Callback between threads in python

Question

I am having a bit of trouble understanding callbacks. I have the following usecase:

From my gui i start a new thread called videoDown (which is its own class). From this thread i want to push the data back to the gui which i have implemented a callback system for. BUT to update the GUI i need to be in the GUI thread and not the thread i currently reside in (which is the videoDown thread).

Code where i start my thread (class Downloader):

def download_single(self, json_data):
    form_data = json.loads(json_data)
    print self.app
    url = form_data["name"]
    dt = form_data["dt"]  # Download type is audio or video
    videoDown = videoDownload(url, dt, self.dd,callback=self.cb,callback_args=("hello", "world",self.app))
    videoDown.start()

videoDownload thread:

class videoDownload(threading.Thread):
def __init__(self,url, dt, dd,callback=None, callback_args=None, *args, **kwargs):
    threading.Thread.__init__(self)
    self.callback = callback
    self.url = url
    self.dt = dt
    self.dd = dd
    self.callback_args = callback_args
    if self.callback is not None:
        self.callback(*self.callback_args)

def run(self):
    if self.url.__contains__("https://www.youtube.com/watch?v="):
        if self.dt == 'audio':
            self._downloadVid(self.url, vid=False)
        else:
            self._downloadVid(self.url, vid=True)
    else:
        print "Incorrect url"

def _downloadVid(self, url, vid=False, order_reverse=False, vinName=None):
    video = pafy.new(url)

    if self.callback is not None: //<--CALLBACK IN HERE
        self.callback(*self.callback_args)
    streams = video.allstreams
    for stream in streams:
        print stream

    print video
    name = u''.join(video.title).encode('utf8')
    name = re.sub("[<>:\"/\\|?*]", "", name)
    if not vid:
        file = video.getbestaudio()
    else:
        file = video.getbest()
    if (order_reverse):
        file.download(self.dd + vinName + name + ".mp4", quiet=False, callback=self.mycb)
    else:
        file.download(self.dd + name + ".mp4", quiet=False, callback=self.mycb)

Callback (also in class Downloader):

def cb(self,param1, param2,param3):
    print threading.current_thread()

How exactly do i implement it so i can give data from my video download thread back to the gui thread while the current thread is set to this video download thread.

What do i need to change, i have been struggling with this for hours.

~Greetings

Luke Smith · Accepted Answer · 2017-10-31 20:26:03Z

1

Maybe use a Queue instead of a callback. Initialise the queue in your main thread, pass the object to the download thread and call get() on the Queue object to block until an item is passed to it.

q = Queue.Queue()
d = ("hello", "world", self.app)
videoDown = videoDownload(url, dt, self.dd, queue=q, data=d)
returned_data = q.get()

In the videoDownload object pass the data back to the main thread via the Queue

if self.queue and self.data:
    self.queue.put(self.data)

answered Oct 31, 2017 at 20:26

Luke Smith

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8 Comments

Ellisan Over a year ago

Wont this block my GUI? And im trying to do it with callbacks cause i want to change stuff inside the gui with returned data.

Luke Smith Over a year ago

If you cannot block like this in the GUI thread you can call get(block=False) and poll for the returned data.

Ellisan Over a year ago

How do i poll for returned data

Luke Smith Over a year ago

Does your GUI have an event loop? If so get() should probably be called there. Either use a mechanism like select to allow you to wait on multiple events, or add a timeout to the wait on GUI events and poll the queue with q.get(block=False).

Ellisan Over a year ago

I use htmlpy so i honestly have no idea.

|

Ellisan · Accepted Answer · 2017-11-01 19:14:20Z

With a lot of help from luke. I came up with the following answer.

In my javascript i created the following function:

    function start() {
        Downloader.trydasd(); //ignore the nameconvention. We all try sometimes
        setTimeout(start, 3000);
    }

    start();

Which is implemented in python like this:

@htmlPy.Slot()
def trydasd(self):
    if not self.q.empty():
        print self.q.get(block=False)

After that I changed my init to create a queue like this:

    def __init__(self, app):
        super(Downloader, self).__init__()
        # Initialize the class here, if required.
        self.app = app
        self.q = Queue.Queue()

After that i changed my download_single function to this:

 def download_single(self, json_data):
    form_data = json.loads(json_data)
    print self.app
    url = form_data["name"]
    dt = form_data["dt"]  # Download type is audio or video
    d = ("hello", "world", self.app)
    videoDown = videoDownload(url, dt, self.dd, queue=self.q, data=d)
    videoDown.start()

And lastly my download thread is now this:

class videoDownload(threading.Thread):
def __init__(self,url, dt, dd,queue=None,data=None, *args, **kwargs):
    threading.Thread.__init__(self)
    self.queue = queue
    self.data = data
    self.url = url
    self.dt = dt
    self.dd = dd

def run(self):
    if self.url.__contains__("https://www.youtube.com/watch?v="):
        if self.dt == 'audio':
            self._downloadVid(self.url, vid=False)
        else:
            self._downloadVid(self.url, vid=True)
    else:
        print "Incorrect url"

def _downloadVid(self, url, vid=False, order_reverse=False, vinName=None):
    video = pafy.new(url)
    if self.queue and self.data:
        self.queue.put(video)

    name = u''.join(video.title).encode('utf8')
    name = re.sub("[<>:\"/\\|?*]", "", name)
    if not vid:
        file = video.getbestaudio()
    else:
        file = video.getbest()
    if (order_reverse):
        file.download(self.dd + vinName + name + ".mp4", quiet=False, callback=self.mycb)
    else:
        file.download(self.dd + name + ".mp4", quiet=False, callback=self.mycb)

This solved my issue, any other way does NOT work and will block the UI.

Thank you a lot luke!

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