On Sun, Oct 18, 2009 at 10:50 AM, Darren Dale dsdale24@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."