title: Why doesn't "is not None" work with dataframe.loc, but "!= None" works fine?
body: <p>I'm currently playing with Pandas dataframe, I wanted to select all data entries in the dataframe which has not None entity attribute. </p>
  
  <pre class="lang-py prettyprint-override"><code>df_ = df.loc[df['entities'] != None]
  </code></pre>
  
  <p>seems work pretty well
  but </p>
  
  <pre class="lang-py prettyprint-override"><code>df_ = df.loc[df['entities'] is not None]
  </code></pre>
  
  <p>will raise KeyError
  which is </p>
  
  <blockquote>
    <p>File "pandas_libs\index.pyx", line 107, in >pandas._libs.index.IndexEngine.get_loc
     File "pandas_libs\index.pyx", line 128, in >pandas._libs.index.IndexEngine.get_loc
     File "pandas_libs\index_class_helper.pxi", line 91, in >pandas._libs.index.Int64Engine._check_type
    KeyError: True</p>
  </blockquote>
  
  <p>Well I already know the solution to my original problem, I'm just curious why this happens</p>