title: New column in DataFrame based on rows and columns from another DataFrame
body: <p>I have two pandas DataFrames.</p>
  <pre><code>df1
  
      col1    col2
  0   E8      K4
  1   E6      K3
  2   E8      K4
  3   E8      K4
  4   E8      K2
  
  df2
  
      group   K1  K2  K3  K4
  0   E6      -2  -90 24  -23
  1   E7      94  -34 3   22
  2   E8      7   30  100 -9
  </code></pre>
  <p>In <code>df2</code>, for <code>E8 K4</code>, the value is</p>
  <pre><code>df2.loc[df2['group'] == 'E8']['K4'].item()
  -9
  </code></pre>
  <p>But, how could I create a new column in <code>df1</code> based on <code>df2</code>?</p>
  <p>The result will look like this:</p>
  <pre><code>    col1    col2    col3
  0   E8      K4      -9
  1   E6      K3      24
  2   E8      K4      -9
  3   E8      K4      -9
  4   E8      K2      30
  </code></pre>