[pypy-dev] special methods

[Armin Rigo]

Hi Simon,

On Mon, Mar 26, 2007 at 02:30:15PM -0700, Simon Burton wrote:
> The attached script is an attempt to get __str__ to work in rpython.
> After annotation, it looks for str(someinstance), rewriting the block to call __str__.
> Then the modified blocks are fed back into the annotator.

The "official" approach is quite different.  It would involve a
consider_op_str() on SomeInstance, as you also thought.  It is in some
sense harder, but more robust - I certainly wouldn't be happy to check
in code in PyPy that adds a rewriting pass in the middle of
annotation...  For example, your approach only supports direct 'str(x)'
calls, which is somehow the easy case - because they can be manually
replaced by 'x.__str__()' in the source code anyway - but not indirect
cases like 'str([x, y, z])' where the x, y and z have a custom __str__()
method.  To do this properly you need a consider_op_str() using
bookkeeper.emulate_pbc_call(), a lot of patience understanding what
rpbc.py is doing, and probably a call to hlinvoke() in the ll_str() of
rclass.py...  and then the same for the oo type system, if you want to
be complete.  Argh.

All in all I'll stick to the point of view that adding support for
special methods in RPython is a very dangerous direction to go: where do
you stop?  Is __add__() RPython?  Is the full logic of __add__() versus
__radd__() RPython?


A bientot,

Armin.