Tags Python , Qt

In most of the PyQt code samples you find online and in books (including, I confess, my examples and blog posts) the "old-style" signal-slot connection mechanism is used. For example, here's a basic push-button window:

from PyQt4.QtCore import *
from PyQt4.QtGui import *

class MyForm(QMainWindow):
    def __init__(self, parent=None):
        super(MyForm, self).__init__(parent)
        the_button = QPushButton('Hello')
        self.connect(the_button, SIGNAL('clicked()'), self.on_hello)
        self.setCentralWidget(the_button)

    def on_hello(self):
        print('hello!!')

if __name__ == "__main__":
    import sys
    app = QApplication(sys.argv)
    form = MyForm()
    form.show()
    app.exec_()

The relevant code is:

self.connect(the_button, SIGNAL('clicked()'), self.on_hello)

Apart from being verbose and un-Pythonic, this syntax has a serious problem. You must type in the C++ signature of the signal exactly. Otherwise, the signal just won't fire, without an exception or any warning. This is a very common mistake. If you think that clicked() is a simple enough signature to write, how about these ones (taken from real code):

SIGNAL("currentRowChanged(QModelIndex,QModelIndex)")
SIGNAL('marginClicked(int, int, Qt::KeyboardModifiers)')

The "new-style" signal-slot connection mechanism is much better. Here's how the button click connection is done:

the_button.clicked.connect(self.on_hello)

This mechanism is supported by PyQt since version 4.5, and provides a safer and much more convenient way to connect signals and slots. Each signal is now an attribute that gets automatically bound once you access it. It has a connect method that simply accepts the slot as a Python callable. No more C++ signatures, yay!

PyQt 4.5 was released almost 2 years ago - it's time to switch to the new mechanism.


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