title: What's the error mean for `$in` in Pymongo?
body: <pre><code>for element in self.table1.find({'ji': {'$ne': ""}}):
      gongLiNian = int(element['gongLiNian'])
      gongLiNianScope = [str(gongLiNian-1), str(gongLiNian), str(gongLiNian+1)]
      res = self.table2.find_one({'guanZhi' : element['guanZhi'],
                               'gongLiNian' : {'$in', gongLiNianScope},
                                  'name'    :  element['name']})
  </code></pre>
  
  <p>For this code, here is the error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "C:\Users\elqstux\Desktop\study\History\oneJi.py", line 172, in &lt;module&gt;
      oneJi.run()
    File "C:\Users\elqstux\Desktop\study\History\oneJi.py", line 158, in run
      res1 = self.step1()
    File "C:\Users\elqstux\Desktop\study\History\oneJi.py", line 44, in step1
      'gongLiNian' : {'$in', gongLiNianScope},
  TypeError: unhashable type: 'list'
  </code></pre>
  
  <p>But i can't find any clue from the error.Could you give me some advise?</p>