Qprocess not working on Windows (works fine on Linux and OSX)

Dost · 16 Apr 2022, 09:36

I am trying to spawn a subprocess from my main programm using the QProcess module. This process should send information to the main programm. My code works fine for Linux and OSX but doesn't work on Windows. It seems like the commands for executing the the process aren't even read since a wrong path does not trigger any Error messages.

I have already asked this question on stackoverflow but so far no suggestion worked out.
link: https://stackoverflow.com/questions/71881833/qprocess-wont-start-process-on-windows-but-works-on-linux-and-osx?noredirect=1#comment127039137_71881833

here my code for the main program:

import sys
from PyQt5 import QtCore, QtWidgets, QtGui
from PyQt5 import uic
import pickle

class MyGuiApp(QtWidgets.QMainWindow):
    def __init__(self):
        QtWidgets.QMainWindow.__init__(self)
        self.ui = uic.loadUi('process_gui.ui', self)

        
        # Set puchButtons
        self.stop_pB.setEnabled(False)
        self.start_pB.clicked.connect(self.start_measurement)
        self.stop_pB.clicked.connect(self.stop_measurement)

        # Initialising Subprocess
        self.my_process = QtCore.QProcess()
        self.my_process.readyReadStandardOutput.connect(self.new_data)

    def start_measurement(self):
        self.stop_pB.setEnabled(True)
        self.start_pB.setEnabled(False)

        self.my_process.start('python3', ['C:Users/me/desktop/dummyfolder/main_subprocess.py'])

    def stop_measurement(self):
        self.stop_pB.setEnabled(False)
        self.start_pB.setEnabled(True)

        self.my_process.terminate()

    def new_data(self):
        data = self.my_process.readAllStandardOutput()
        # unpickle data -> string
        stdout_str = pickle.loads(data)

        self.label.setText(stdout_str)


if __name__=='__main__':
    app = QtWidgets.QApplication(sys.argv)
    mainWindow = MyGuiApp()
    mainWindow.show()
    sys.exit(app.exec_())

here the code for my subprocess:

 import time
   import sys
   import pickle

   index = 0
   while True:
      start = time.time()

      sys.stdout.buffer.write(pickle.dumps(str(index)))
      sys.stdout.flush()

      index = index + 1
      time.sleep(0.5-(time.time()-start)) # sets loop runtime to 0.5 secs

and in case someone wants to run the code here the .ui code:

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>800</width>
    <height>600</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralwidget">
   <layout class="QGridLayout" name="gridLayout_2">
    <item row="0" column="0">
     <widget class="QGroupBox" name="groupBox">
      <property name="title">
       <string>GroupBox</string>
      </property>
      <layout class="QGridLayout" name="gridLayout">
       <item row="0" column="0">
        <layout class="QVBoxLayout" name="verticalLayout">
         <item>
          <widget class="QPushButton" name="start_pB">
           <property name="text">
            <string>Start</string>
           </property>
          </widget>
         </item>
         <item>
          <widget class="QPushButton" name="stop_pB">
           <property name="text">
            <string>Stop</string>
           </property>
          </widget>
         </item>
         <item>
          <widget class="QLabel" name="label">
           <property name="font">
            <font>
             <pointsize>20</pointsize>
            </font>
           </property>
           <property name="text">
            <string>DATA</string>
           </property>
          </widget>
         </item>
        </layout>
       </item>
      </layout>
     </widget>
    </item>
   </layout>
  </widget>
  <widget class="QMenuBar" name="menubar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>800</width>
     <height>22</height>
    </rect>
   </property>
  </widget>
  <widget class="QStatusBar" name="statusbar"/>
 </widget>
 <resources/>
 <connections/>
</ui>

JonB · replied to Dost on 16 Apr 2022, 09:36

@Dost said in Qprocess not working on Windows (works fine on Linux and OSX):

'C:Users/me/desktop/dummyfolder/main_subprocess.py'

This is not an absolute path, and doubtless not what you intended, so hardly surprising.

If you had read from the QProcess standard error, and checked for an error, as you should always do, you would doubtless have seen the appropriate error message saying "it cannot be found". Before you say it's not the path but something else, connect to QProcess::errorOccurred() & readyReadStandardError(); if still unclear, connect to started(), stateChanged() & finished().

Put a print("Reached\n") in start_measurement(). And create a file somewhere as the first statement in your .py subprocess script, so that we know whether it has actually run or not rather than relying on whether you think you see any output from it.

