More Flexible Decorators

Hi and thank you for this explanation of the new features in 3.9

I am using PyQt5 and wanted to ask if you can give an example of how the new decorators can be used. Let’s say that my code looks like this:

class MainWindow(QtWidgets.QMainWindow):
    def __init__(self):
        super().__init__()

        self.button_qpb = QtWidgets.QPushButton("Button")
        self.button_qpb.clicked.connect(self.on_button_clicked)
        self.setCentralWidget(self.button_qpb)
        self.show()

    def on_button_clicked(self):
        print("clicked!")

How then can i instead use decorators to connect the button to the “slot function” (on_button_clicked)?

Grateful for help and with kind regards, Tord

Hi Tord,

The changes to how decorators worked would allow for dot-delimited names in the decorators, but you’d still need to write a decorator to take advantage of it. I’ve not written any Qt before, so I don’t know if their registration mechanism use decorators or whether they’re using the Py39 syntax changes.

If you’re curious about writing your own decorators, you might find the following useful:

realpython.com/primer-on-python-decorators/

Otherwise, maybe post in the Slack channels and you can find someone who has done Qt and provide specific advice.

Happy coding. …ct

Hi Tord,

This is a slightly tricky use case for the new decorator syntax because the decorator function, in this case button_qpb.clicked.connect(), must be available in the scope where you define the “slot function”.

One way to do this would be to drag the button out as a class variable:

class MainWindow(QtWidgets.QMainWindow):
    button_qpb = QtWidgets.QPushButton("Button")

    def __init__(self):
        super().__init__()
        self.setCentralWidget(self.button_qpb)
        self.show()

    @button_qpb.clicked.connect
    def on_button_clicked(self):
        print("clicked!")

I’ll leave it to you to judge whether this is an improvement :) I like the visibility the decorator brings. Having the button as a class variable could bring issues down the line, especially if you need several instances of the class (since those would share the same button).

Hi again, Tord.

A small follow-up to my earlier comment. A possibly better way to use the dotted decorator would be to define the “slot function” as an inner function:

class MainWindow(QtWidgets.QMainWindow):
    def __init__(self):
        super().__init__()
        self.button_qpb = QtWidgets.QPushButton("Button")
        self.setCentralWidget(self.button_qpb)
        self.show()

        @self.button_qpb.clicked.connect
        def on_button_clicked(self):
            print("clicked!")

This way, you’re keeping the button as an instance variable, while still using the decorator.

Hi and thanks for your help!

I’ve tried them and they both work, with the small change that the decorated functions cannot take self as an argument. The inner function approach seems best for me since that will have access to self through the outer scope (the __init__ function), which means that i can call other functions in the class.

I now understand decorators better and have a new+better way to write my PyQt/Pyside code!

Kind regards, Tord

For reference, here is the complete code i ended up with

import sys
import logging
from PyQt5 import QtWidgets
from PyQt5 import QtCore

logging.basicConfig(level=logging.DEBUG)


class MainWindow(QtWidgets.QMainWindow):
    def __init__(self):
        super().__init__()
        self.button_qpb = QtWidgets.QPushButton("Button")
        self.setCentralWidget(self.button_qpb)
        self.show()

        @self.button_qpb.clicked.connect
        def on_button_clicked():
            print("clicked!")
            print(self.button_qpb.text())
            self.my_print()

    def my_print(self):
        print("my_print entered")


if __name__ == "__main__":
    app = QtWidgets.QApplication(sys.argv)
    logging.debug(f"Python version: {sys.version!r}")
    logging.debug(f"Qt version: {QtCore.qVersion()!r}")
    main_window = MainWindow()
    main_window.show()
    app.exec_()

Output at start:

DEBUG:root:Python version: '3.9.5 (default, May 11 2021, 08:20:37) \n[GCC 10.3.0]'
DEBUG:root:Qt version: '5.15.2'

Output at click:

clicked!
Button
my_print entered