How Can I put a QMdiSubWindow into a QMdiArea QMainWindow?

I'm new in pyqt. I want to do an application with a Qmainwindow with its menubar and menu items. when I click in some menu item A QMdiSubWindow should appear into the qmdiarea, How can I do that? this is my qmainwindow code:

class Ui_mainForm(QMainWindow):
    def __init__(self):
        QtGui.QMainWindow.__init__(self)
        self.resize(928, 695)
        self.qareaMdi = QtGui.QWidget(self)
        #all the design code...
        self.actionFriends.triggered.connect(self.actionFriends_click)
        #more code...
    def actionFriends_click(self):
        #how can I call a qmdisubwindow here?

and this is my QMdiSubWindow code:

class Ui_friendForm(QMdiSubWindow):
    def __init__(self):
        QtGui.QMdiSubWindow.__init__(self)
        self.resize(878, 551)
        QtCore.QMetaObject.connectSlotsByName(self)

thanks in advance

Update: I modified the actionFriends_click function like this:

def actionFriends_click(self):
    subwindow_friend = Ui_friendForm()
    self.mdiArea.addSubWindow(subwindow_friend)
    subwindow_friend.show()

Update 2: I forgot It. We have to add a reference into Ui_mainForm

from VIEW.friendsForm import Ui_friendForm

In this case the QMdiSubWindows Ui_friendForm class is in the VIEW package.

We have to add a reference into Ui_mainForm (In this case the QMdiSubWindows Ui_friendForm class is in the VIEW package.) and I modified the actionFriends_click function like this:

from VIEW.friendsForm import Ui_friendForm
def actionFriends_click(self):
    subwindow_friend = Ui_friendForm()
    self.mdiArea.addSubWindow(subwindow_friend)
    subwindow_friend.show()