title: Using a list as a value in a pandas dataframe
body: <p>I have some data that I want to insert in a dataframe. The data is <code>columns= ['Title', 'Category']</code>. For each Titles I have one or more Categories, and I decided to insert the categories as a list. So my df looks like this:</p>
  
  <pre><code>In [39]: title_cat_df
  Out[39]: 
      Title      Category
  0  Title1  [Cat1, Cat2]
  1  Title3        [Cat5]
  2  Title2  [Cat3, Cat4]
  ...
  ...
  ...
  </code></pre>
  
  <p>However I don't know if this is a pythonic/pandaionic(?!) approach, since I have stumbled upon problems such as looking for specific categories using <code>isin</code>:</p>
  
  <pre><code>In [41]: test_df['Category'].isin(cat_list)
  Out[41]: TypeError: unhashable type: 'list'
  </code></pre>
  
  <p>What would be a better way to represent categories in this case, and hopefully be able to look for titles in a specific category or categories?</p>