QIODevice::read (QProcess): device not open

[FoxBandyKoot]

Hello guys !

I'm working on QtCreator and I want to use a python script with a xml file from QT Windows.

The file is open, but I can't use the script :

QString p_stdout = _process.readAll();

show me : QIODevice::read (QProcess): device not open

Here is my code :

// EXEC PYTHON SCRIPT


    QStringList args = (QStringList() << script_path << employesDesires << file_path) ; // parameters
    QString program = "Python";                                                         // program


    int exitCode = _process.execute( program, args );

    _process.waitForFinished(-1);

//      _process.open(QIODevice::OpenMode()) ;

    QString p_stdout = _process.readAll();

    QString s = QString::number(exitCode);

    ui->labelResult->setText(s);

exitCode return 0

I searched on many forums but can't find a solution that I can understand..Someone have an idea ?

[JonB]

@FoxBandyKoot
http://doc.qt.io/qt-5/qprocess.html#execute is static. It is supposed to be invoked via QProcess::execute(). You do not even show your declaration of _process, but assuming it's a QProcess instance you cannot use it to access anything from the static call.

You need to use the member function http://doc.qt.io/qt-5/qprocess.html#start on your QProcess instance.

[FoxBandyKoot]

Okey I don't know static notion, but I will try start() function.

QProcess _process;

Yes it is a Qprocess instance ! :)

Thanks you for help me !

[FoxBandyKoot]

So I tried many things since the last time...
I do not have the error anymore, but I can not recover the data from my python scrypt which processes an excel file.

That's where I am, I do not understand how to use signals despite the documentation...

.h file simplified

namespace Ui {
    class TabtStatEmp_CountWorkTogether;
}

class TabtStatEmp_CountWorkTogether : public QDialog
{
    Q_OBJECT

public:
    // constructor
    explicit TabtStatEmp_CountWorkTogether(QString file_path, QString script_path, QString employesDesires, QWidget *parent = nullptr);

    //functions
    QString OpenFile();

    QProcess::ProcessChannel StandardOutput;

    void setReadChannel(QProcess::ProcessChannel StandardOutput);


private:
    
    Ui::TabtStatEmp_CountWorkTogether *ui;

    QProcess _process;

};

My .cpp simplified :

TabtStatEmp_CountWorkTogether::TabtStatEmp_CountWorkTogether(QString file_path, QString script_path, QString employesDesires, QWidget *parent) :
    QDialog(parent),
    ui(new Ui::TabtStatEmp_CountWorkTogether)
{

    ui->setupUi(this);

// OPEN XLS FILE

    QFile xslFile(file_path);
    QString test   ;
    xslFile.open(QIODevice::ReadOnly | QIODevice::Text);


// EXEC PYTHON SCRIPT


    QStringList args = (QStringList() << script_path << employesDesires << file_path) ; // parameters
//    QStringList args = (QStringList() << employesDesires ) ; // parameters

    QString program = "Python";                                                         // program



    _process.start(program, args );

//   _process.readAllStandardOutput();

    _process.setReadChannel(StandardOutput);


    QString p_stdout = _process.readAllStandardOutput(); // get output


    ui->labelResult->setText(p_stdout); // display output
}

It is a simplified version where the python script know already path of xml file.

And the python script return :

# version with 1 parameter
def __main__(employesChoisis):
    total = Tabstats(employesChoisis)
    str(total)
    
    return total

[SGaist]

Hi,

That's because you're not waiting for your process to finish. See the Synchronous Process API part of QProcess's documentation.

[FoxBandyKoot]

So I used your link to place signals. Program start but now I have this error in debbuging mode :

This my code :

// OPEN XLS FILE

    QFile xslFile(file_path);
    xslFile.open(QIODevice::ReadOnly | QIODevice::Text);


// EXEC PYTHON SCRIPT


   // QStringList args = (QStringList() << script_path << employesDesires << file_path) ; // parameters
    QStringList args = (QStringList() << script_path << employesDesires ) ; // parameters

    QString program = "Python";                                                         // program

    QByteArray p_stdout;

    _process.start(program, args );                                 // START PROG

    if (_process.waitForStarted()){                                 // Check if python is running

        QMessageBox::warning(this,tr("A COMMENCE"), tr("A COMMENCE"));

        _process.setReadChannel(StandardOutput);                    // OPEN CHANNEL FOR READ


        p_stdout = _process.readAllStandardOutput();                // get output

    }

    _process.closeReadChannel(StandardOutput);                      // Stop read

    QMessageBox::warning(this,tr("A FINIT"), tr("A FINIT"));

//    QString AA = QString::QByteArray(p_stdout[0]); // Doesn't exist

     ui->labelResult->setText(p_stdout); // display output

    if (!_process.waitForFinished()){                               // If process crash, stop it
        return;
    }

This error happens when the following instruction is running :

p_stdout = _process.readAllStandardOutput();                // get output

[SGaist]

Can you run your application through the debugger to get more details about what is happening ?

[FoxBandyKoot]

I'm not sure about what you wanna see, so I take so captures :
BREAK POINT 1

BREAK POINT 2

AFTER BREAK POINT 2

[SGaist]

What does the script you launch do ?

