title: Sort by certain order (Situation: pandas DataFrame Groupby)
body: <p>I want to change the day of order presented by below code.<br>
  What I want is a result with the order (Mon, Tue, Wed, Thu, Fri, Sat, Sun) <br> - should I say, sort by key in certain predefined order?</p>
  
  <hr>
  
  <p>Here is my code which needs some tweak:</p>
  
  <pre><code>f8 = df_toy_indoor2.groupby(['device_id', 'day'])['dwell_time'].sum()
  
  print(f8)
  </code></pre>
  
  <p>Current result:</p>
  
  <pre><code>device_id                         day
  device_112                        Thu     436518
                                    Wed     636451
                                    Fri     770307
                                    Tue     792066
                                    Mon     826862
                                    Sat     953503
                                    Sun    1019298
  device_223                        Mon    2534895
                                    Thu    2857429
                                    Tue    3303173
                                    Fri    3548178
                                    Wed    3822616
                                    Sun    4213633
                                    Sat    4475221
  </code></pre>
  
  <p>Desired result:</p>
  
  <pre><code>device_id                         day
  device_112                        Mon     826862  
                                    Tue     792066
                                    Wed     636451 
                                    Thu     436518
                                    Fri     770307
                                    Sat     953503
                                    Sun    1019298
  device_223                        Mon    2534895
                                    Tue    3303173
                                    Wed    3822616
                                    Thu    2857429
                                    Fri    3548178
                                    Sat    4475221
                                    Sun    4213633
  </code></pre>
  
  <hr>
  
  <p>Here, <code>type(df_toy_indoor2.groupby(['device_id', 'day'])['dwell_time'])</code> is a class 'pandas.core.groupby.SeriesGroupBy'.</p>
  
  <p>I have found <code>.sort_values()</code> , but it is a built-in sort function by values. <br>
   I want to get some pointers to set some order to use it further data manipulation.<br>
  Thanks in advance.</p>