title: python pandas dataframe join two dataframes
body: <p>I am trying to join to data frames.  They look like this</p>
  
  <pre><code>DF1 = ID     COUNTRY     YEAR     V1     V2     V3    V4
        12     USA         2012     x      y      z      a
        13     USA         2013     x      y      z      a
        14     RUSSIA      2012     x      y      z      a
  
  DF2 = ID     COUNTRY     YEAR     TRACT
        9      USA         2000       A
        13     USA         2013       B
  </code></pre>
  
  <p>The desired end goal is:</p>
  
  <pre><code>DF3 = ID     COUNTRY     YEAR     V1     V2     V3    V4    TRACT    
        9      USA         2000                                 A
        12     USA         2012     x      y      z      a
        13     USA         2013     x      y      z      a      B
        14     RUSSIA      2012     x      y      z      a
  </code></pre>
  
  <p>I've been trying to use the pd.merge and the .join function with the on='outer' setting to no success</p>
  
  <pre><code>df3 = pd.merge(df1,df2,how='outer',left_on=['ID','Country','Year'],right_on=['ID',"Country","Year"])
  </code></pre>