title: Returning an alternate column in pandas dataframe
body: <p>I have two dataframes. Dataframe A is an experimental dataframe which contains a list of things which have been used (along with date etc). Dataframe B is a reference dataframe. Dataframe A and B have matching index numbers. I want to update Dataframe A with extra information from Dataframe B, where the index numbers match. </p>
  
  <p>For example </p>
  
  <p>dfA</p>
  
  <pre><code>REF
  ABC
  DEF
  DEF
  XYZ 
  </code></pre>
  
  <p>dfB </p>
  
  <pre><code>REF VALUE
  ABC 1.23
  DEF 2.22
  XYZ 3.33
  </code></pre>
  
  <p>In reality the reference dataframe is much larger than the experimental dataframe. I would like to create a new column in dataframe A with the value from dataframe B based on matching references. I have tried 'is in' and where but the mis matched lengths of the series creating an error. 
  I have tried using merge but as dataframe A has repetitions of the reference value the merged dataframe has too many rows. 
  Is there an effective way to do this without creating a new series or column for each reference?</p>