title: pymysql select in with variable number of parameters
body: <p>I read several examples that show how pymysql "select in" should work. So, this example works just fine:</p>
  
  <pre><code> sql_select = 'SELECT a.user_id, AVG(a.rcount) AS \'average\' ' \
                   'FROM (SELECT user_id, item_id, count(*) AS rcount ' \
                   'FROM submission AS qsm ' \
                   'JOIN metadata as qm   ' \
                   'ON qsm.item_id = qm.id ' \
                   'WHERE qsm.item_id NOT IN (1, 2, 5, 6, 7, 147, 148) ' \
                   'AND DATE(FROM_UNIXTIME(submission_time)) BETWEEN %s AND %s ' \
                   'AND qm.type != \'survey\' ' \
                   'GROUP BY user_id, item_id ' \
                   'ORDER BY user_id) a ' \
                   'GROUP BY a.user_id'
      args = [course_start, course_end]
      cur.execute(sql_select, args)
  </code></pre>
  
  <p>But, I would also like to add another argument for this "NOT IN" part. The problem here is that this list is variable, so not quite sure how to handle this one. </p>