How to signal slots in a GUI from a different process?

Question

Context: In Python a main thread spawns a 2nd process (using multiprocessing module) and then launches a GUI (using PyQt4). At this point the main thread blocks until the GUI is

Answer 1

Hy all,

I hope this is not considered to much of a necro-dump however I thought it would be good to update Nizam's answer by adding updating his example to PyQt5, adding some comments, removing some python2 syntax and most of all by using the new style of signals available in PyQt. Hope someone finds it useful.

"""
Demo to show how to use PyQt5 and qt signals in combination with threads and
processes.

Description:
Text is entered in the main dialog, this is send over a queue to a process that 
performs a "computation" (i.e. capitalization) on the data. Next the process sends 
the data over a pipe to the Emitter which will emit a signal that will trigger 
the UI to update.

Note:
At first glance it seems more logical to have the process emit the signal that 
the UI can be updated. I tried this but ran into the error 
"TypeError: can't pickle ChildProc objects" which I am unable to fix.
"""

import sys
from multiprocessing import Process, Queue, Pipe

from PyQt5.QtCore import pyqtSignal, QThread
from PyQt5.QtWidgets import QApplication, QLineEdit, QTextBrowser, QVBoxLayout, QDialog


class Emitter(QThread):
    """ Emitter waits for data from the capitalization process and emits a signal for the UI to update its text. """
    ui_data_available = pyqtSignal(str)  # Signal indicating new UI data is available.

    def __init__(self, from_process: Pipe):
        super().__init__()
        self.data_from_process = from_process

    def run(self):
        while True:
            try:
                text = self.data_from_process.recv()
            except EOFError:
                break
            else:
                self.ui_data_available.emit(text.decode('utf-8'))


class ChildProc(Process):
    """ Process to capitalize a received string and return this over the pipe. """

    def __init__(self, to_emitter: Pipe, from_mother: Queue, daemon=True):
        super().__init__()
        self.daemon = daemon
        self.to_emitter = to_emitter
        self.data_from_mother = from_mother

    def run(self):
        """ Wait for a ui_data_available on the queue and send a capitalized version of the received string to the pipe. """
        while True:
            text = self.data_from_mother.get()
            self.to_emitter.send(text.upper())


class Form(QDialog):
    def __init__(self, child_process_queue: Queue, emitter: Emitter):
        super().__init__()
        self.process_queue = child_process_queue
        self.emitter = emitter
        self.emitter.daemon = True
        self.emitter.start()

        # ------------------------------------------------------------------------------------------------------------
        # Create the UI
        # -------------------------------------------------------------------------------------------------------------
        self.browser = QTextBrowser()
        self.lineedit = QLineEdit('Type text and press ')
        self.lineedit.selectAll()
        layout = QVBoxLayout()
        layout.addWidget(self.browser)
        layout.addWidget(self.lineedit)
        self.setLayout(layout)
        self.lineedit.setFocus()
        self.setWindowTitle('Upper')

        # -------------------------------------------------------------------------------------------------------------
        # Connect signals
        # -------------------------------------------------------------------------------------------------------------
        # When enter is pressed on the lineedit call self.to_child
        self.lineedit.returnPressed.connect(self.to_child)

        # When the emitter has data available for the UI call the updateUI function
        self.emitter.ui_data_available.connect(self.updateUI)

    def to_child(self):
        """ Send the text of the lineedit to the process and clear the lineedit box. """
        self.process_queue.put(self.lineedit.text().encode('utf-8'))
        self.lineedit.clear()

    def updateUI(self, text):
        """ Add text to the lineedit box. """
        self.browser.append(text)


if __name__ == '__main__':
    # Some setup for qt
    app = QApplication(sys.argv)

    # Create the communication lines.
    mother_pipe, child_pipe = Pipe()
    queue = Queue()

    # Instantiate (i.e. create instances of) our classes.
    emitter = Emitter(mother_pipe)
    child_process = ChildProc(child_pipe, queue)
    form = Form(queue, emitter)

    # Start our process.
    child_process.start()

    # Show the qt GUI and wait for it to exit.
    form.show()
    app.exec_()