What should C return to Python call()?

Question

Continuing on my previous question at Executing a C program in python?; what returns one from C to get usable data in Python??

Currently my program returns this:

int main (int argc, char *argv[])
{
    spa_data spa;  //declare the SPA structure
    int result;
    float min, sec;

    //enter required input values into SPA structure

    spa.year          = 2003;
    spa.month         = 10;
    spa.day           = 17;
    spa.hour          = 12;
    spa.minute        = 30;
    spa.second        = 30;
    spa.timezone      = -7.0;
    spa.delta_t       = 67;
    spa.longitude     = -105.1786;
    spa.latitude      = 39.742476;
    spa.elevation     = 1830.14;
    spa.pressure      = 820;
    spa.temperature   = 11;
    spa.slope         = 30;
    spa.azm_rotation  = -10;
    spa.atmos_refract = 0.5667;
    spa.function      = SPA_ALL;

    //call the SPA calculate function and pass the SPA structure

    result = spa_calculate(&spa);

    if (result == 0)  //check for SPA errors
    {
        //display the results inside the SPA structure

        printf("Julian Day:    %.6f\n",spa.jd);
        printf("L:             %.6e degrees\n",spa.l);
        printf("B:             %.6e degrees\n",spa.b);
        printf("R:             %.6f AU\n",spa.r);
        printf("H:             %.6f degrees\n",spa.h);
        printf("Delta Psi:     %.6e degrees\n",spa.del_psi);
        printf("Delta Epsilon: %.6e degrees\n",spa.del_epsilon);
        printf("Epsilon:       %.6f degrees\n",spa.epsilon);
        printf("Zenith:        %.6f degrees\n",spa.zenith);
        printf("Azimuth:       %.6f degrees\n",spa.azimuth);
        printf("Incidence:     %.6f degrees\n",spa.incidence);

        min = 60.0*(spa.sunrise - (int)(spa.sunrise));
        sec = 60.0*(min - (int)min);
        printf("Sunrise:       %02d:%02d:%02d Local Time\n", (int)(spa.sunrise), (int)min, (int)sec);

        min = 60.0*(spa.sunset - (int)(spa.sunset));
        sec = 60.0*(min - (int)min);
        printf("Sunset:        %02d:%02d:%02d Local Time\n", (int)(spa.sunset), (int)min, (int)sec);

    } else printf("SPA Error Code: %d\n", result);

    return 0;
}

I read some articles about structs and Pythons'pack, but I couldn't quite grasp it yet, so maybe somebody can point the right direction.

After looking at the C code, it would probably be easy enough to generate a module using it, bypassing having to call it at all. — Ignacio Vazquez-Abrams, Dec 07 '10 at 15:20

David Wolever · Accepted Answer · 2010-12-07T15:18:56.650

The simplest way to return data to Python would be to print it out in some sensible format. The one you've got there decent, but a simple CSV would be a bit easier.

Then you'll use subprocess.Popen:

p = subprocess.Popen(["./spa", "args", "to", "spa"], stdout=subprocess.PIPE)
(stdout, stderr) = p.communicate()
data = parse_output(stdout.read())

And, for example, if the output was CSV:

printf("%.6f, %.6e, %.6e, %.6f, %.6f, %.6e, %.6e, %.6f, %.6f, %.6f, %.6f\n",
       spa.jd, spa.l, spa.b, spa.r, spa.h, spa.del_psi, spa.del_epsilon, spa.epsilon,
       spa.zenith, spa.azimuth, spa.incidenc)

Then parse_output could be written:

def parse_output(datastr):
    return [ float(value.strip()) for value in datastr.split(",")

Now, this does make a bunch of assumptions… Specifically:

That you're dealing with a fairly small number of data (Popen.communicate() stores all the output in memory before returning it to your program)
That you won't be calling ./spa too often (spawning a process is very, very slow)

But if that's fine, this will work for you.

Thanks! That really worked instantly! @edit: For now, this really helped me. Up to this day I had never even used C, so i'll worry about performance and memory usage later ;) Again, thanks! — Izz ad-Din Ruhulessin, Dec 07 '10 at 15:33
Glad I could help :) Also, that's the spirit with performance and memory usage! — David Wolever, Dec 07 '10 at 16:22

What should C return to Python call()?

1 Answers1