[Interest] QThreadPool functionality for QThreads

Elvis Stansvik elvstone at gmail.com
Wed Jan 18 20:12:28 CET 2017

2017-01-18 20:11 GMT+01:00 Elvis Stansvik <elvstone at gmail.com>:
> 2017-01-18 20:04 GMT+01:00 Elvis Stansvik <elvstone at gmail.com>:
>> 2017-01-18 8:17 GMT+01:00 Frank Rueter | OHUfx <frank at ohufx.com>:
>>> Hi Thiago,
>>> thanks for your quick reply. I will try and give some more context:
>>> I use the python requests module inside my PySide app to post requests to a
>>> website. Some of those requests return a lot of data that I need to parse to
>>> be able to show progress, other requests are file downloads that I need
>>> progress bars for as they stream onto disk.
>>> I had tried to not use threads and use QApplication.processEvents() for each
>>> data chunk downloaded, but that made the download about 4-5 times slower.
>>> Introducing threading made a huge difference.
>> Right, you won't get a good result with that approach. It's always
>> good to be up front with any special circumstances like this (using a
>> networking API that does not run on top of the Qt event loop).
>>> My app can download a list of files at the same time. Depending on the
>>> situation and the user request, the list of files to be downloaded can
>>> happen asynchronously, in other situations they need to be downloaded one
>>> after the other.
>> I think this is what confused Thiago: What you mean is sequential (as
>> opposed to parallell), not synchronous.
>> If you have one QThread that depends on the completion of another,
>> then no, I don't think there's a convenient API in Qt to express that
>> relationship and run the threads infrom sequence. You'll have to string
>> them together yourself, or maybe use something else like Threadweaver.
>> I could be wrong of course :)
> Another approach is of course to ditch the threading and use the
> asyncronous QNetworkAccessManager from Qt, instead of using the Python
> requests module. You can use the downloadProgress/uploadProgress of

downloadProgress/uploadProgress *signals*.

> the QNetworkReply you get from get(..) or post(..) if you want to show
> progress (haven't used it myself).
> Elvis
>> Elvis
>>>>>All I can tell you is that you don't need to do what you're trying to do,
>>>>> since you don't need threads in the first place.
>>> If I can avoid threads to do the above I would be more than happy to adjust
>>> and get rid of them again, but I haven't managed to find a non-threaded
>>> approach that doesn't slow down the download significantly.
>>> Cheers,
>>> frank
>>> On 18/01/17 6:26 PM, Thiago Macieira wrote:
>>>> On quarta-feira, 18 de janeiro de 2017 17:21:46 PST Frank Rueter | OHUfx
>>>> wrote:
>>>>> Hi,
>>>>> I got another threading question for the pros out there:
>>>>> In  my current application I am using QThread objects and
>>>>> QObject.moveToThread() to enable my GUI to download multiple files while
>>>>> updating progress bars in the main event loop. This is the respective
>>>> As usual, the usual disclaimer: you do not need threads to do networking
>>>> or
>>>> file I/O. The combined overhead of the networking I/O and saving of the
>>>> files is
>>>> unlikely to overwhelm the event loop to the point that the progress bar
>>>> can't
>>>> update smoothly.
>>>> I'm not saying impossible, but it's unlikely.
>>>>> snippet of code:
>>>>>           self.worker = MyClass()
>>>>>           self.workerThread = QtCore.QThread()
>>>>>           self.worker.moveToThread(self.workerThread)
>>>>> The trouble is when the user wants to download multiple files at once.
>>>>> In my current implementation that all works fine and I see multiple
>>>>> progress bars do there thing.
>>>>> However, there are cases when I need to force the download threads to be
>>>>> synchronous. I had hoped that I can use QThreadPool with QThreads, but
>>>>> turns out I need QRunnables in this case, and those don't have the same
>>>>> signals as QThread.
>>>> Why do you need to force them to be synchronous? And synchronous with
>>>> what?
>>>> With each other? Or do you mean sync() in the file saving?
>>>> Finally, what does being synchronous have to do with signals?
>>>>> So my question is:
>>>>> Is there a good way to use QThreads in a queue which is controlled by
>>>>> the main thread, or should I re-write my code and subclass QRunnable to
>>>>> add the signals I need (i.e. the ones that QThread has by default)?
>>>> The whole point of QThread is that the code it runs is independent of
>>>> anything
>>>> else. Only the OS scheduler decides when it's time to run it or suspend
>>>> it.
>>>>> In the latter case I guess I'd have to inherit from both QObject and
>>>>> QRunnable, is this ok?
>>>> Right.
>>>> But we still don't understand what you're trying to do. All I can tell you
>>>> is
>>>> that you don't need to do what you're trying to do, since you don't need
>>>> threads in the first place.
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