title: Pandas Groupby - No objects to concatenate
body: <p>I have a Dataframe with following values:</p>
  
  <pre><code>    df = 
  route    stop_code      stop_name
  900      92072          Eastbound @ 257 Kingston Road East   
  900      1590           Kingston Westbound @ Wicks   
  900      2218           Kingston Eastbound @ Wicks   
  900      93152          Salem Northbound @ Kingston   
  92       728            Kingston Rd. @ Salem Rd.   
  224      92071          Salem Southbound @ Twilley   
  215      92071          Salem Southbound @ Twilley   
  215      92054          Northbound @ 133 Salem   
  224      92054          Northbound @ 133 Salem   
  215      93152          Salem Northbound @ Kingston   
  </code></pre>
  
  <p>What I want is to group routes by stop_code or stop_name, something like:</p>
  
  <pre><code>df2 = 
      route         stop_code      stop_name
      900           92072          Eastbound @ 257 Kingston Road East   
      900           1590           Kingston Westbound @ Wicks   
      900           2218           Kingston Eastbound @ Wicks   
      92            728            Kingston Rd. @ Salem Rd.   
      224, 215      92071          Salem Southbound @ Twilley   
      215, 215      92054          Northbound @ 133 Salem   
      215, 900      93152          Salem Northbound @ Kingston 
  </code></pre>
  
  <p>I tried to do the following:</p>
  
  <pre><code>df2 = df.groupby(['stop_code']).agg(set).reset_index()
  </code></pre>
  
  <p>while it did work fine in my test environment, when I deployed it in Django (Python Anywhere), I got the following error (maybe due to different versions of Pandas / Python / Django):</p>
  
  <pre><code>ValueError: No objects to concatenate
  </code></pre>
  
  <p>can anyone please guide me sort it out? TIA</p>