title: python pandas not concat'ing into empty DataFrame
body: <p>I'm iterating over a <code>DataFrame</code>, evaluating each row and then sticking it into another <code>DataFrame</code> by using the <code>concat()</code> method. However, the receiving <code>DataFrame</code> is still empty.</p>
  
  <pre><code>import pandas as pd
  
  empty = DataFrame(columns=('col1', 'col2'))
  
  d = {'col1' : Series([1, 2, 3]),
      'col2' : Series([3, 4, 5])
  }
  
  some_data = DataFrame(d)
  
  print empty
  print some_data
  print 'concat should happen below'
  
  for index, row in some_data.iterrows():
      pd.concat([empty, DataFrame(row)])
  
  print empty # should contain 3 rows of data
  </code></pre>
  
  <p>OUTPUT:</p>
  
  <pre><code>Empty DataFrame
  Columns: [col1, col2]
  Index: []
     col1  col2
  0     1     3
  1     2     4
  2     3     5
  concat should happen below
  Empty DataFrame
  Columns: [col1, col2]
  Index: []
  </code></pre>