PYQT: How to send a custom signal from the mainwindow(parent) to a child window?

Question

How do I send a signal from a parent/mainwindow to a child window?

For example, how would I register and send mainSignal down to either widgetA or widgetB ?

from PyQt5 import QtCore, QtGui, QtWidgets


class widgetB(QtWidgets.QWidget):

    procDone = QtCore.pyqtSignal(str)

    def __init__(self, parent=None):
        super(widgetB, self).__init__(parent)

        self.lineEdit = QtWidgets.QLineEdit(self)
        self.button = QtWidgets.QPushButton("Send Message to A", self)
        self.layout = QtWidgets.QHBoxLayout(self)
        self.layout.addWidget(self.lineEdit)
        self.layout.addWidget(self.button)

        self.button.clicked.connect(self.on_button_clicked)

    @QtCore.pyqtSlot()
    def on_button_clicked(self):
        self.procDone.emit(self.lineEdit.text())

    @QtCore.pyqtSlot(str)
    def on_procStart(self, message):
        self.lineEdit.setText("From A: " + message)

        self.raise_()


class widgetA(QtWidgets.QWidget):
    procStart = QtCore.pyqtSignal(str)

    def __init__(self, parent=None):
        super(widgetA, self).__init__(parent)

        self.lineEdit = QtWidgets.QLineEdit(self)
        self.lineEdit.setText("Hello!")

        self.button = QtWidgets.QPushButton("Send Message to B", self)
        self.button.clicked.connect(self.on_button_clicked)

        self.layout = QtWidgets.QHBoxLayout(self)
        self.layout.addWidget(self.lineEdit)
        self.layout.addWidget(self.button)

    @QtCore.pyqtSlot()
    def on_button_clicked(self):
        self.procStart.emit(self.lineEdit.text())

    @QtCore.pyqtSlot(str)
    def on_widgetB_procDone(self, message):
        self.lineEdit.setText("From B: " + message)

        self.raise_()


class mainwindow(QtWidgets.QMainWindow):
    mainSignal = QtCore.pyqtSignal(str)

    def __init__(self, parent=None):
        super(mainwindow, self).__init__(parent)

        self.button = QtWidgets.QPushButton("Click Me", self)
        self.button.clicked.connect(self.on_button_clicked)

        self.setCentralWidget(self.button)

        self.widgetA = widgetA()
        self.widgetB = widgetB()

        self.widgetA.procStart.connect(self.widgetB.on_procStart)
        self.widgetB.procDone.connect(self.widgetA.on_widgetB_procDone)

    @QtCore.pyqtSlot()
    def on_button_clicked(self):
        self.widgetA.show()
        self.widgetB.show()

        self.widgetA.raise_()


if __name__ == "__main__":
    import sys

    app = QtWidgets.QApplication(sys.argv)
    main = mainwindow()
    main.show()
    sys.exit(app.exec_())

Answer 1

The logic is the same, here it does not matter if they are parents, children, etc. It is only necessary that both objects can be accessed at the same time. The design of the signals and slots are made so that the classes are independent.

from PyQt5 import QtCore, QtGui, QtWidgets


class widgetB(QtWidgets.QWidget):

    procDone = QtCore.pyqtSignal(str)

    def __init__(self, parent=None):
        super(widgetB, self).__init__(parent)

        self.lineEdit = QtWidgets.QLineEdit(self)
        self.button = QtWidgets.QPushButton("Send Message to A", self)
        self.layout = QtWidgets.QHBoxLayout(self)
        self.layout.addWidget(self.lineEdit)
        self.layout.addWidget(self.button)

        self.button.clicked.connect(self.on_button_clicked)

    @QtCore.pyqtSlot()
    def on_button_clicked(self):
        self.procDone.emit(self.lineEdit.text())

    @QtCore.pyqtSlot(str)
    def on_procStart(self, message):
        self.lineEdit.setText("From A: " + message)

        self.raise_()

    @QtCore.pyqtSlot(str)
    def on_message_from_main(self, text):
        self.lineEdit.setText("From Main: " + text)

class widgetA(QtWidgets.QWidget):
    procStart = QtCore.pyqtSignal(str)

    def __init__(self, parent=None):
        super(widgetA, self).__init__(parent)

        self.lineEdit = QtWidgets.QLineEdit(self)
        self.lineEdit.setText("Hello!")

        self.button = QtWidgets.QPushButton("Send Message to B", self)
        self.button.clicked.connect(self.on_button_clicked)

        self.layout = QtWidgets.QHBoxLayout(self)
        self.layout.addWidget(self.lineEdit)
        self.layout.addWidget(self.button)

    @QtCore.pyqtSlot()
    def on_button_clicked(self):
        self.procStart.emit(self.lineEdit.text())

    @QtCore.pyqtSlot(str)
    def on_widgetB_procDone(self, message):
        self.lineEdit.setText("From B: " + message)
        self.raise_()

    @QtCore.pyqtSlot(str)
    def on_message_from_main(self, text):
        self.lineEdit.setText("From Main: " + text)


class mainwindow(QtWidgets.QMainWindow):
    mainSignal = QtCore.pyqtSignal(str)

    def __init__(self, parent=None):
        super(mainwindow, self).__init__(parent)

        self.button = QtWidgets.QPushButton("Click Me", self)
        self.button.clicked.connect(self.on_button_clicked)

        self.setCentralWidget(self.button)

        self.widgetA = widgetA()
        self.widgetB = widgetB()

        self.widgetA.procStart.connect(self.widgetB.on_procStart)
        self.widgetB.procDone.connect(self.widgetA.on_widgetB_procDone)

        self.mainSignal.connect(self.widgetA.on_message_from_main)
        self.mainSignal.connect(self.widgetB.on_message_from_main)

    @QtCore.pyqtSlot()
    def on_button_clicked(self):
        self.widgetA.show()
        self.widgetB.show()

        self.widgetA.raise_()

        self.mainSignal.emit("Hello")


if __name__ == "__main__":
    import sys

    app = QtWidgets.QApplication(sys.argv)
    main = mainwindow()
    main.show()
    sys.exit(app.exec_())