cin >> input;  //input is a QString here
cout << input.toStdString() << flush;

proc = subprocess.Popen(["file.exe"], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, cwd=FULL_PATH)

for line in proc.stdout: print line

proc.stdin.write("My string")
proc.stdin.flush()
print proc.stdout.readline()