title: How do I join two dataframes (pandas) with different indices?
body: <p>I'm working on a way to transform sequence/genotype data from a csv format to a genepop format.</p>
  
  <p>I have two dataframes: <code>df1</code> is empty, <code>df1.index</code> (rows = samples) consists of almost the same as <code>df2.index</code>, except I inserted "POP" in several places (to specify the different populations). <code>df2</code> holds the data, with <code>Loci</code> as columns.</p>
  
  <p>I want to insert the values from <code>df2</code> into <code>df1</code>, keeping empty rows where <code>df1.index = 'POP'</code>.</p>
  
  <p>I tried <code>join</code>, <code>combine</code>, <code>combine_first</code> and <code>concat</code>, but they all seem to take the rows that exist in both df's. </p>
  
  <p>Is there a way to do this?</p>