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

0 votes

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)


class Scanner(QObject):

    def scan(self):
        scan_value(start, stop, step)

thread = QThread()
scanner = Scanner()


But if I change the last part to this:

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

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.

Nov 27, 2018 in Python by Anirudh
• 2,050 points

1 answer to this question.

Your answer

Your name to display (optional):
Privacy: Your email address will only be used for sending these notifications.
0 votes

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::DirectConnectionThe 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.

answered Nov 27, 2018 by Nymeria
• 3,500 points

Related Questions In Python

0 votes
2 answers

How do I connect to a MySQL Database in Python?

connect mysql database with python import MySQLdb db = ...READ MORE

answered Mar 28 in Python by rajesh
• 1,210 points
0 votes
1 answer

How to add a new line in Python?

You can use '\n' for a next ...READ MORE

answered May 2, 2018 in Python by aayushi
• 750 points
0 votes
2 answers

How to calculate square root of a number in python?

calculate square root in python >>> import math ...READ MORE

answered Apr 2 in Python by anonymous
0 votes
1 answer

How to print first character of each word in upper case of a string in Python

You can use split() using space as ...READ MORE

answered Jun 1, 2018 in Python by george
• 200 points
0 votes
1 answer

how can i count the items in a list?

suppose you have a list a = [0,1,2,3,4,5,6,7,8,9,10] now ...READ MORE

answered May 2 in Python by Mohammad
• 1,400 points
+4 votes
6 answers
0 votes
1 answer

How do you add a background thread to flask in Python?

The example below creates a background thread ...READ MORE

answered Nov 19, 2018 in Python by Nymeria
• 3,500 points
0 votes
1 answer

List comprehension on a nested list - How to do it in Python?

Not sure what your desired output is, ...READ MORE

answered Nov 21, 2018 in Python by Nymeria
• 3,500 points

© 2018 Brain4ce Education Solutions Pvt. Ltd. All rights Reserved.
"PMP®","PMI®", "PMI-ACP®" and "PMBOK®" are registered marks of the Project Management Institute, Inc. MongoDB®, Mongo and the leaf logo are the registered trademarks of MongoDB, Inc.