[Python] python calling executable with popen;, file written by executable can not be accessed

I have a C++ file

#include <cstdio>

int main(int iArgC, char *apArgV[]) {
int iResult = -1;
if (iArgC > 1) {
FILE *fOut = fopen(apArgV[1], "wt");
if (fOut != NULL) {
fprintf(fOut, "hello\n");
fprintf(fOut, "goodbye\n");
fclose(fOut);
printf("Written `%s`\n", apArgV[1]);
iResult = 0;
} else {
printf("Coudn´t open %s\n", apArgV[1]);
}
} else {
printf("I need an argument\n");
}
return 0;
}


which i compile like this

g++ -g -o hello hello.cpp


When i run ./hello /home/jody/hihi,txt the file hihi.txt is written.

Then i have this python script

#!/usr/bin/python

import subprocess as sp
import time

def callHello(file_name) :
command = ["/home/jody/progs/polyhedronLED/version_c/hello", file_name]
proc = sp.Popen(command,
stdout=sp.PIPE,
stderr=sp.PIPE)
sOut, sError = proc.communicate()
iResult = proc.returncode
print("return code: %d"%iResult)
print("Output: %s"%sOut)
# -- end def

if __name__ == '__main__':
file_name0 = "/home/jody/hihi.txt"
callHello(file_name0)

ff = open(file_name0, "wt")
lines = ff.readlines()
ff.close()
print("output\n%s"%lines)
# -- end if


When i run this script, i get the output:

return code: 0
Output: b'Written `/home/jody/hihi.txt`\n'
Traceback (most recent call last):
File "/home/jody/progs/polyhedronLED/version_c/./hellocall.py", line 22, in <module>
lines = ff.readlines()
^^^^^^^^^^^^^^
io.UnsupportedOperation: not readable


But when i subsequently check the output directory, the file hihi.txt has been written.

I also tried it with a delay time.sleep(5) after the call to callHello() but the result was the same.

I then wrote a bash script which calls hello and subsequently reads the contents of the file: this worked as expected.

When (or how) can a file written by a subprocess be actually accessed in python?

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