[FoxBandyKoot]

It makes a count of how many times employes choosed have worked together, based on a xlsx file. Finally return this number (that I cast to string before)

Sorry for faults, I'm not english :/ And thanks you really for taking time to help me ^^

# version with 1 parameter
def __main__(employesChoisis):
    total = Tabstats(employesChoisis)
    str(total)
    
    return total

[JonB]

@FoxBandyKoot

Finally return this number (that I cast to string before)

This may not be relevant to whatever your problem is, but:

    total = Tabstats(employesChoisis)
    str(total)
    
    return total

This does not "cast the string before returning as a result". Python str() is a function returning a string. It does not change its argument. Your standalone str(total) line has no side-effects. Your function returns whatever type is returned from Tabstats(). If you need to force-convert return result to a string: return str(total).

[SGaist]

Do you have the same problem if you run a dummy python application ?

[FoxBandyKoot]

@JonB

Okay so I changed it, thanks you !

@SGaist
I created a test.py just for test :

def __main__(employesChoisis):
    
    
    return "TEST"

Same error :/

[SGaist]

And if you just start a normal command line tool like dir ?

[JonB]

@FoxBandyKoot
The problem is what I see in the debugger output window is happening while you try to run/debug your program, right? You could put messages in/tell us where exactly you receive the warnings as you step through the code....

Right, I am now peering at the screenshot code you are using.

1. Remove/comment out

 _process.setReadChannel(StandardOutput);                    // OPEN CHANNEL FOR READ
    _process.closeReadChannel(StandardOutput);                      // Stop read

I am hoping that will get rid of the nasty output?

2. Your parent program is intended to read whatever the sub-process sends to its stdout. Yours is sending nothing. You cannot "return a string" like "TEST" from any program, Python or otherwise (you can only return an integer). Your test.py should probably be going print("HELLO") (Python 3), for what you want.

[FoxBandyKoot]

@SGaist
I tried to launch "dir" but nothing is happening

@JonB
Yes that's it, when I debug I can see this error, on instruction readAllStandardOutput();
And the application crash...

If I'm not in debug mode, program continues... that's really weird
And when this instruction happens :

ui->labelResult->setText(p_stdout); // display output

Nothing appears. I don't understand why I should comment out functions which open and close channels... My true script return henceforth a integer :

You want me to show you the code that precedes the script?

[JonB]

@FoxBandyKoot
1.

I tried to launch "dir" but nothing is happening

How do you know "nothing is happening" from such a command? Since you're not collecting the output, what do you expect to see?

I don't understand why I should comment out functions which open and close channels...

Because I don't think you want them, and I don't think it should be done that way, period. I've never used it and had no problems. Why not try?

3. _process.readAllStandardOutput()

this function returns all data available from the standard output of the process

Since you have only done _process.waitForStarted(), how do you know there is any output yet available to read?

[FoxBandyKoot]

1. I thinked that I could see content of directory where the script is located in the pyzo interpreter :
   

2. So I tried to comment out these functions :

        _process.setReadChannel(StandardOutput);                    // OPEN CHANNEL FOR READ

        _process.closeReadChannel(StandardOutput);                      // Stop read

And the application doesn't crash anymore either in debug mode or not ! But the return value in "p_stdout" is null.

3. how do you know there is any output yet available to read?

In my new code I try to implement

waitForReadyRead and waitForBytesWritten but I don't pass into these functions, I don't know why :/

Only waitForStarted return true, so the first readAllStandardOutput happens but return nothing from true script or test script.

#include "ui_tabtstatemp_countworktogether.h"

#include <QMessageBox>
#include <QDebug>

TabtStatEmp_CountWorkTogether::TabtStatEmp_CountWorkTogether(QString file_path, QString script_path, QString employesDesires, QWidget *parent) :
    QDialog(parent),
    ui(new Ui::TabtStatEmp_CountWorkTogether)

{

    ui->setupUi(this);

// OPEN XLS FILE
    
    QFile xslFile(file_path);
    xslFile.open(QIODevice::ReadOnly | QIODevice::Text);


// EXEC PYTHON SCRIPT


   // QStringList args = (QStringList() << script_path << employesDesires << file_path) ; // parameters on final version
//    QStringList args = (QStringList() << script_path << employesDesires ) ; // second try with just one parameter
    QStringList args = (QStringList() << script_path << employesDesires ) ; // first try without parameter

    QString program = "Python";                                                         // program

    QString p_stdout;

    _process.start(program, args );                                 // START PROG

    if (_process.waitForStarted()){                                 // Check if python is running

        QMessageBox::warning(this,tr("Have started"), tr("Have started"));

//        _process.setReadChannel(StandardOutput);                    // OPEN CHANNEL FOR READ

        p_stdout = _process.readAllStandardOutput();                // get output

        if (_process.waitForReadyRead(10000)){                                 // 10 sec to check if data is ready
            QMessageBox::warning(this,tr("Data ready"), tr("Data ready"));

            p_stdout = _process.readAllStandardOutput();                // get output


            if (_process.waitForBytesWritten(10000)){     // 10 sec to check if data is written
                QMessageBox::warning(this,tr("Data written"), tr("Data written"));

            }
        }


    }

//    _process.closeReadChannel(StandardOutput);                      // CLOSE CHANNEL FOR READ

    QMessageBox::warning(this,tr("Have ended"), tr("Have ended"));

    ui->labelResult->setText(p_stdout); // display output

    if (!_process.waitForFinished()){                               // If process crash, stop it
        return;
    }
}




// DESTRUCTOR
TabtStatEmp_CountWorkTogether::~TabtStatEmp_CountWorkTogether()
{
    delete ui;
}

[FoxBandyKoot]

**My first problem has been resolved, so I will close this, and open a topic for my new problem.

For future people who come to this topic : The solution for "Device not open" was to use "start function" instead of "execute function".

Thank you people for helping me.**