PyQT : How to activate command link button if at least one element is checked in a treeview

I want to activate command link button if at least one element is checked in a treeview.

I tried this method but it is not working well :

def activate_launch_button(self):

    model = self.scenarios.model()
    checked_indexes = model.match(model.index(0, 0), QtCore.Qt.CheckStateRole,QtCore.Qt.Checked, -1,QtCore.Qt.MatchExactly | QtCore.Qt.MatchRecursive)
    print checked_indexes
    if checked_indexes != []:
        self.launch_btn.setEnabled(True)

The problem with this method is the button is activated only after closing the window. I don't understand why. For caling this method I put it in two methods :

def show(self):
    self.project.load()
    if self.project.tranus_project:
        self.tranus_folder.setText(self.project.tranus_project.path)
    self.activate_launch_button()
    self.launch_options_TRANUS()
    super(OptionsTRANUSDialog, self).show()


def select_tranus_folder(self):

    folder = QtGui.QFileDialog.getExistingDirectory(self, "Select directory")
    if folder:
        self.tranus_folder.setText(folder)
        if not self.project.load_tranus_folder(folder):
            self.tranus_folder.setText('')
        self.reload_scenarios()
        self.activate_launch_button()

I really want that the command link button should be activated when at least one element of treeview is checked and to be inactive if there is no element checked.

Thanks.