TypeError: cannot pickle 'weakref' object

Question

Quite new to multiprocessing here. I have a code that runs two processes. One to continuously receive data blocks from the server and put it inside a queue and the other to remove the data blocks from the queue and process it.

Below is my client code:

import socket
import turtle
import multiprocessing
from multiprocessing import Process, Queue
from tkinter import *


class GUI:

    def __init__(self, master):
        rec_data = recv_data()
        self.master = master
        master.title("Collision Detection")

        self.input_label = Label(root, text="Input all the gratings set straight wavelength values in nm")
        self.input_label.grid(row=0)

        self.core_string = "Core "
        self.entries = []


        self.label_col_inc = 0
        self.entry_col_inc = 1
        self.core_range = range(1, 5)

        for y in self.core_range:
            self.core_text = self.core_string + str(y) + '_' + '25'
            self.core_label = Label(root, text=self.core_text)
            self.entry = Entry(root)
            self.core_label.grid(row=1, column=self.label_col_inc, sticky=E)
            self.entry.grid(row=1, column=self.entry_col_inc)
            self.entries.append(self.entry)
            self.label_col_inc += 2
            self.entry_col_inc += 2

        self.threshold_label = Label(root, text="Threshold in nm")
        self.entry_threshold = Entry(root)

        self.threshold_label.grid(row=2, sticky=E)
        self.entry_threshold.grid(row=2, column=1)

        self.light_label = Label(root, text='Status')
        self.light_label.grid(row=3, column=3)

        self.canvas = Canvas(root, width=150, height=50)
        self.canvas.grid(row=4, column=3)

        # Green light
        self.green_light = turtle.RawTurtle(self.canvas)
        self.green_light.shape('circle')
        self.green_light.color('grey')
        self.green_light.penup()
        self.green_light.goto(0, 0)

        # Red light
        self.red_light = turtle.RawTurtle(self.canvas)
        self.red_light.shape('circle')
        self.red_light.color('grey')
        self.red_light.penup()
        self.red_light.goto(40, 0)

        self.data_button = Button(root, text="Get data above threshold", command=rec_data.getData)
        self.data_button.grid(row=5, column=0)


class recv_data:
    def __init__(self):
        self.buff_data = multiprocessing.Queue()
        self.p1 = multiprocessing.Process(target=self.recvData)
        self.p2 = multiprocessing.Process(target=self.calculate_threshold)
        self.host = '127.0.0.1'
        self.port = 5001
        self.s = socket.socket()
        self.s.connect((self.host, self.port))

    # function to receive TCP data blocks
    def getData(self):
        len_message = self.s.recv(4)
        bytes_length = int(len_message.decode('utf-8'))  # for the self-made server
        recvd_data = self.s.recv(bytes_length)
        self.buff_data.put(recvd_data)
        self.p1.start()
        self.p2.start()

        self.p1.join()
        self.p2.join()

    def recvData(self):
        len_message = self.s.recv(4)
        while len_message:
            bytes_length = int(len_message.decode('utf-8'))  # for the self-made server
            recvd_data = self.s.recv(bytes_length)
            self.buff_data.put(recvd_data)
            len_message = self.s.recv(4)

        else:
            print('out of loop')
        self.s.close()

    def calculate_threshold(self):
        rmv_data = self.buff_data.get()
        stringdata = rmv_data.decode('utf-8')
        rep_str = stringdata.replace(",", ".")
        splitstr = rep_str.split()

        # received wavelength values
        inc = 34
        wav_threshold = []
        for y in gui.entries:
            straight_wav = float(y.get())
            wav = float(splitstr[inc])
            wav_diff = wav - straight_wav
            if wav_diff < 0:
                wav_diff = wav_diff * (-1)
            wav_threshold.append(wav_diff)
            inc += 56

        threshold = float(gui.entry_threshold.get())

        for x in wav_threshold:
            if (x > threshold):
                gui.red_light.color('red')
                gui.green_light.color('grey')

            else:
                gui.red_light.color('grey')
                gui.green_light.color('green')
    

