Cannot open file - QProcess

Chaki

Hello, I am trying to start a python script with QProcess on my Raspberry. I have tried the docu, but the following code doesn't start my python file:
The python-file does only create a .txt-file and works if I run it.

communication.cpp:

#include "communication.h"
#include <QProcess>

communication::communication()
{
}
communication::~communication()
{
}

void communication::conPy()
{
    QProcess *process = new QProcess(this);
    QString program = "test.py";
    QString folder = "/home/myFolder/test.py";
    process->start(program, QStringList() << folder);```

communication.h:


#ifndef COMMUNICATION_H
#define COMMUNICATION_H

#include <QObject>

class communication : public QObject
{
    Q_OBJECT
public:
    communication();
    ~communication();
    void conPy();
};

#endif // COMMUNICATION_H

main.cpp:

#include <QCoreApplication>
#include <communication.h>

int main(int argc, char *argv[])
{
     QCoreApplication a(argc, argv);
     communication comm;
     comm.comPy();
     return a.exe();
}

test.py:

fp = open('test.txt', 'x')
fp.close()

jsulm

@Chaki said in Cannot open file - QProcess:

QString program = "test.py";
QString folder = "/home/myFolder/test.py";
process->start(program, QStringList() << folder);

Should rather be:

QString program = "/home/myFolder/test.py";
process->start(program, QStringList());

Don't forget to delete the QProcess instance when not needed anymore, else you will have a memory leak.

Chaki

Thank you for your answer, I know that it works with following code:

void communication::conPy()
{
    QProcess *process = new QProcess(this);
    QString program = "/home/someFolder/test.py";
    QStringList arguments;
    arguments << QDir::currentPath()<< "/home/someFolder/test.py";
    process->start("python", arguments);

    if(!process->waitForFinished(10000))
        {
            qDebug() << "Error: " <<process->errorString();
        }
        else
        {
            qDebug() << "Analyzing done. ";
        }

}

jsulm

@Chaki Is test.py marked as executable? Does it have the shebang line?

#/usr/bin/env python3

Can you start it directly from a terminal? Does that work?
Also, you should really add error handling, see QProcess documentation.

jsulm

@Chaki said in Cannot open file - QProcess:

arguments << QDir::currentPath()<< "/home/someFolder/test.py";

Why do you pass the path to the executable as parameter?!

JonB

@Chaki
Please first answer @jsulm's questions. If you have not set up your script file to be a properly executable Python file as he states that would be the reason for your problem.

However, additionally:

I know that it works with following code:

process->start("python", arguments);

I do not know about RPi, but some Linux distributions require you to use python3 as the executable. python may either run Python 2.x or does not have any such executable installed.

arguments << QDir::currentPath()<< "/home/someFolder/test.py";

You say that "it works with following code", but that should give you an error like:

/usr/bin/python3: can't find '__main__' module in '/the/QDir/currentPath/directory'

You are sure what you say works without this error, you did actually test it?

Chaki

@jsulm yes, i was able to start the python-script from a terminal. I posted the wrong code earlier - it works with the following lines, which I also tested:

 QProcess *process = new QProcess(this);
 process->start("python3 /home/someFolder/test.py");

    if(!process->waitForFinished(10000))
        {
            qDebug() << "Error: " <<process->errorString();
        }
        else
        {
            qDebug() << "Analyzing done. ";
        }

JonB

@Chaki
That is not what @jsulm is asking you. You have already said that you can run your script, even from QProcess, with python3 .... What you have asked is why you cannot execute it with just QString program = "/home/someFolder/test.py";.

If you want to know the answer to that, go into a terminal and type

/home/someFolder/test.py

and see what happens.

Chaki

@JonB I will have a look at it - thank you very much for your answers!

jsulm

@Chaki said in Cannot open file - QProcess:

process->start("python3 /home/someFolder/test.py");

This should be

process->start("python3", QStringList() << "/home/someFolder/test.py");