title: Dummy variables when not all categories are present
body: <p>I have a set of dataframes where one of the columns contains a categorical variable. I'd like to convert it to several dummy variables, in which case I'd normally use <code>get_dummies</code>.</p>
  
  <p>What happens is that <code>get_dummies</code> looks at the data available in each dataframe to find out how many categories there are, and thus create the appropriate number of dummy variables. However, in the problem I'm working right now, I actually know in advance what the possible categories are. But when looking at each dataframe individually, not all categories necessarily appear.</p>
  
  <p>My question is: is there a way to pass to <code>get_dummies</code> (or an equivalent function) the names of the categories, so that, for the categories that don't appear in a given dataframe, it'd just create a column of 0s?</p>
  
  <p>Something that would make this:</p>
  
  <pre><code>categories = ['a', 'b', 'c']
  
     cat
  1   a
  2   b
  3   a
  </code></pre>
  
  <p>Become this:</p>
  
  <pre><code>  cat_a  cat_b  cat_c
  1   1      0      0
  2   0      1      0
  3   1      0      0
  </code></pre>