title: Pandas: join a row of data on specific index number
body: <pre><code>import pandas
  a=[['Date', 'letters', 'numbers', 'mixed'], ['1/2/2014', 'a', '6', 'z1'], ['1/2/2014', 'a', '3', 'z1'], ['1/3/2014', 'c', '1', 'x3']]
  df = pandas.DataFrame.from_records(a[1:],columns=a[0])
  
  b= [['a', 'b', 'c'], ['a', 'b', 'c']]
  df2 = pandas.DataFrame.from_records(b[1:],columns=b[0])
  </code></pre>
  
  <p>I want to join df2 on df so it looks like this:</p>
  
  <pre><code>       Date letters numbers mixed    a    b    c
  0  1/2/2014       a       6    z1  
  1  1/2/2014       a       3    z1    a    b    c
  2  1/3/2014       c       1    x3  
  </code></pre>
  
  <p>Looking through the docs, I got as close as <code>df=df.join(df2,how='outer')</code></p>
  
  <p>which gives you this:</p>
  
  <pre><code>       Date letters numbers mixed    a    b    c
  0  1/2/2014       a       6    z1    a    b    c
  1  1/2/2014       a       3    z1  NaN  NaN  NaN
  2  1/3/2014       c       1    x3  NaN  NaN  NaN
  </code></pre>
  
  <p>I want something like <code>df=df.join(df2,how='outer', on_index = 1)</code></p>