PySide: QMetaObject.connectSlotsByName emits warnings "No matching signal..." but still works..?

Question

In Qt Designer, I created a QDialog window and used pysideuic to compile that to a base class which contains a setupUi method initialising all GUI elements and which I extend to implement the functionality, as so:

class MyDialog(QtGui.QDialog, ui_file.Ui_main_dialog):
    def __init__(self, parent=None):
        QtGui.QDialog.__init__(self, parent)
        ui_file.Ui_main_dialog.__init__(self)
        self.setupUi(self)

This setupUi method has calls to QtCore.QObject.connect for the signal-slot connections I created in Qt Designer, where I also added new slots to the GUI. These slots are non-existent in the base class generated by pysideuic and I added them to the MyDialog class, e.g.

    def on_list_selection_changed(self):
        self.run_btn.setEnabled(len(self.modules_list.selectedIndexes()) > 0)

For this example, the slot is called on_list_selection_changed() (with empty parameter list) in Qt Designer.

On initialisation, MyDialog.__init__ calls Ui_main_dialog.setupUi, which eventually calls QtCore.QMetaObject.connectSlotsByName (the latter two with the MyDialog instance's self which is currently being created). This emits the following line on sys.stderr:

QMetaObject::connectSlotsByName: No matching signal for on_list_selection_changed()

Still, the signal behaves correctly and the slot is called when the connected modules_list.itemSelectionChanged() (modules_list is a QListWidget).

So here is my question: why do I receive this warning? What can I do so it doesn't appear, given that it seems to be irrelevant?

---

Edit: Since I didn't receive any answers in the last 5 months, I thought I give a complete example to make reproducing the problem easier.

This example differs from the question above in that it only uses a QLineEdit instance. Here is the code:

import sys
from PySide import QtCore, QtGui

class Ui_Dialog(object):
    def setupUi(self, Dialog):
        Dialog.setObjectName("Dialog")
        self.lineEdit = QtGui.QLineEdit(Dialog)
        self.lineEdit.setObjectName("lineEdit")
        QtCore.QObject.connect(self.lineEdit, QtCore.SIGNAL("textChanged(QString)"), Dialog.on_lineEdit_changed)
        QtCore.QMetaObject.connectSlotsByName(Dialog)

class MainWindow(QtGui.QMainWindow, Ui_Dialog):
    def __init__(self, parent=None):
        QtGui.QMainWindow.__init__(self, parent)
        Ui_Dialog.__init__(self)
        Ui_Dialog.setupUi(self, self)
    @QtCore.Slot(unicode) # signal with no arguments
    def on_lineEdit_changed(self, text):
        print 'Changed to', text

if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    mw = MainWindow()
    mw.show()
    sys.exit(app.exec_())

Note that the code for the Ui_Dialog class is generated by the pysideuic from the Qt Designer's .ui file, but I shortened it a bit to better highlight the problem.

Answer 1

I'm late but in case someone else comes here for this same question.

PySide auto connects the signals, this is how it worked for me since I didn't want to be connecting manually.

Here I use python 3.9 and PySide2, but it is the same for PySide6 and also for other versions of python.

import sys
import typing

from PySide2.QtCore import QObject, SignalInstance, Signal, Slot
from PySide2.QtWidgets import QMainWindow, QApplication

from .ui_mainwindow import Ui_MainWindow  # Don't forget to add your import


class MyQObject(QObject):
    mySignal: SignalInstance = Signal()
    otherSignal: SignalInstance = Signal(str)

    def __init__(self, parent: typing.Optional[QObject] = None) -> None:
        super().__init__(parent)


class MainWindow(QMainWindow, Ui_MainWindow):
    def __init__(self):
        QMainWindow.__init__(self)

        # Instantiate before self.setupUI
        self.myQObject = MyQObject(self)
        self.myQObject.setObjectName("syncProduct")  # do not forget this

        # Load UI from designer & init
        self.setupUi(self)

    @Slot()
    def on_myQObject_mySignal(self):
        print("Handle mySignal")

    @Slot(str)
    def on_myQObject_otherSignal(self, text: str):
        print(f"Handle otherSignal {text}")


if __name__ == '__main__':
    app = QApplication(sys.argv)
    window = MainWindow()
    window.show()
    sys.exit(app.exec_())

Answer 2

I have the same issue in C++. connectSlotsByName is hard-coded to output that message if it finds a slot it can't connect automatically, even though you don't need the automatic connection because you've done it explicitly yourself. You can use qInstallMsgHandler to suppress warnings in general or write the messages somewhere better but I don't think there's any way to tell connectSlotsByName that you don't care about this case in particular.