Function is not run when connecting Button to it in PyQt4 - python-3.x

Question

I m new to PyQt4 and having problems when trying to connect a button to a function.
I m using PyQt 4.11.4.
Neither the clicked.connect method nor the line in the comment below seemed to work.
from PyQt4 import QtGui, QtCore
import sys
class Window(QtGui.QMainWindow):
def __init__(self):
super(Window, self).__init__()
def setupUi(self, Window):
MainWindow.setObjectName("MainWindow")
MainWindow.resize(200, 200)
self.centralwidget = QtGui.QWidget(MainWindow)
self.centralwidget.setGeometry(0,0,100,100)
MainWindow.setCentralWidget(self.centralwidget)
self.pushButton = QtGui.QPushButton(self.centralwidget)
self.pushButton.setGeometry(QtCore.QRect(100, 50, 55, 20))
self.pushButton.setText("Run")
self.pushButton.clicked.connect(self.run)
#OR QtCore.QObject.connect(self.pushButton, QtCore.SIGNAL('clicked()'),self.run)
def run(self):
print("ok")
if __name__ == "__main__":
app = QtGui.QApplication(sys.argv)
MainWindow = QtGui.QMainWindow()
Window().setupUi(MainWindow)
MainWindow.show()
sys.exit(app.exec_())

Answer 1

First of all, by placing statements directly after the __name__ == "__main__" test you introduce global variables. For instance the MainWindow variable is global and is subsequently altered in the Window.setupUi method. This is undesirable. It's better to create a main function and call only that. For example by changing those lines into this...
def main():
app = QtGui.QApplication(sys.argv)
MainWindow = QtGui.QMainWindow()
Window().setupUi(MainWindow)
MainWindow.show()
sys.exit(app.exec_())
if __name__ == "__main__":
main()
you will get a run-time error and thus notice something's wrong.
By the way, it is the convention in Python to start class names with an upper case character and objects with a lower case. Therefore, at first glance I thought that MainWindow was a class. It is, however, an object and should better be renamed to mainWindow. Notice how the StackOverflow syntax highlighting colors all identifiers that start with an capital letter light blue?
So, the issue with the signal is that you create two main window objects: first by MainWindow = QtGui.QMainWindow() and then on the next line by Window().setupUi(MainWindow). On that second line you call setupUi with the self parameter set to a temporary Window object, and the Window parameter set to a QtGui.QMainWindow object (MainWindow). It's not surprising that this doesn't work, it is actually a bit surprising it does something at all.
Finally, it is strongly recommended to use layouts in Qt to arrange the widgets. This makes the calls to setGeometry() superfluous. Adding widgets to a layout also set's their parent, so you don't need to do this separately then. I think you definitely should read the docs on layout management (except the last section on writing your own layout manager).
Apply all the advice above and you get something like this...
from PyQt4 import QtGui, QtCore
import sys
class Window(QtGui.QMainWindow):
def __init__(self):
super(Window, self).__init__()
self._setupUi()
def _setupUi(self):
self.resize(200, 200)
# the central widget is called my widget to prevent clashes with
# the centralWidget() function.
self.mainWidget = QtGui.QWidget()
self.setCentralWidget(self.mainWidget)
self.mainLayout = QtGui.QVBoxLayout()
self.mainWidget.setLayout(self.mainLayout)
self.pushButton = QtGui.QPushButton()
self.pushButton.setText("Run")
self.mainLayout.addWidget(self.pushButton)
self.pushButton.clicked.connect(self.run)
def run(self):
print("ok")
def main():
app = QtGui.QApplication(sys.argv)
mainWindow = Window()
mainWindow.setObjectName("mainWindow") # can be omitted.
mainWindow.show()
sys.exit(app.exec_())
if __name__ == "__main__":
main()