[Python-Dev] right-side binary operations

Oleg Broytman phd at phd.pp.ru
Fri Mar 19 14:07:34 CET 2010

On Fri, Mar 19, 2010 at 03:46:07PM +0300, Alex A. Naanou wrote:
> >>> class C(object):
> ...     def __radd__(self, other):
> ...         print 'C: adding C and %s objects.' % other.__class__.__name__
> ...
> >>> 1 + c
> C: adding C and int objects.
> My first expectation would be to get a TypeError here, as ints indeed
> have an __add__ method, and they do not know anything about C objects
> (obviously :) ). On second thought, giving client code priority to
> handle things has it's merits.
> The problem is that I found no mention of this behavior in the docs.

   It's well-known and documented behavior. It's what r-methods are for.
See http://docs.python.org/reference/datamodel.html#emulating-numeric-types

   "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 and the operands are of
different types. [3] 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 if x.__sub__(y) returns NotImplemented."

     Oleg Broytman            http://phd.pp.ru/            phd at phd.pp.ru
           Programmers don't die, they just GOSUB without RETURN.

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