title: Coalesce values from 2 columns into a single column in a pandas dataframe
body: <p>I'm looking for a method that behaves similarly to coalesce in T-SQL. I have 2 columns (column A and B) that are sparsely populated in a pandas dataframe. I'd like to create a new column using the following rules:</p>
  
  <ol>
  <li>If the value in column A <strong><em>is not null</em></strong>, use that value for the new column C</li>
  <li>If the value in column A <strong><em>is null</em></strong>, use the value in column B for the new column C</li>
  </ol>
  
  <p>Like I mentioned, this can be accomplished in MS SQL Server via the coalesce function. I haven't found a good pythonic method for this; does one exist?</p>