The reflected operators, e.g. __radd__, are used when
the left operand does not have the non-reflected
operator, *unless* the right operand is of the same type.

The documentation on the "Emulating numeric types" page
doesn't mention this peculiar exclusion.  Of the
reflected operators it says:
"These methods are called to implement the binary
arithmetic operations (+, -, *, /, %, divmod(), pow(),
**, <<, >>, &, ^, |) with reflected (swapped) operands.
These functions are only called if the left operand
does not support the corresponding operation. For
instance, to evaluate the expression x-y, where y is an
instance of a class that has an __rsub__() method,
y.__rsub__(x) is called."

This code demonstrates the correct behavior and then
the problem:

class A(object):
    def __radd__(self, other):
        return '__radd__', other

print None + A()
print A() + A()

giving....

('__radd__', None)

Traceback (most recent call last):
  File "C:/Temp/reflectedbug.py", line 6, in -toplevel-
    print A() + A()
TypeError: unsupported operand type(s) for +: 'A' and 'A'

I've replaced None in the first print statement with
many kinds of builtin objects, instances of other
classes, and with instances of subclasses of A.  In all
these cases the __radd__ operator is used as
documented.  I've only seen the problem when the two
operands are of the same type.

This problem also occurs during the backing-off to
plain operators that occurs when augmented operators,
e.g. __iadd__, aren't implemented by the type and the
operands are of the same type.

This problem is present in 2.4 on Linux and Windows,
and in the current CVS version (2.5a0, 27-Feb-05) on Linux.

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Someone remove this accidental repost.  Thanks.