title: Difference of use of [] or not in variable
body: <p>What is the difference of the below codes?</p>
  <p>When I executed python codes, I got different results.</p>
  <p>1st code is easy for me, but 2nd code is what I want.</p>
  <p>I want to know the reason.</p>
  <ol>
  <li>variable.columns.isnull()  eg. <code>boat_data.gear.isnull()</code></li>
  <li>variable[variable.columns.isnull()] eg. <code>boat_data[boat_data.gear.isnull()]</code></li>
  </ol>