Question

I am using scipy’s weave.inline to perform computationally expensive tasks. I have problems returning an one-dimensional array back into the python scope. Weave.inline uses a special argument called “return_val” for the purpose of returning values back into the python scope.
The following example returning an integer value works well:

>>> from scipy.weave import inline
>>> print inline(r'''int N = 10; return_val = N;''')
10

However the following example, which indeed compiles without prompting an error, does not return the array i would expect:

>>> from scipy.weave import inline
>>> code =\
    r'''                                                              
       int* pairs;                                                       
       int  lenght = 0;                                                      
       for (int i=0;i<N;i++){                                            
         lenght   += 1;                                                     
         pairs     = (int *)malloc(sizeof(int)*lenght);                       
         pairs[i]  = i;
         std::cout << pairs[i] << std::endl;              
       }                                                                 
       return_val = pairs;                                               
    '''
 >>> N  = 5
 >>> R = inline(code,['N'])
 >>> print "RETURN_VAL:",R
 0
 1
 2
 3
 4
 RETURN_VAL: 1    

I need to reallocate the size of the array “pairs” dynamically which is why I can’t pass a numpy.array or python list per se.

Answer 1

All you need to do is use the raw python c-api calls, or if you’re looking for something a bit more convenient, the built in scipy weave wrappers.

No guarantees about leaks or efficiency, but it should look something a bit like this:

from scipy.weave import inline

code = r'''
    py::list ret; 
    for(int i = 0; i < N; i++) {
        py::list item;
        for(int j = 0; j < i; j++) {
            item.append(j);
        }
        ret.append(item);
    }
    return_val = ret;
    '''
N  = 5 
R = inline(code,['N'])
print R