[Python-Dev] Coercion and slots handling mixed types

[M.-A. Lemburg]

Greetings everybody,

During the course of implementing the Unicode proposal we
discussed here in early December I stumbled into an old
problem which has caused me some troubles already in the
early stages of writing mx.DateTime:

Most of the C slot functions are not capable of dealing
with mixed type arguments. Even worse, most expect to be
passed "their" type of arguments without even checking
them. In Python things are much better: you have the __r***___
methods, can do mixed type arguments, etc. 

In short: we need the same things for the C level !

To start this discussion, I would like to point you to
an old patch I wrote for Python 1.5. It focusses mainly
on the numeric slots, but does provide some insights into
possible ways out of the current problems with C level
coercions (the business of dealing with mixed type argument):

  http://starship.skyport.net/~lemburg/CoercionProposal.html

Here are some starters for the discussion. I think we need:

· slots which are able to deal with mixed type argument

· slots which do not expect to find a certain type without
  checking for it

· coercion implemented at two levels:
  1. a generic mixed type abstract operation mechanism (such as
     the one included in the above patch for numeric types)
  2. slots which can handle mixed type argument and have a
     way to signal "I can't handle this type combination"
     (the patch uses a special singleton for this instead of
     exceptions because the latter cause too much of a perfomance
     hit -- yes I've tested this).

· all of the above for the numeric slots as well as most
  of the other slots dealing with PyObject* arguments

Note that these things are also needed for David Ascher's rich
comparisons.

-- 
Marc-Andre Lemburg
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