Why does pyqtSlot decorator cause “TypeError: connect() failed”?

Question

I have this Python 3.5.1 program with PyQt5 and a GUI created from a QtCreator ui file where the pyqtSlot decorator causes \"TypeError: connect() failed between textChanged(

Answer 1

I don't know what was wrong, but I found a workaround: Connect the textChanged signal to a pyqtSlot decorated MainApp method that calls LineEditHandler.edited():

import sys
from PyQt5 import uic
from PyQt5.QtWidgets import *
from PyQt5.QtCore import pyqtSlot


Ui_MainWindow, QtBaseClass = uic.loadUiType("mainwindow.ui")


class MainApp(QMainWindow, Ui_MainWindow):

    def __init__(self):
        # noinspection PyArgumentList
        QMainWindow.__init__(self)
        Ui_MainWindow.__init__(self)
        self.setupUi(self)

        # Instantiate the QLineEdit handler.
        self._line_edit_handler = LineEditHandler(self, self.lineEdit)
        self.lineEdit.textChanged.connect(self._line_edited)

        @pyqtSlot(name="_line_edited")
        def _line_edited(self):
            self._line_edit_handler.edited()

class LineEditHandler:

    def __init__(self, main_window, line_edit_obj):
        self._line_edit = line_edit_obj
        self._main_window = main_window

    def edited(self):
        # Copy the entry box text to the label box below.
        self._main_window.label.setText(self._line_edit.text())


def main():
    app = QApplication(sys.argv)
    window = MainApp()
    window.show()
    sys.exit(app.exec_())


if __name__ == "__main__":
    main()

Answer 2

Why do you want to use @pyqtSlot?

The reason it fails is that LineEditHandler is not a QObject. What @pyqtSlot does is basically creating a real Qt slot instead of internally using a proxy object (which is the default behavior without @pyqtSlot).