On Fri, Mar 19, 2010 at 12:46 PM, Alex A. Naanou <alex.nanou@gmail.com> wrote:
A friend of mine stumbled upon the following behavior:
---cut---
class A(object): pass ... class B(object): ... def __add__(self, other): ... print 'B: adding B and %s objects.' % other.__class__.__name__ ... class C(object): ... def __radd__(self, other): ... print 'C: adding C and %s objects.' % other.__class__.__name__ ... a, b, c = A(), B(), C()
b + c B: adding B and C objects.
a + c C: adding C and A objects.
# so far, quite logical. now let's do this:
1 + c C: adding C and int objects.
--uncut--
My first expectation would be to get a TypeError here, as ints indeed have an __add__ method, and they do not know anything about C objects (obviously :) ).
Yes: the int.__add__ method is tried first. Since it doesn't know anything about C objects it returns NotImplemented, and then C.__radd__ is given a chance to do the addition. The rules are documented here: http://docs.python.org/reference/datamodel.html#coercion-rules Mark