Question

I am writing a binding system that exposes classes and functions to python in a slightly unusual way.

Normally one would create a python type and provide a list of functions that represent the methods of that type, and then allow python to use its generic tp_getattro function to select the right one.

For reasons I wont go into here, I can’t do it this way, and must provide my own tp_getattro function, that selects methods from elsewhere and returns my own ‘bound method’ wrapper. This works fine, but means that a types methods are not listed in its dictionary (so dir(MyType()) doesn’t show anything interesting).

The problem is that I cannot seem to get __add__ methods working. see the following sample:

>>> from mymod import Vec3
>>> v=Vec3()
>>> v.__add__
<Bound Method of a mymod Class object at 0xb754e080>
>>> v.__add__(v)
<mymod.Vec3 object at 0xb751d710>
>>> v+v
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'mymod.Vec3' and 'mymod.Vec3'

As you can see, Vec3 has an __add__ method which can be called, but python’s + refuses to use it.

How can I get python to use it? How does the + operator actually work in python, and what method does it use to see if you can add two arbitrary objects?

Thanks.

(P.S. I am aware of other systems such as Boost.Python and SWIG which do this automatically, and I have good reason for not using them, however wonderful they may be.)

Answer 1

Do you have an nb_add in your type’s number methods structure (pointed by field tp_as_number of your type object)?