Jason Baker, 31.05.2010 18:15:

I'm working on a Python-to-mzscheme binding using mzscheme's ffi library (mzscheme's rough equivalent of the Python ctypes library). I'm looking for a way to convert Python objects into Scheme values. Essentially what I'm trying to do is say "Is this a Python integer? Ok, convert it to a Scheme integer." or "Is this a Python string? Ok, convert it into a Scheme string." ... etc.

What is the best way to do this? My first intuition was to call Py*_Check to determine the type, but it turns out that's a macro that can't be used in non-c code. I'm sure I could translate that into the appropriate C code, but that gives me a bad feeling.

Those are macros because that makes them really, really fast, especially in Py3. However, their definition changes between Python versions, so replacing them by regular C code is really not a good idea.

In general, I wouldn't try to make the wrapper too generic. You might want to inspect the underlying Scheme code (potentially at runtime), and infer the possible types (or the required conversion) from that.

Right now, I'm looking for simple and easy more than efficient and complete (but I would also like to know what the efficient and complete approach would be).

Have you considered integrating this with Cython via a thin C bridge, instead of talking to Python's C-API directly? Without knowing anything about the FFI you are using here, I suspect that it likely wouldn't be a generic wrapper in that case. Instead, it would allow users to write specialised and well optimised Scheme wrappers for their specific use case. So far, Cython supports calling into C, C++ and Fortran, where the Fortran wrapper is also implemented as a C-bridge (via fwrap). A similar Scheme binding would allow users to take advantage of Cython's static typing features, so that your code wouldn't have to guess types in the first place.

Stefan