Qprocess exiting with exit code 255 when trying to run python script

Adithya

I am trying to create a Qprocess to run a python script and it is throwing error with exit code 255 .
Code :

int main(int argc, char *argv[])
{
QApplication a(argc, argv);

    // Path to the directory containing your Python script and other files
    QString workingDirectory = QDir::currentPath() + "/Tools/script";
    qDebug() << "Path = " <<workingDirectory;

    // Path to the Python script you want to execute
    QString pythonScript = QDir::currentPath() + "/Tools/script/main.py";

    // Arguments to pass to the Python script
    QStringList arguments;
    arguments << pythonScript << "1" << "2";

    // Create a QProcess instance
    QProcess process;

    // Set the working directory
    process.setWorkingDirectory(workingDirectory);

    // Set the command to execute
    process.start("python", arguments);

    if (!process.waitForStarted()) {
         qDebug() << "Failed to start Python process";
         //return 1;
     }

    // Wait for the process to finish (optional)
    process.waitForFinished();

    // Get the exit code of the process
    int exitCode = process.exitCode();

    // Handle the exit code if needed
    if (exitCode != 0) {
        qDebug() << "Python script execution failed with exit code:" << exitCode;
        //return 1;
    }
    return a.exec();

}

main.py

if name == 'main':
print("Hey there")

error:
Failed to start Python process
Python script execution failed with exit code: 255

Thanks for the help in advance

jsulm

@Adithya What does https://doc.qt.io/qt-6/qprocess.html#error return? On what platform are you? Is Python executable in PATH?

Adithya

@jsulm Hi , the start() exit code is 255 and I am not using execute() . I am trying this in ubuntu 22 qt c++ .Python interpreter is in the path /usr/bin/python3.exe

echo $PATH
/opt/ros/noetic/bin:/home/adithya/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin

So ideally python should take from PATH

Adithya

@jsulm I tried using QProcess::execute() aswell it is giving return value as -2

jsulm

@Adithya said in Qprocess exiting with exit code 255 when trying to run python script:

Can you please answer my first question?

"Python interpreter is in the path /usr/bin/python3.exe" - in your code you're using "python" not python3 - is there a "python" executable?

J.Hilk

@Adithya

Exit code 255 generally indicates that the program couldn't start. This can happen due to various reasons.

in this case, I think its a typical case of a wrong path.

currentPath() should be the location of your generated executable, not your source files.
Check that QDir::currentPath() + "/Tools/script/main.py" is actually a valid file and the correct file.

Adithya

@jsulm I dont have python.exe , but I have python3.exe .I changed process.start("python", arguments); to process.start("python3", arguments); aswell .I was not getting the log from py file but there was no 255 exit code . But -2 when I used QProcess::execute().

Adithya

Hi @J-Hilk , This is not the issue path. the py file preset in the specified path

jsulm

@Adithya I will ask for the third time (I will stop here if you do not answer): What does https://doc.qt.io/qt-6/qprocess.html#error return?

Adithya

@jsulm I am getting QProcess::error as "ERROR: QProcess::UnknownError"

jsulm

@Adithya Then maybe your Python script exits with -2?
You can also connect a slot to https://doc.qt.io/qt-6/qprocess.html#stateChanged and log state changes.
And you can also connect slots to https://doc.qt.io/qt-6/qprocess.html#readyReadStandardError and https://doc.qt.io/qt-6/qprocess.html#readyReadStandardOutput and print out stderr/stdout from your process there.

J.Hilk

@Adithya did you really check, or did you just type this answer ?

If I had a € for every time I heard "ThE PaTh Is CoRrEcT!!111eleven" and it turns out it's not, because currentPath() or applicationDirPath() etc. did not actually point to the assumed directory, I would have a surprising amount of money :D.

qDebug() << (QDir::currentPath() + "/Tools/script/main.py");

Adithya

@J-Hilk // Path to the Python script you want to execute
QString pythonScript = QDir::currentPath() + "/Tools/Extractor_OD/main_od_for_rci.py";
qDebug() << "Script Path = " <<pythonScript;

Log : Script Path = "/home/adithya/DVT/DVT-3.0/DistanceVerificationTool/Tools/Extractor_OD/main_od_for_rci.py"

adithya@adithya:~/DVT/DVT-3.0/DistanceVerificationTool/Tools/Extractor_OD$ pwd
/home/adithya/DVT/DVT-3.0/DistanceVerificationTool/Tools/Extractor_OD
adithya@adithya:~/DVT/DVT-3.0/DistanceVerificationTool/Tools/Extractor_OD$ ls
copy_script RCI_Front_LUT.h RCI_Rear_LUT.h src
main_od_for_rci.py RCI_Left_LUT.h RCI_Right_LUT.h

I reallt checked multiple time .The oath is not the issue . You are not gettiing the € this time :p

Adithya

@jsulm I tried using stateChanged something like this
QObject::connect(&process, &QProcess::stateChanged, [](QProcess::ProcessState newState) {
qDebug() << "Process = "<< newState;
});
Log :Process = QProcess::Starting
ERROR: QProcess::UnknownError
Process = QProcess::NotRunning
Failed to start Python process
Python script execution failed with exit code: 255

jsulm

@Adithya Please share your current code, so we can see what exactly you're trying now.
Also try with an absolute path to the Python executable to make sure that is not the issue.