        # function to write into the file

    def write_file(self, data):
        with open("Output.txt", "a") as text_file:
            text_file.write('\t'.join(data[0:]))
            text_file.write('\n')

if __name__ == '__main__':
    root = Tk()
    gui1 = GUI(root)

    root.mainloop()

The error I get is shown below:

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Users\AppData\Local\Programs\Python\Python38-32\lib\tkinter\__init__.py", line 1883, in __call__
    return self.func(*args)
  File "C:/Users/PycharmProjects/GUI/GUI_multiprocess.py", line 85, in getData
    self.p2.start()
  File "C:\Users\AppData\Local\Programs\Python\Python38-32\lib\multiprocessing\process.py", line 121, in start
    self._popen = self._Popen(self)
  File "C:\Users\AppData\Local\Programs\Python\Python38-32\lib\multiprocessing\context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "C:\Users\AppData\Local\Programs\Python\Python38-32\lib\multiprocessing\context.py", line 326, in _Popen
    return Popen(process_obj)
  File "C:\Users\AppData\Local\Programs\Python\Python38-32\lib\multiprocessing\popen_spawn_win32.py", line 93, in __init__
    reduction.dump(process_obj, to_child)
  File "C:\Users\AppData\Local\Programs\Python\Python38-32\lib\multiprocessing\reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
TypeError: cannot pickle 'weakref' object
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\AppData\Local\Programs\Python\Python38-32\lib\multiprocessing\spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "C:\Users\AppData\Local\Programs\Python\Python38-32\lib\multiprocessing\spawn.py", line 126, in _main
    self = reduction.pickle.load(from_parent)
EOFError: Ran out of input

What am I doing wrong here and how can I fix it? Any help is appreciated. Thank you!

Answer 1

I just came to the same traceback and managed to solve it. It was due to that an object had a running or exited Process as a variable and it was starting another Process using that object.

Problem

This is a minimal code to produce your error:

import multiprocessing

class Foo:
    def __init__(self):
        self.process_1 = multiprocessing.Process(target=self.do_stuff1)
        self.process_2 = multiprocessing.Process(target=self.do_stuff2)

    def do_multiprocessing(self):
        self.process_1.start()
        self.process_2.start()

    def do_stuff1(self):
        print("Doing 1")

    def do_stuff2(self):
        print("Doing 2")

if __name__ == '__main__':
    foo = Foo()
    foo.do_multiprocessing()

[out]:
Traceback (most recent call last):
  File "myfile.py", line 21, in <module>
    foo.do_multiprocessing()
  File "myfile.py", line 11, in do_multiprocessing
    self.process_2.start()
  File "...\lib\multiprocessing\process.py", line 121, in start
    self._popen = self._Popen(self)
  File "...\lib\multiprocessing\context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "...\lib\multiprocessing\context.py", line 327, in _Popen
    return Popen(process_obj)
  File "...\lib\multiprocessing\popen_spawn_win32.py", line 93, in __init__
    reduction.dump(process_obj, to_child)
  File "...\lib\multiprocessing\reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
TypeError: cannot pickle 'weakref' object
Doing 1
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "...\lib\multiprocessing\spawn.py", line 116, in spawn_main
    exitcode = _main(fd, parent_sentinel)
  File "...\lib\multiprocessing\spawn.py", line 126, in _main
    self = reduction.pickle.load(from_parent)
EOFError: Ran out of input

So the issue is that Foo contains also the running/exited process foo.process_1 when it starts foo.process_2.

Solution 1

Set foo.process_1 to None or something else. Or store the Processes somewhere else than in foo to prevent being passed when starting process_2.

...
def do_multiprocessing(self):
    self.process_1.start()
    self.process_1 = None # Remove exited process
    self.process_2.start()
...

Solution 2

Remove the problematic variable (process_1) from pickling:

class Foo:
    def __getstate__(self):
        # capture what is normally pickled
        state = self.__dict__.copy()

        # remove unpicklable/problematic variables 
        state['process_1'] = None
        return state
...

This seems to be problem in newer Python versions. My own code worked fine for 3.7 but failed due to this issue in 3.9.

I tested your code (from recv_data). Since you join the processes and need them, you should do the solution 2 or store the processes somewhere else than in recv_data. Not sure what other problems your code has.