import sys
from functools import partial

from PyQt5 import QtWidgets, QtGui, QtCore


class MainWindow(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        self.central_widget = QtWidgets.QWidget(self)
        self.setCentralWidget(self.central_widget)
        self.mw_open_new_dialog_button = QtWidgets.QPushButton('Open New dialog', self)
        self.mw_line_edit = QtWidgets.QLineEdit(self)
        self.hlayout = QtWidgets.QHBoxLayout(self)
        self.hlayout.addWidget(self.mw_open_new_dialog_button)
        self.hlayout.addWidget(self.mw_line_edit)
        self.central_widget.setLayout(self.hlayout)

        self.mw_open_new_dialog_button.clicked.connect(self.open_new_dialog)


    def open_new_dialog(self):
        self.dlg = Dialog()
        #self.dlg.clicked.connect(partial(self.write_something, self.dlg.line_edit.text())) # <<<<<<< This does not work
        self.dlg.clicked.connect(lambda: self.write_something(self.dlg.line_edit.text()))
        #self.dlg.clicked.connect(self.wrapper(self.dlg.line_edit.text()))# <<<<<<<<<<This does not work
        self.dlg.exec()

    @QtCore.pyqtSlot()
    def write_something(self, text):
        self.mw_line_edit.setText(text)

    def wrapper(self, text):
        return lambda: self.write_something(text)



class Dialog(QtWidgets.QDialog):

    clicked = QtCore.pyqtSignal()
    def __init__(self, parent=None):
        super(QtWidgets.QDialog, self).__init__(parent)
        self.hlayout = QtWidgets.QHBoxLayout(self)
        self.line_edit = QtWidgets.QLineEdit(self)
        self.push_button = QtWidgets.QPushButton('Click me', self)
        self.hlayout.addWidget(self.line_edit)
        self.hlayout.addWidget(self.push_button)
        self.label = QtWidgets.QLabel('I am a Qlabel', self)
        self.hlayout.addWidget(self.label)
        self.setLayout(self.hlayout)
        self.push_button.clicked.connect(self.clicked)

    def write_something(self, text):
        print(text)



app = QtWidgets.QApplication(sys.argv)

main_window = MainWindow()
main_window.show()
sys.exit(app.exec())