Set the active window to a QmdiArea

What you try in the end is the correct way to do it. The only weird thing I see is window_vt = QMdiSubWindow(self.win_vt)that it should be window_vt = QMdiSubWindow(). If not, the cause of it not working for you may be due to a name conflict. Note that as-is window_ts, window_vtand window_normare local variables to the method where you define them. If you use self.mdiArea.setActiveSubWindow(window_ts)outside of said method it won't work logically. On the other hand, using the same name for your and your widget instances QMdiSubWindowcan be confusing and even misleading. It would be necessary to see your complete app or at least the error that throws you (if it does).

Having said that, I am going to explain a little how it should be done and give a reproducible example to show its use.

You must create QMdiSubWindowsinside the QMdiarea, then add the widgets you want to each subwindow as if it were any other window (as we would in one QMainWidgetfor example).

The method setActiveSubWindowmust receive an instance of QMdiSubWindowsin any case.

In the following example, three windows are created and we place the focus on the second one that is created. Each window has an QTextEditinside, this is just as an example and so that they are not empty windows. It would be the same for any other frame.

import sys
from PyQt4.QtGui import QApplication, QMainWindow, QMdiSubWindow, QMdiArea, QTextEdit

class MainWindow(QMainWindow):
    def __init__(self, parent = None):
        super(MainWindow, self).__init__(parent)
        self.setWindowTitle("MDI demo")
        self.mdi = QMdiArea()
        self.setCentralWidget(self.mdi)

        self.win_ts = QMdiSubWindow()
        self.win_ts.setWindowTitle("Window TS")
        self.win_ts.setWidget(QTextEdit())


        self.win_vt = QMdiSubWindow()
        self.win_vt.setWindowTitle("Window VT")
        self.win_vt.setWidget(QTextEdit())

        self.win_norm = sub = QMdiSubWindow()
        self.win_norm.setWindowTitle("Window Norm")
        self.win_norm.setWidget(QTextEdit())

        self.mdi.addSubWindow(self.win_ts)
        self.mdi.addSubWindow(self.win_vt)
        self.mdi.addSubWindow(self.win_norm)

        self.win_ts.show()
        self.win_vt.show()
        self.win_norm.show()

        self.mdi.cascadeSubWindows() # Ordena las ventanas en cascada

        self.mdi.setActiveSubWindow(self.win_vt) #<<<<<<<<


def main():
    app = QApplication(sys.argv)
    ex = MainWindow()
    ex.show()
    sys.exit(app.exec_())

if __name__ == '__main__':
    main()

Which should produce something like:

We can see how the focus appears in the second window ( self.win_vt) just as we wanted.