'Test that setting the progress handler to None clears the previously set handler.'

def CheckClearHandler(self):

    con = sqlite.connect(':memory:')
    action = []
    def progress():
        action.append(1)
        return 0
    con.set_progress_handler(progress, 1)
    con.set_progress_handler(None, 1)
    con.execute('select    1    union    select    2    union    select    3').fetchall()
    self.assertEqual(len(action), 0, 'progress    handler    was    not    cleared')
