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:
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.