Dost · 17 Apr 2022, 06:32

Thank for your awnser. Sadly i was slopy when editing the path, in my program i am actually using an absolute path. I also did the trick with the file already so i knew that the subprocess didn't even start.

I added what you have suggested now, with the result: error code 0 (The process failed to start. Either the invoked program is missing, or you may have insufficient permissions to invoke the program.)

I double checked the file path so there should be no problem with that. Concerning the insufficient permissions, what to do about that?

updated code:

import sys
from PyQt5 import QtCore, QtWidgets
from PyQt5 import uic
import pickle


class MyGuiApp(QtWidgets.QMainWindow):
    def __init__(self):
        print('too far')
        QtWidgets.QMainWindow.__init__(self)
        self.ui = uic.loadUi('process_gui.ui', self)

        # Settings pushButtons
        self.stop_pB.setEnabled(False)
        self.start_pB.clicked.connect(self.start_measurement)
        self.stop_pB.clicked.connect(self.stop_measurement)

        # Initialising Subprocess
        self.my_process = QtCore.QProcess()
        self.my_process.readyReadStandardOutput.connect(self.new_data)
        self.my_process.readyReadStandardError.connect(self.process_error)
        self.my_process.started.connect(self.started)
        self.my_process.errorOccurred.connect(self.error)
        # tried another way, both didn't work
        self.my_process.setProgram('python3')
        self.my_process.setArguments([r'C:\Users\Me\Desktop\dummyfolder\main_subprocess.py'])
        print(str(self.my_process.state()), 'State (init)')

    def start_measurement(self):
        self.stop_pB.setEnabled(True)
        self.start_pB.setEnabled(False)


        #self.my_process.start('python3', [r'C:\Users\Me\Desktop\dummyfolder\main_subprocess.py'])
        self.my_process.start()
        print(str(self.my_process.state()), 'State (start_measurement)')

        print('reached')

    def stop_measurement(self):
        self.stop_pB.setEnabled(False)
        self.start_pB.setEnabled(True)

        self.my_process.terminate()

    def new_data(self):
        data = self.my_process.readAllStandardOutput()
        # unpickle data -> string
        stdout_str = pickle.loads(data)

        self.label.setText(stdout_str)

    def process_error(self):
        error = self.my_process.readAllStandardError()
        print(error)

    def started(self):
        print('process started')

    def error(self):
        print('error occurred {}'.format(self.my_process.error()))



if __name__ == '__main__':
    app = QtWidgets.QApplication(sys.argv)
    mainWindow = MyGuiApp()
    mainWindow.show()
    sys.exit(app.exec_())

JonB · replied to Dost on 17 Apr 2022, 06:32

@Dost said in Qprocess not working on Windows (works fine on Linux and OSX):

I added what you have suggested now, with the result: error code 0 (The process failed to start. Either the invoked program is missing, or you may have insufficient permissions to invoke the program.)

Well whatever the reason that seems to be the cause if you are saying that is the error text it reports.

I am not clear from what you write where that message came from? From process_error()? From error()? Did you get any message from started()?

The "invoked program" in this case is python3, not your .py script file argument to it.

Let's start by removing the script. I believe that, say, python3 -V just reports some stuff and exits. Try that, or something similar. Does it run, do you get its output? Assuming not, we know where we are. Are you sure python3 is in some directory on your PATH when invoked from your Qt app? Can it find whatever DLLs it needs correctly from that PATH? Where is python3, is it a .exe or a .bat file? Does it help to give the full path to the desired python3.exe instead of just your plain python3 to run it?

Go find something completely different from python3 to execute, and verify that does work as expected.

Dost · 18 Apr 2022, 06:00

Again thank you for your answer. As you already assumed the problem was that i had to add the path leading to the python.exe.

The error was triggered by errorConnect and there was no message from started().

A new problem occured as the Qprocess wont stop when terminate() (state stays 2 before and after the call) is called, but it works with kill() for me.

JonB · replied to Dost on 18 Apr 2022, 06:00

@Dost
Good for the PATH issue. Consider putting python3 on your PATH rather than having to typing the full path to it.

A new problem occured as the Qprocess wont stop when terminate() (state stays 2 before and after the call) is called, but it works with kill() for me.

This is usual for Windows, for any command-line program like python3:

Console applications on Windows that do not run an event loop, or whose event loop does not handle the WM_CLOSE message, can only be terminated by calling kill().

Really it's naughty to need to use either terminate() or particularly kill(). Presumably your Python script runs forever and does not have a nice means to exit it.