def callback(self, event):
        """
        Selects cells on click.
        """
        self.init_width()

        if len(self.initial) > 0:
            for cell in self.initial:
                self.color_square(cell[0], cell[1], True)
            self.initial = []
        self.begin_drag = event
        self.color_square(event.x, event.y)
Selects cells on click.