[ python-Bugs-1153163 ] reflected operator not used when operands have the same type

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Bugs item #1153163, was opened at 2005-02-27 20:09
Message generated for change (Comment added) made by hughsw
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Category: Python Interpreter Core
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Submitted By: HughSW (hughsw)
Assigned to: Nobody/Anonymous (nobody)
Summary: reflected operator not used when operands have the same type

Initial Comment:

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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>Comment By: HughSW (hughsw)
Date: 2005-02-28 01:09

Message:
Logged In: YES 
user_id=1146279

I've looked into this a little.  Newbie that I am, I don't
know where the 
 x = slotw(v, w);
call goes (in binary_op1() in abstract.c near line 377)....
 AFAICT, this code in abstract.c behaves reasonably, so
problem would seem to be in the tp_as_number slot-function
that's getting called.  And whereever that is, it's not the
binary-op functions in classobject.c that I thought it would
be....


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