support the corresponding operation and the
> operands are of different types. [3] For instance, to evaluate the
> expression x - y, where y is an instance of a class that has an
> __rsub__() method, y.__rsub__(x) is called if x.__sub__(y) returns
> NotImplemented."
>
> Consider the following simple example:
>
> ==========================
> class Quantity(object):
>
>    def __add__(self, other):
>        return '__add__ called'
>
>    def __radd__(self, other):
>        return '__radd__ called'
>
> class UnitQuantity(Quantity):
>
>    def __add__(self, other):
>        return '__add__ called'
>
>    def __radd__(self, other):
>        return '__radd__ called'
>
> print 'Quantity()+Quantity()', Quantity()+Quantity()
> print 'UnitQuantity()+UnitQuantity()', UnitQuantity()+UnitQuantity()
> print 'UnitQuantity()+Quantity()', UnitQuantity()+Quantity()
> print 'Quantity()+UnitQuantity()', Quantity()+UnitQuantity()
> ==========================
>
> The output should indicate that __add__ was called in all four trials,
> but the last trial calls __radd__. Interestingly, if I comment out the
> definition of __radd__ in UnitQuantity, then the fourth trial calls
> __add__ like it should.
>
> I think this may be an important bug. I'm running Python 2.6.4rc1
> (r264rc1:75270, Oct 13 2009, 17:02:06) an ubuntu Karmic. Is it a known
> issue, or am I misreading the documentation?

I'm sorry, I should have read further down the page in the documentation:

"Note: If the right operand’s type is a subclass of the left operand’s
type and that subclass provides the reflected method for the
operation, this method will be called before the left operand’s
non-reflected method. This behavior allows subclasses to override
their ancestors’ operations."