JonB

@Adithya said in Qprocess exiting with exit code 255 when trying to run python script:

I am trying this in ubuntu 22 qt c++ .Python interpreter is in the path /usr/bin/python3.exe

You are under Ubuntu/Linux and the executable you want to run ends in .exe, right? Even though that is for Windows? So you can type /usr/bin/python3.exe in a terminal and that executes fine, right?

That aside. Since the Ubuntu executable is indeed python3 why are you trying to start a process named python?

#include <QCoreApplication>
#include <QDebug>
#include <QProcess>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QProcess process;
    process.start("python", QStringList());
    QObject::connect(&process, &QProcess::errorOccurred, &process, [&process]() { qDebug() << process.errorString();  } );
    if (!process.waitForStarted()) {
         qDebug() << "Failed to start Python process";
         return 1;
     }
    process.waitForFinished();
    int exitCode = process.exitCode();
    if (exitCode != 0) {
        qDebug() << "Python script execution failed with exit code:" << exitCode;
        return 1;
    }

    return a.exec();
}

shows me

"execvp: No such file or directory"
Failed to start Python process

So execvp() on python complains No such file or directory under Ubuntu 22.04, as I would expect.

Changing to process.start("python3", ...); produces no such error and runs the python3 interpreter....

Adithya

@jsulm
//trying it in main funtion itself for now , have commented everything else.
main.cpp
int main(int argc, char *argv[])
{
QApplication a(argc, argv);

// Path to the Python interpreter
QString pythonInterpreter = "/usr/bin/python3.exe"; // or the full path to python.exe

// Path to the directory containing your Python script and other files
QString workingDirectory = QDir::currentPath() + "/Tools/Extractor_OD";
qDebug() << "Path = " <<workingDirectory;

// Path to the Python script you want to execute
QString pythonScript = QDir::currentPath() + "/Tools/Extractor_OD/main_od_for_rci.py";
qDebug() << "Script Path = " <<pythonScript;

// Arguments to pass to the Python script
QStringList arguments;
arguments << pythonScript << "1" << "2";

QString command = pythonScript + " 1 2";

// Create a QProcess instance
QProcess process;

QObject::connect(&process, &QProcess::stateChanged, [](QProcess::ProcessState newState) {
           qDebug() << "Process = "<< newState;
   });

// Set the working directory
//process.setWorkingDirectory(workingDirectory);

// Set the command to execute
process.start(pythonInterpreter, arguments);

QProcess::ProcessError error = process.error();
qDebug() << "ERROR: " << error;

if (!process.waitForStarted()) {
     qDebug() << "Failed to start Python process";
     //return 1;
 }

// Wait for the process to finish (optional)
process.waitForFinished();

// Get the exit code of the process
int exitCode = process.exitCode();

// Handle the exit code if needed
if (exitCode != 0) {
    qDebug() << "Python script execution failed with exit code:" << exitCode;
    //return 1;
}
return a.exec();

}

//Have commented out everything just trying to log for now
main_od_for_rci.py
if name == 'main':
print("Hey there")

jsulm

@Adithya said in Qprocess exiting with exit code 255 when trying to run python script:

/usr/bin/python3.exe

This is for sure wrong.
You're on Linux, not Windows...

JonB

@Adithya said in Qprocess exiting with exit code 255 when trying to run python script:

QString pythonInterpreter = "/usr/bin/python3.exe"; // or the full path to python.exe

You read my answer where it said .exe is for Windows not Linux, didn't you? What happened when you tried my:

So you can type /usr/bin/python3.exe in a terminal and that executes fine, right?

In a terminal please copy & paste:

ls -l /usr/bin/python3.exe

and paste the output here.

Adithya

@JonB @jsulm using python3 actually worked .But not sure whether the script is being executed or not
I changed .py file something like this (running in infinite loop)

if name == 'main':
while True:
print("Hey there")

The process is still running
Log :
Process = QProcess::Starting
ERROR: QProcess::UnknownError
Process = QProcess::Running

But any idea why I am not getting the log .