Calling a C/C++ code from Python with an argument

Question

I read similar posts but nothing specific to this. Or maybe my logic is incorrect and you can correct me please.

What I'm trying to do:

Write python code, that then calls an already compiled C or C++ code. But how would I pass an argument to that C/C++ script given that it's already been compiled?

I have a program in Python that manipulates an Excel file. And then at some point I need to search through, append, and create an array of 10,000 cells. I figured C/C++ would make this faster since python is taking some time to do that.

Help please.

This sounds somewhat like an [XY problem](http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). — Christian Dean, Dec 01 '16 at 22:13
Perhaps...but before you guys flag this, please anyone any pointers? Would greatly appreciate it. — Michael 1022, Dec 01 '16 at 22:18
No, no. Your misunderstanding my point. I'm not saying your question should be flagged, I'm simply saying you should ask about how to solve your problem, not how to fix your solution. — Christian Dean, Dec 01 '16 at 22:19
Are you sure the problem is with Python, and not an inefficient implementation? Have you looked at `pandas` and `numpy`? — brianpck, Dec 01 '16 at 22:19
compiled c and c++ functions still take arguments. Maybe take a look at [this](http://stackoverflow.com/questions/145270/calling-c-c-from-python?rq=1) after making sure your problem isn't with the algorithm. — jaggedSpire, Dec 01 '16 at 22:22
I'm using openpyxl. All I do is iterate over rows/columns and manipulate the data, split text, and put it together in different order etc, extract things from it, store in new column but I have 10,000 of those and it takes longer than I thought to get this done. Not sure where I'd implement numpy with openpyxl — Michael 1022, Dec 01 '16 at 22:26
Thank you. Boost Python seems like a good solution. A very simple follow up: if I had written and compiled a program in C/C++ into an executable. I can just call os.startfile() in python to run it, correct? — Michael 1022, Dec 01 '16 at 22:29

score 2 · Accepted Answer · answered Dec 02 '16 at 03:02

Let's say we want to write a Python script that acts as a wrapper on top of a C binary and passes arguments from the terminal to the C binary. first, create a test.c C program as follows:

#include <stdio.h>

int main(int argc, char *argv[])
{
   if(argc > 1)
   {
      int i;
      printf("C binary: ");
      for(i = 0; i < argc; i++)
         printf("%s ", argv[i]);
      printf("\n");
   }
   else
      printf("%s: No argument is provided!\n", argv[0]);
   return(0);
}

then compile it using:

gcc -o test test.c

and run it for two dummy arguments using:

./test arg1 arg2

Now, going back to your question. How I could pass arguments from Python to a C binary. First you need a Python script to read the arguments from the terminal. The test.py would do that for you:

import os
import sys

argc = len(sys.argv)
argv =  sys.argv

if argc > 2:
   cmd = './'
   for i in range(1,argc):
      cmd = cmd + argv[i] + ' '
   if os.path.isfile(argv[1]):
      print('Python script: ', cmd)
      os.system(cmd)
   else:
      print('Binary file does not exist')
      bin = 'gcc -o ' + argv[1] + ' '+ argv[1] + '.c'
      print(bin)
      os.system(bin)
      if os.path.isfile(argv[1]):
         os.system(cmd)
      else:
         print('Binary source does not exist')
         exit(0)
else:
   print('USAGE: python3.4', argv[0], " BINARY_FILE INPUT_ARGS");
   exit(0)

Having test.c and test.py in the same directory. Now, you can pass arguments from the terminal to the test C binary using:

python3.4 test.py test arg1 arg2

Finally, the output would be:

Python script:  ./test arg1 arg2
C binary: ./test arg1 arg2

Two last remarks:

Even if you don't compile the source code, the test.py will look for the test.c source file and try to compile it.
If you don't want to pass arguments from the terminal, you can always define the arguments in the Python script and pass them to the C binary.

Calling a C/C++ code from Python with an argument

1 Answers1