[Numpy-discussion] newbie for writing numpy/scipy extensions

[Travis Oliphant]

Gennan Chen wrote:

> Hi! All,
>
> I just start writing my own python extension based on numpy. Couple  
> of questions here:
>
> 1. I have some utility functions, such as wrappers for  
> PyArray_GETPTR* needed be access by different extension modules. So,  
> I put them in utlis.h and utlis.c. In utils.h, I need to include  
> "numpy/arrayobject.h". But the compilation failed when I include it  
> again in my extension module function, wrap.c:
>
> #include "numpy/arrayobject.h"
> #include "utils.h"
>
> When I remove it and use
>
> #include "utils.h"
>
> the compilation works. So, is it true that I can only include  
> arrayobject.h once?


No, you can include arrayobject.h more than once.  However, if you make 
use of C-API functions (not just macros that access elements of the 
array) in more than one file for the same extension module, you need to 
do a couple of things to make it work.

In the original file you must define PY_ARRAY_UNIQUE_SYMBOL to something 
unique to your extension module before you include the arrayobject.h file.

In the helper c file you must define PY_ARRAY_UNIQUE_SYMBOL and define 
NO_IMPORT_ARRAY prior to including the arrayobject.h

Thus, in wrap.c you do (feel free to change the name from 
_chen_extension to something else)

#define PY_ARRAY_UNIQUE_SYMBOL  _chen_extension 
#include "numpy/arrayobject.h"

and in

utils.c  you do

#define PY_ARRAY_UNIQUE_SYMBOL  _chen_extension 
#define NO_IMPORT_ARRAY
#include "numpy/arrayobject.h"


>
> 2.  which import I should use in my initial function:
>
> import_array()


import_array()

-Travis