def _binary_operator_error(t1, t2, name):
    raise TypeError("unsupported operand type(s) for %s: '%s' and '%s'" %
                    (t1, t2, name))

def binary_operator(name, rname, op1, op2):
    t1 = type(op1)
    t2 = type(op2)
    if issubclass(t2, t1):
        if rname in t2.__dict__:
            result = t2.__dict__[rname](op2, op1)
            if result is not NotImplemented:
                return result
        if name in t1.__dict__:
            result = t1.__dict__[name](op1, op2)
            if result is not NotImplemented:
                return result
    else:
        if name in t1.__dict__:
            result = t1.__dict__[name](op1, op2)
            if result is not NotImplemented:
                return result
        if rname in t2.__dict__:
            result = t2.__dict__[rname](op2, op1)
            if result is not NotImplemented:
                return result
    _binary_operator_error(t1, t2, name)        


# In CPython this fails for 2.5, 2.6, 3.1 (and Unladen Swallow)
# PyPy and Jython both succeed
assert binary_operator('__add__', '__radd__', 3.0, 4j) == 3.0 + 4j

# I believe the problem is in the function wrap_binaryfunc_r
# in typeobject.c
# wrap_binaryfunc_r returns NotImplemented for op(a, b) 
# if b is not subtype of a, but it should let the slot 
# function handle subtyping.