title: Zip pandas dataframes into a new dataframe
body: <p>I have 2 dataframes:</p>
  
  <p>df_A</p>
  
  <pre><code>   country_codes
  0              4
  1              8
  2             12
  3             16
  4             24
  </code></pre>
  
  <p>and df_B</p>
  
  <pre><code>   continent_codes
  0                4
  1                3
  2                5
  3                6
  4                5
  </code></pre>
  
  <p>Both dataframes have same length, but no common column. I want to concatenate the two but since not all values are common, I get lots of NaNs. How do I concatenate or zip them up into a combined dataframe?</p>
  
  <p>-- EDIT desired output is this:</p>
  
  <pre><code>   country_codes   continent_codes
  0              4      4
  1              8      3
  2             12      5
  3             16      6
  4             24      5
  </code></pre>