Multithreaded python application not hitting breakpoints in Visual Studio

Question

I am currently developing a PyQt application in Visual Studio. Debugging has been working great, until I decided to keep my UI responsive by moving stuff to a worker thread with Qt Threads.

class MainWindow(base, form):

    start_work = QtCore.pyqtSignal()

    def __init__(self, parent=None):
        # Create a seperate thread in which the update information is polled.
        self.thread = QtCore.QThread()
        # Create Worker object and move it to new thread
        self.worker = Worker()
        self.worker.moveToThread(self.thread)
        # connect signal to start work in the extra tread
        self.start_work.connect(self.worker.get_work_done)
        self.thread.start()

    #function emit a signal to start doing the work
    def do_work(self):
        self.startWork.emit()

Any function that is invoked on my worker object is connected via signal slots

class Worker(QtCore.QObject):
    @QtCore.pyqtSlot()
    def get_work_done(self):
        #lets do some time consuming work.

The code works fine. The only problem is now, I cannot debug anything that is happening inside get_work_done. Visual studio won't break at those breakpoints.

When I break inside any MainWindow function, the Visual Studio Debugger shows only one thread. It seems unaware of any other threads created by the application.

Answer 1

As said already all threads not created using Python need to be registered with PTVS. To do this for a QThread, call this in the threads run method:

import pydevd;pydevd.settrace(suspend=False)

Some more info here: Can I put break points on background threads in Python?

Answer 2

Debugger needs to play some tricks to detect new threads and set up its hooks (which are needed to hit breakpoints etc). It does so by hijacking the standard Python _thread module. If you're creating the threads in some way that circumvents that module altogether, which is what I suspect Qt does here, the debugger will not be aware of those threads.

Try using the standard threading module instead of QThread, and see if that helps.