How to make an extension class have a __getitem__ interface?

John Machin sjmachin at
Wed Mar 9 02:35:28 CET 2005

Steve Juranich wrote:
> I'm in the process of writing a few extension types, and there's one
> that I'd sort of like to have getitem, setitem, getslice, setslice
> functionality for.
> I've been looking through the docs and how things are done in
> Objects/listobject.c, and it appears tha there are a couple of ways
> do it and I'd like to get some opinions on what's the best way.
> As I see it, here are my options (please correct me if I'm wrong):
> 1) Register my getitem handler as "__getitem__", my setitem handler
> "__setitem__", etc with my PyMethodDef struct for my new type.
> 2) Give my new type a full-blown sequence interface and stick my
> handlers in the PySequenceMethods struct.
> 3) Give my new type a full-blown mapping interface and stick my
> handlers in the PyMappingMethods struct.
> I kind of want to shy away from (2) and (3) simply because there's
> still a lot of stuff going on inside those
> structs that I don't understand and I don't want to be responsible
> learning right now.  But I'm also not sure if it's just as easy as
> setting some functions to respond to __getitem__, __getslice__, etc.
> As I'm still new to the world of writing extension modules, any
> corrections or clarifications would be greatly appreciated.  I've
> mostly been using SWIG up to this point, but I've got some code here
> where I really care what the Python interface looks like, and SWIG
> just won't do what I want it to do (with nested templated classes and
> the like).

1. Define the interface you want; it doesn't have to be a full blown
sequence or a full blown mapping -- the main thing is that you know
what you want it to do; like what is the input to __getitem__, an index
or a key??

2. Implement it in Python. Write some code that will actually use it.
Refine your interface.

3. Make the minimal changes so that it can be compiled by pyrex. Read
the C code generated by pyrex. At this stage a masochist woulld say "Oh
goodie I know what to do now" and go off and hand code a C extension. A
sensible person would send Greg Ewing an appreciative e-mail message
and keep the extension in pyrex.



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