Cannot subclass multiprocessing Queue in Python 3.5

My eventual goal is to redirect the `stdout` from several subprocesses to some queues, and print those out somewhere (maybe in a little GUI). The first step is to subclass `Queue` into an object that behaves much like the `stdout`. But that is where I got stuck. Subclassing the multiprocessing `Queue` seems impossible in Python v3.5. ``` # This is a Queue that behaves like stdout # Unfortunately, doesn't work in Python 3.5 :-( class StdoutQueue(Queue): def __init__(self,*args,**kwargs): Queue.__init__(self,*args,**kwargs, ctx='') def write(self,msg): self.put(msg) def flush(self): sys.__stdout__.flush() ``` I found this snippet in the following post (probably Python 3.5 did not yet exist at that moment): Python multiprocessing redirect stdout of a child process to a Tkinter Text In Python v3.5 you stumble on strange error messages when subclassing the multiprocessing `Queue` class. I found two bug reports describing the issue: https://bugs.python.org/issue21367 https://bugs.python.org/issue19895 I have 2 questions: - Suppose I want to stick to Python v3.5 - going to a previous version is not really an option. What workaround can I use to subclass the multiprocessing Queue somehow? - Is the bug still around if I upgrade to Python v3.6? EDIT : There is a known issue when you try to subclass the `Queue` class found in here: ``` from multiprocessing import Queue # <- known issue: you cannot subclass # this Queue class, because it is # not a genuine python class. ``` But the following should work: ``` from multiprocessing.queues import Queue # <- from this Queue class, you # should be able to make a # subclass. But Python 3.5 # refuses :-( ``` Sadly, even that doesn't work in Python v3.5. You get the following error: ``` C:\Users\..\myFolder > python myTest.py Traceback (most recent call last): File "myTest.py", line 49, in <module> q = StdoutQueue() File "myTest.py", line 22, in __init__ super(StdoutQueue,self).__init__(*args,**kwargs) TypeError: __init__() missing 1 required keyword-only argument: 'ctx' ``` EDIT : Thank you Darth Kotik for solving the problem! Here is the complete code, updated with his solution. Now it works. ``` import sys import time import multiprocessing as mp import multiprocessing.queues as mpq from threading import Thread from tkinter import * '''-------------------------------------------------------------------''' ''' SUBCLASSING THE MULTIPROCESSING QUEUE ''' ''' ''' ''' ..and make it behave as a general stdout io ''' '''-------------------------------------------------------------------''' # The StdoutQueue is a Queue that behaves like stdout. # We will subclass the Queue class from the multiprocessing package # and give it the typical stdout functions. # # (1) First issue # Subclassing multiprocessing.Queue or multiprocessing.SimpleQueue # will not work, because these classes are not genuine # python classes. # Therefore, you need to subclass multiprocessing.queues.Queue or # multiprocessing.queues.SimpleQueue . This issue is known, and is not # the reason for asking this question. But I mention it here, for # completeness. # # (2) Second issue # There is another problem that arises only in Python V5 (and beyond). # When subclassing multiprocessing.queues.Queue, you have to provide # a 'multiprocessing context'. Not doing that, leads to an obscure error # message, which is in fact the main topic of this question. Darth Kotik # solved it. # His solution is visible in this code: class StdoutQueue(mpq.Queue): def __init__(self,*args,**kwargs): ctx = mp.get_context() super(StdoutQueue, self).__init__(*args, **kwargs, ctx=ctx) def write(self,msg): self.put(msg) def flush(self): sys.__stdout__.flush() '''-------------------------------------------------------------------''' ''' TEST SETUP ''' '''-------------------------------------------------------------------''' # This function takes the text widget and a queue as inputs. # It functions by waiting on new data entering the queue, when it # finds new data it will insert it into the text widget. def text_catcher(text_widget,queue): while True: text_widget.insert(END, queue.get()) def test_child(q): # This line only redirects stdout inside the current process sys.stdout = q # or sys.stdout = sys.__stdout__ if you want to print the child to the terminal print('child running') def test_parent(q): # Again this only redirects inside the current (main) process # commenting this like out will cause only the child to write to the widget sys.stdout = q print('parent running') time.sleep(0.5) mp.Process(target=test_child,args=(q,)).start() if __name__ == '__main__': gui_root = Tk() gui_txt = Text(gui_root) gui_txt.pack() q = StdoutQueue() gui_btn = Button(gui_root, text='Test', command=lambda:test_parent(q),) gui_btn.pack() # Instantiate and start the text monitor monitor = Thread(target=text_catcher,args=(gui_txt,q)) monitor.daemon = True monitor.start() gui_root.mainloop() ```