title: Filter a Pandas dataframe by a condition and a minimum value in a column
body: <p>I have a dataframe similar to </p>
  
  <pre><code>  test_a test_b  metric_e
  0     OK    NOK        12
  1     OK     OK         7
  2     OK    NOK         2
  3     OK     OK        55
  </code></pre>
  
  <p>and I want to filter by one condition, meaning that <code>test_a == OK</code> and capture the minimum value on <code>metric_e</code>. I can accomplish that with two lines, copying a dataframe:</p>
  
  <pre><code>df_t = df[df.test_a == 'OK'].reset_index(drop=True)
  df_t.iloc[df_t.metric_e.idxmin()].to_frame()
  
  test_a | test_b | metric_e
  OK     |  NOK   | 2
  </code></pre>
  
  <p>Is there a way to do it without having to use an intermediate dataframe?</p>