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?
Thanks,
Darren
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