def mouse_down(self, button='left'):
        """Simulates a `mousedown` event.

        :param button: A string indicating which mouse button to be pressed.
                       Available mouse button options are:
                       {'left', 'middle', 'right'}.
        """
        buttons = {'left', 'middle', 'right'}
        if not isinstance(button, str) or button not in buttons:
            raise ValueError('invalid literal for mouse_down(): {}'.format(button))
        else:
            coords = self._action_pos(win32api.GetCursorPos())
            self.mouse.press(button=button, coords=coords)
Simulates a `mousedown` event.

        :param button: A string indicating which mouse button to be pressed.
                       Available mouse button options are:
                       {'left', 'middle', 'right'}.
        
airtest/core/win/win.py