QProcess will not open python script

Question

I cannot figure out why this does not work:

void Controller::on_buttonVisualTracking_clicked()
{
    QProcess *trackingProcess = new QProcess();
    trackingProcess->start("python C:\\visualTracking.py");
}

The specific script here is a pychart script, and if I simply run it from the command line, it executes correctly, opening up a window that displays the chart. That is something that I should emphasize, I'm expecting a new window to open displaying the pychart, which is what I get if I run the script myself.

I also tried this code to see if QProcess simply wasn't working. However this worked as expected and an empty notedpad window appeared.

void Controller::on_buttonVisualTracking_clicked()
{
    QProcess *trackingProcess = new QProcess();
    trackingProcess->start("notepad");
}

So then I thought maybe something is wrong with how I'm supplying arguments, so I tried this, which opened a notepad window with the visualTracking.py text as you would expect.

void Controller::on_buttonVisualTracking_clicked()
{
    QProcess *trackingProcess = new QProcess();
    trackingProcess->start("notepad C:\\visualTracking.py");
}

Thus, I'm completely at a loss. Why will QProcess not open up the python script?

UPDATE: Per suggestions I've now tried these two options, neither one worked.

void Controller::on_buttonVisualTracking_clicked()
{
    QString run = "C:\\Development\\Anaconda3\\python.exe";
    QStringList args;
    args << "C:\\visualTracking.py";

    QProcess *trackingProcess = new QProcess();
    trackingProcess->start(run, args);
}

and

void Controller::on_buttonVisualTracking_clicked()
{
    QString commands = "python C:\\visualTracking.py";

    QProcess *trackingProcess = new QProcess();
    trackingProcess->start("cmd");
    trackingProcess->write(commands.toLatin1().data());
    if(!trackingProcess->waitForStarted()){};
}

UPDATE: I thought I had a solution to this, but unfortunately I'm once again, on the development machine, unable to run python scripts again. I've no idea why the behavior changes randomly. My only guess is some windows security setting blocking my app from running a script, but I do not have any evidence to suggest this

score 0 · Answer 1 · answered May 08 '17 at 16:48

0

With QProcess, you can only start real executables, not scripts. Therefore, you need to run the python interpreter python.exe and give your script as an argument.

See this answer for an example on how to do that. You might need to specify the full path to python.exe to get it to work for you, for example "C:\\Python26\\python.exe".

answered May 08 '17 at 16:48

Thomas McGuire

5,308
26
45

this did not work either: `void Controller::on_buttonVisualTracking_clicked() { QString run = "C:\\Development\\Anaconda3\\python.exe"; QStringList args; args << "C:\\visualTracking.py"; QProcess *trackingProcess = new QProcess(); trackingProcess->start(run, args); }` – granitdev May 08 '17 at 18:33
Well, try seeing what went wrong by checking the error codes, like `QProcess::errorString()`. – Thomas McGuire May 08 '17 at 19:15
Codes I get are 1 then 2, though I'm not sure I'm checking these correctly, there doesn't appear to be a straightforward way to get these codes. – granitdev Jun 15 '17 at 16:09

score 0 · Answer 2 · answered May 08 '17 at 16:53

0

I see this problem.

But able to resolve by starting cmd first.

Also call ::waitForStarted to block until the process has started.

Check if it works for you (Details in comments).

    //YOUR PY COMMAND    
    QString pyCommand("python C:\\visualTracking.py \n"); //try with out " \n" also...

    //CREATE A PROCESS OBJECT
    QProcess *qprocess = new QProcess(this);

    //START THE CMD
    qprocess->start("cmd");

    //WRITE YOUR PY COMMAND TO PROCESS
    qprocess->write(pyCommand.toLatin1().data());

    //BLOCK THE PROCESS UNTILL IT STARTED
    if (!qprocess->waitForStarted())
    {


    }

answered May 08 '17 at 16:53

Pavan Chandaka

11,671
5
26
34

I tried this one first as it looked easier to impliment, but this did not work either `void Controller::on_buttonVisualTracking_clicked() { QString commands = "python C:\\visualTracking.py \n"; QProcess *trackingProcess = new QProcess(); trackingProcess->start("cmd"); trackingProcess->write(commands.toLatin1().data()); if(!trackingProcess->waitForStarted()); }` – granitdev May 08 '17 at 18:12
Hmmm.. I did not see anything wrong. Do you have the python executable path in "PATH" environment variable. – Pavan Chandaka May 08 '17 at 18:32
Yes, it points to the anaconda folder, and if I double click any .py file it runs it without issue. Also from a command prompt "python anyscript.py" will also run without issue. – granitdev May 08 '17 at 18:35

Richard · Answer 3 · 2018-02-11T06:09:28.030

0

Are you using Qt5.8.0 MinGW version? I encountered the same issue and after switching to the Qt5.8.0 MSVC version everything worked just fine.

I haven't tried other version yet, but I think this may be the problem. Hope this help

UPDATE
I tried the QT 5.10.0 MinGW version, the bug still exist. However, using the gcc and g++ of manual installed MinGW won't have the same problem. I guess it's because the MinGW version shipped with the Qt is probably too old?

edited Feb 11 '18 at 06:09

answered Jan 25 '18 at 04:53

Richard

25
2
8

to be honest, I'm not sure, how do I check this? – granitdev Feb 06 '18 at 21:09
@granitdev Are you using windows visual studio? if so, then it's Qt MSVC. If you're using Qt creator as your IDE then check your build setting. – Richard Feb 11 '18 at 06:07

score 0 · Answer 4 · answered May 03 '18 at 18:33

I've had a similar problem with an interactive script; the solution was to force it to run interactively:

auto *process = new QProcess{this};
connect(process, &QProcess::errorOccurred, []{
  qFatal("process error occurred");
});
process->start("python", {"-i", "myscript.py"});

QProcess will not open python script

4 Answers4