Where does my embedded python stdout go?

Question

Consider the following MWE:

#include <Python.h>
#include <stdio.h>

int main(void) {
  printf("Test 1\n");
  Py_Initialize();
  printf("Test 2\n");
  PyRun_SimpleString("print('Test 3')");
  printf("Test 4\n");
  return 0;
}

When I compile and run this as normal i get the expected output:

$ ./test
Test 1
Test 2
Test 3
Test 4

But when I redirect the output I get nothing from the python code:

$ ./test | cat
Test 1
Test 2
Test 4

What is happening? And more importantly how do I get my python output written to stdout like expected?

Interesting that if you add `'import sys; sys.stdout.flush()'` to the Python script, you get the redirected output, but _before_ everything else. — , Jul 02 '15 at 09:50

score 4 · Accepted Answer · answered Jul 02 '15 at 17:21

4

When stdout refers to a terminal, the output is line buffered otherwise which is block or fully buffered, won't output until the block is full.

To make the output line buffered when stdout refers to non-terminal, set the mode with setvbuf
And you have to call Py_Finalize() to have libpython close its I/O handle.

#include <Python.h>
#include <stdio.h>

int
main(int argc, char **argv) {
    //setlinebuf(stdout);
    setvbuf(stdout, NULL, _IOLBF, 0);
    printf("Test 1\n");
    Py_Initialize();
    printf("Test 2\n");
    PyRun_SimpleString("print('Test 3')");
    Py_Finalize();
    printf("Test 4\n");
    return 0;
}

answered Jul 02 '15 at 17:21

Nizam Mohamed

8,751
24
32

You can call `sys.stdout.flush()` in Python. Calling `Py_Finalize()` might not be intenional for his use case. – tynn Jul 02 '15 at 20:11
@tynn if you don't call `Py_Finalize()` the output is out of order, that's Python's is first. – Nizam Mohamed Jul 03 '15 at 00:30
Ah... of course, it all seams so obvious now :) Thank you! – A. Nilsson Jul 03 '15 at 09:32

score 4 · Answer 2 · answered Mar 13 '21 at 07:58

4

In current Python, you can use Py_UnbufferedStdioFlag.

answered Mar 13 '21 at 07:58

Yu Kobayashi

386
3
6

Where does my embedded python stdout go?

2 Answers2