[Python-Dev] binary operation heuristics -- bug or undocumented?
dickinsm at gmail.com
Fri Mar 19 13:55:58 CET 2010
On Fri, Mar 19, 2010 at 12:46 PM, Alex A. Naanou <alex.nanou at gmail.com> wrote:
> A friend of mine stumbled upon the following behavior:
>>>> 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.
> 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
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