including external widget in my Qt GUI [python]

Here's the steps needed to add an AttitudeIndicator widget both in code, and via promotion in Qt Designer.

Firstly, you will need to make some changes to the "mainwindow.ui" file. So, in Qt designer, click on the central-widget in the Object Inspector, and then in the Property Editor, change the objectName to central_widget. This is important, because the current name is shadowing the QMainWindow.centralWidget() method that we need to use later. Secondly, with the central-widget still selected, click Lay Out Horizontally on the toolbar (the icon has three blue vertical bars) - and save the changes.

Now right-click the viz_widget (either in the Object Inspector or on the form), choose Promote to ..., and enter MyAttitudeIndicator in Promoted class name, and ai_widget in Header file. Then click Add and Promote - and save the changes. After doing that, you should see the Class change from QWidget to MyAttitudeIndicator in the Object Inspector. (But note that the actual widget on the form will not be displayed as an AttitudeIndicator. To do that, you'd need to write a custom designer plugin, which goes way beyond the scope of this question).

To make use of these changes, some code needs to be added. Firstly, you need to create a module called ai_widget.py for the promoted widget class. The module should contain this code:

from ai import AttitudeIndicator

class MyAttitudeIndicator(AttitudeIndicator):
    def __init__(self, parent, hz=30):
        super(MyAttitudeIndicator, self).__init__(hz=hz)
        self.setParent(parent)

The main reason why this class is needed is because the original AttitudeIndicator has a buggy API. The constructor really needs to take a parent widget (which is fixed in the above code). However, you could also use this class to add your own custom features.

The final step is to add some changes to the main script, which is shown below. Note that all the widgets added in Qt Designer will become attributes of the object passed to setupUi() (i.e. the main-window, self, in this case).

import sys
from PyQt4 import QtCore, QtGui, uic
from ai import AttitudeIndicator

qtCreatorFile1 = "mainwindow.ui" # Enter file here.

Ui_MainWindow, QtBaseClass = uic.loadUiType(qtCreatorFile1)

class OperatorGUI(QtGui.QMainWindow, Ui_MainWindow):
    def __init__(self, parent=None):
        super(OperatorGUI, self).__init__(parent)
        self.setupUi(self)

        # promoted widget from qt designer
        self.viz_widget.setPitch(10)
        self.viz_widget.setRoll(20)
        self.viz_widget.setHover(500/10.)
        self.viz_widget.setBaro(500/10.)

        # optional second widget
        self.viz_widget2 = AttitudeIndicator()
        self.viz_widget2.setPitch(10)
        self.viz_widget2.setRoll(-40)
        layout = self.centralWidget().layout()
        layout.addWidget(self.viz_widget2)

    # Key press functions
    def keyPressEvent(self, event):
        if event.key() == QtCore.Qt.Key_Q: #Q: close the window
            print "pressed Q: exit by keyboard"
            self.close()

if __name__ == "__main__":

    app = QtGui.QApplication(sys.argv)
    window = OperatorGUI()
    window.show()
    sys.exit(app.exec_())