title: Pandas lookup, mapping one column in a dataframe to another in a different dataframe
body: <p>I have two pandas dataframes:  df1 and df2.</p>
  
  <p>df1 has columns X and Y and weeknum.
  df2 has columns Z, weeknum, and datetime.  </p>
  
  <p>I want to basically keep df1 and have an extra column in it that is corresponding datetime for weeknum.</p>
  
  <p>I can use merge but there must be a cleaner way, without having to drop column Z.  </p>