[Numpy-discussion] How to call methods from a class with custom matrices parameters with numpy arrays ?

Bill Baxter wbaxter@gmail....
Thu Apr 19 16:19:10 CDT 2007

This may be of no help at all but I see mentions of C++/Python/OO/SWIG
and it triggers me to mention something I heard about recently called
I /think/ the idea behind it is to basically make a C++ version of the
Python C API.  You still do all the wrapping yourself, but it's much
less painful.


On 4/20/07, Matthieu Brucher <matthieu.brucher@gmail.com> wrote:
> > Doing the wrapping in an object oriented way is difficult, and maybe
> > not that useful. This does not prevent the API exposed to python to be
> > OO, of course.
> I have some difficulties to do this in an automated way...
> I'm trying now to make a derived object from my function, without templates
> and, I'm hoping so, with a correct interface i.e. double + 2*int to save
> make the conversions with numpy arrays.
> > > That works great for C then, not that well for C++...
> > Well, this is an inherent problem of C++ when you try to use it from
> > other languages, but let's not start this (again :) ),
> :D
> > The example shows basic wrapping, and some facilities provided by
> > numpy to help. Again, ctype is pretty efficient as long as you do not
> > need to do convertion. If you call it thousand of times, it will be
> > slow, but this is more or less inherent to python (function calls are
> > nowhere near as fast as in a compiled language, at least for now).
> It's for optimization, so the function will be called several hundreds of
> times, I suppose, and I tried porting the whole function to Python, but I'm
> not sure that the Python version behaves like the C++ version - the results
> are not identic, so... -, thus the wrapping.
> > SWIG may be better in your case because it is aware of C++ classes,
> > and is *compiling* an extension, whereas ctypes does not compile
> > anything. This means that you do not have to care about binary
> > interfaces problemes between foreign object codes. SWIG parses a prett
> > good deal of C++, and is aware of classes (template is another matter,
> > obviously). numpy sources contain swig code to process automatically
> > numpy arrays (that is convert C representation of a numpy array to a
> > simple C array and vice et versa), if I remember correctly.
> Yes, I will try to use this solution, I have trouble figuring how passing
> the output numpy array, Bill Baxter asked the same question today, at
> exactly the same time ;) Well, I saw on the docs that such arrays must be
> passed to the function, and already allocated, and that is a problem for
> me...
> > There is also boost.python, but I don't know if its situation towards
> > numpy array has changed (eg there was some code lying around to
> > represent numpy arrays in C++).
> That will be my next quest during the summer ;)
> > If I were you, and if there are only a few classes and a few member
> > functions, I would try the C wrapper called from python first.
> It's only one class and one method + the constructor. Not much but I'm a
> real beginner in that domain. True, I could use the C API directly...
> Matthieu
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