Mark Dickinson wrote:
On Sep 21, 2:07 am, Steven D'Aprano <steve
+comp.lang.pyt...@pearwood.info> wrote:
After playing around with various combinations of C1, C2, D1 and D2, it
seems to me that the rule is:
If the right-hand argument is a subclass of the left-hand argument, AND also
defines __radd__ directly rather than inheriting it, then its __radd__
method is called before the left-hand argument's __add__ method.
which strikes me as a strangely specific and not very useful rule. I suspect
it might be an accident of implementation rather than a deliberate feature.
I'm 99.9% sure it's deliberate rather than an accident of
implementation. See the note in the docs at:
http://docs.python.org/reference/datamodel.html#emulating-numeric-types
Support that you're subclassing int, (class MyInt(int): ...) and you
want to intercept additions of the form 3 + MyInt(4) (perhaps because
you want MyInt to be 'contagious', so that an arithmetic operation
that combines an int and a MyInt returns a MyInt). How would you
achieve this without this rule?
I think Steven's objection was in not calling subclass.__radd__ when
__radd__ is inherited rather than directly overridden. Interestingly
enough, the following works as expected (at least, as I expected ;)...
class C1(object):
def __add__(self, other):
print "C1.__add__(%r, %r)" % (self, other)
class C2(C1):
def __radd__(self, other):
print "C2.__radd__(%r, %r)" % (self, other)
class C3(C2):
pass
C1() + C2() # --> C2.__radd__(<...C2...>, <...C1...)
C1() + C3() # --> C2.__radd__(<...C3...>, <...C1...>)
C2() + C3() # --> C1.__add__(<...C2...>, <...C3...>)
~Ethan~
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