Signals and Missing Positional Arguments

Question

I developed two windows in QtDesigner (SourceForm, DestinationForm) and used pyuic5 to convert their .ui pages. I am using a third class WController as a way to

Answer 1

There's already an accepted answer to this question but I'll give mine anyway.

The problematic slot is connected to the itemChanged(QTreeWidgetItem *item, int column) signal, so the pyqtSlot should look like @pyqtSlot(QTreeWidgetItem, int).

Now, as ekhumoro pointed out, PyQt accepts to connect a signal to any Python callable, be it a method, a lambda, or an functor having a __call__ method. But it's less safe to do so rather than use @pyqtSlot.

For example, Qt automatically disconnects when either the source QObject (who would emit the signal) is destroyed, or the target QObject (who has the Qt slot) is destroyed. For example, if you remove a widget, it's not necessary to signal to it that something happened elsewhere. If you use @pyqtSlot, a real Qt slot is created in your class, so this disconnection mechanism can apply. Also, Qt doesn't hold a strong reference to the target QObject, so it can be deleted.

If you use any callable, for example a non-decorated, bound method, Qt will have no way to identify the target QObject of the connection. Worse, since you pass a Python callable, it will hold a strong reference to it, and the callable (the bound method) will in turn hold a reference to the final QObject, so your target QObject will not be garbage collected, until you manually disconnect it, or remove the source QObject.

See this code, you can enable one connection or the other, and observe the difference in behavior, which shows whether the window can be garbage-collected or not:

from PyQt5.QtWidgets import QApplication, QMainWindow

app = QApplication([])

def create_win():
    win = QMainWindow()
    win.show()

    # case 1
    # app.aboutToQuit.connect(win.repaint)  # this is a qt slot, so win can be deleted

    # case 2
    # app.aboutToQuit.connect(win.size)  # this is not a qt slot, so win can't be deleted

    # win should get garbage-collected here

create_win()

app.exec_()

Answer 2

You need to be careful when using pyqtSlot, as it is only too easy to clobber the signature of the slot it is decorating. In your case, it has re-defined the slot as having no arguments, which explains why you are getting that error message. The simple fix is to simply remove it, as your example will work perfectly well without it.

The main purpose of pyqtSlot is to allow several different overloads of a slot to be defined, each with a different signature. It may also be needed sometimes when making cross-thread connections. However, these use-cases are relatively rare, and in most PyQt/PySide applications it is not necessary to use pyqtSlot at all. Signals can be connected to any python callable object, whether it is decorated as a slot or not.