title: nunique into new dataframe
body: <p>I have some data in pandas:</p>
  
  <pre><code>df1
  df1['ID_A'].nunique()
  5
  df2
  df2['ID_B'].nunique()
  6
  df3
  df1['ID_A'].nunique()
  2
  df4
  df2['ID_B'].nunique()
  9
  </code></pre>
  
  <p>and so-on until 200 <code>df</code>.</p>
  
  <p>how to make new <code>dataframe</code> based on this <code>nunique</code></p>
  
  <p>my expected result looks like this:</p>
  
  <pre><code>  combine    ID_A  ID_B
    combine_1  5     6
    combine_2  2     9
  </code></pre>
  
  <p>thank you</p>