Question on PyQt How to connect a signal to a slot to start a background operation in Python

Question

I have the following code that performs a background operation (scan_value) while updating a progress bar in the ui (progress). scan_value iterates over some value in obj, emitting a signal (value_changed) each time that the value is changed. For reasons which are not relevant here, I have to wrap this in an object (Scanner) in another thread. The Scanner is called when the a button scan is clicked. And here comes my question ... the following code works fine (i.e. the progress bar gets updated on time).

# I am copying only the relevant code here.

def update_progress_bar(new, old):
    fraction = (new - start) / (stop - start)
    progress.setValue(fraction * 100)

obj.value_changed.connect(update_progress_bar)

class Scanner(QObject):

    def scan(self):
        scan_value(start, stop, step)
        progress.setValue(100)

thread = QThread()
scanner = Scanner()
scanner.moveToThread(thread)
thread.start()

scan.clicked.connect(scanner.scan)

But if I change the last part to this:

thread = QThread()
scanner = Scanner()
scan.clicked.connect(scanner.scan) # This was at the end!
scanner.moveToThread(thread)
thread.start()

The progress bar gets updated only at the end (my guess is that everything is running on the same thread). Should it be irrelevant if I connect the signal to a slot before of after moving the object receiving object to the Thread.

Answer 1

It shouldn't matter whether the connection is made before or after moving the worker object to the other thread. To quote from the Qt docs:

Qt::AutoConnection - If the signal is emitted from a different thread than the receiving object, the signal is queued, behaving as Qt::QueuedConnection. Otherwise, the slot is invoked directly, behaving as Qt::DirectConnection. The type of connection is determined when the signal is emitted. [emphasis added]

So, as long as the type argument of connect is set to QtCore.Qt.AutoConnection (which is the default), Qt should ensure that signals are emitted in the appropriate way.