You’re missing a key exception to the usual behaviour: when the right-hand operand is an instance of a subclass of the class of the left-hand operand, the special method for the right-hand operand is called first.
See the documentation at:
http://docs.python.org/reference/datamodel.html#coercion-rules
and in particular, the following two paragraphs:
For objects
x
andy
, first
x.__op__(y)
is tried. If this is not
implemented or returns
NotImplemented
,y.__rop__(x)
is
tried. If this is also not implemented
or returnsNotImplemented
, a
TypeError exception is raised. But see
the following exception:Exception to the previous item: if the
left operand is an instance of a
built-in type or a new-style class,
and the right operand is an instance
of a proper subclass of that type or
class and overrides the base’s
__rop__()
method, the right
operand’s__rop__()
method is tried
before the left operand’s__op__()
method.