CODEWITHSUNDEEP
pythonmultithreadingpyqtpyside
tjmgis
tjmgis

Reputation: 1661

How to pass variable to QThread (PySide or PyQT)

I am really struggling to finish writing a GUI app which requires several different threads. At a high level I require:

A GUI Thread, which has a button to open a directory browser which the user selects a directory. On selecting the directory a thread starts which looks for a particular file type. This may take a long time so I know I need to put in a separate QThread.

Then once that browseFile thread has finished searching for files it returns a fileList, which is then chunked into sub fileLists. Each subfileList is then sent to a separate thread for processing which will take lots of time.

This is what I have coded so far:

from PyQt4 import QtCore
from PyQt4 import QtGui
from PyQt4.QtCore import * 
import os

class BrowseThread(QThread):
    processdone = QtCore.pyqtSignal("QString") # Define custom signal.

    def __init__(self, parent, *args, **kw):
        QThread.__init__(self, parent)
        self.search(*args, **kw)

    def search(self, directory_path):
        self.fileList = []
        self.count = 0
        for dirname, dirnames, filenames in os.walk(directory_path):
            for filename in filenames:
                if filename.endswith(".gz"):
                    self.fileList.append(os.path.join(directory_path,filename))

        self.emit( SIGNAL('processdone'), "DONE")

        return     

class MyClass(QObject):

    def __init__(self):            
        super(MyClass, self).__init__()

        directory_path = r'C:\Data'

        thread1 = BrowseThread(self, directory_path) 
        self.connect( thread1, SIGNAL("processdone"), self.thread1done)  
        thread1.start()  

    def thread1done(self, text):
        print(text)
        sys.exit()

if __name__ == "__main__":
    import sys
    app = QtGui.QApplication(sys.argv)  
    a = MyClass()
    sys.exit(app.exec_())

Is there a better way to pass the directory path to the browseThread than using *args, **kw?

How can I return the fileList back to the main thread which I can then pass to a number of new processing threads.

I am sure I making this harder than it should be so hopefully someone can help me

thanks

Upvotes: 0

Views: 3721

Answers (2)

JeffCharter
JeffCharter

Reputation: 1549

You should be able to use QT's signal and slot mechanism.

http://doc.qt.io/qt-5/threads-qobject.html#signals-and-slots-across-threads

You can also use a message passing system.

There is nothing wrong with passing it as an arg either. Just make sure you do it safely: https://web.archive.org/web/20150423115746/http://effbot.org/zone/thread-synchronization.htm

Upvotes: 0

Kirell
Kirell

Reputation: 9798

I suggest to change the design of your code to a solution with a worker class which does the real work. This worker class will be moved to a normal thread like so (I don't know PyQt but it should be fairly similar to the C++ version):

class SearchWorker(QObject):
    finished_sig = QtCore.pyqtSignal('')  # finished signal

    def __init__(self, your_stuff):
        self.directory_path = ...

    def process_slot(self):
        # use self.directory_path
        # your code
        self.emit(SIGNAL('finished_sig'), '')

Now the main part is to connect the start signal to your worker in your class MyClass. Which gives something like this in C++:

QThread* thread = new QThread;
Worker* worker = new Worker();
worker->moveToThread(thread);
// The start signal will actually launch your search method in the worker
connect(thread, SIGNAL(started()), worker, SLOT(process_slot()));
connect(worker, SIGNAL(finished()), thread, SLOT(quit()));

// May be useless in python
connect(worker, SIGNAL(finished_sig()), worker, SLOT(deleteLater()));
connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
thread->start();

The good point is that you can use the worker with or without a thread and you can easily pass parameters to the worker via setter methods.

To know more about Qt signals and slots for threads I suggest you to read:

Upvotes: 1

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