title: Pandas DataFrame concatenation in for loop returns empty DataFrame
body: <p>I am trying to stack several cyclical data elements of a DataFrame on top of each other to change the DataFrame dimensions. E.g. Go from 100x20 to 500x4.</p>
  
  <pre><code>Sample 11x7 input:
      0   1   2   3   4   5   6   7       
  0   1   713 1622    658 1658    620 1734        
  1   2   714 1623    657 1700    618 1735    
  2   3   714 1624    656 1701    617 1736    
  3   4   714 1625    655 1702    615 1738    
  4   5   714 1626    654 1703    614 1739    
  5   6   713 1627    653 1705    612 1740    
  6   7   713 1628    651 1706    610 1741    
  7   8   713 1629    650 1707    609 1742    
  8   9   713 1630    649 1709    607 1744    
  9   10  713 1631    648 1710    605 1745    
  10  11  712 1632    646 1711    604 1746    
  
  Desired 32x3 output:
  0   1   713 1622    
  1   2   714 1623    
  2   3   714 1624    
  3   4   714 1625    
  4   5   714 1626    
  5   6   713 1627    
  6   7   713 1628    
  7   8   713 1629    
  8   9   713 1630    
  9   10  713 1631    
  10  11  712 1632    
  11  1   658 1658    
  12  2   657 1700    
  13  3   656 1701    
  14  4   655 1702    
  15  5   654 1703    
  16  6   653 1705    
  17  7   651 1706    
  18  8   650 1707    
  19  9   649 1709    
  20  10  648 1710    
  21  11  646 1711    
  22  1   620 1734        
  23  2   618 1735    
  24  3   617 1736    
  25  4   615 1738    
  26  5   614 1739    
  27  6   612 1740    
  28  7   610 1741    
  29  8   609 1742    
  30  9   607 1744    
  31  10  605 1745    
  32  11  604 1746
  </code></pre>
  
  <p>I have spent an inordinate amount of time checking this, and I cannot find anything better than </p>
  
  <pre><code>pd.concat([df1, df2], ignore_index=True) 
  or 
  df1.append(df2, ignore_index=True)
  </code></pre>
  
  <p>, which should produce identical solutions inthis case. However, whichever option is used, it is going to be placed at the end of a loop that produces temporary DataFrames to be concatenated with or appended to the permanent DataFrame. The temp df's come out fine, but the allegedly straightforward concatenation step fails consistently. I get an empty DataFrame with a proper header...</p>
  
  <pre><code>
  for l in range(1,13):
      s1 = l * 4 - 4
      s2 = l * 4 
      dft = df0.iloc[:, s1:s2]
      dft.columns = new_col
      #pd.concat([df1, dft], ignore_index=True, axis = 0)
      #df1.append(dft, ignore_index=True)
  df1.head()
  </code></pre>
  
  <p>Either of the commented out lines should produce a stack of 4-wide temp DataFrames... I get an empty DataFrame with a proper header and no error messages...</p>