title: Difference between rows as a list within a column
body: <p>I have a (messy) pd.df like this, </p>
  
  <pre><code>Number    Day        Colour   City
  123       Monday     Red      London
  234       Monday     Blue     Paris
  234       Wednesday  Yellow   Paris
  345       Tuesday    Green    Berlin
  </code></pre>
  
  <p>Is there an elegant way of making a new df which has the same columns but shows the differences between two rows of like ['Number'] as a list within a columns. So the desired output is something like this.   </p>
  
  <pre><code>Number    Day                Colour        City
  123       Monday             Red           London
  234       Monday, Wednesday  Blue, Yellow  Paris
  345       Tuesday            Green         Berlin
  </code></pre>
  
  <p>Even a point in the right direction regarding which functions I should be looking at would be helpful. The values I am interested in will always be a string rather than a float or integer. Many thanks.</p>