title: How to append two pandas.DataFrame with different numbers of columns
body: <p>Based on the fact that directly append two <code>dataframe</code> with different numbers of columns, an error would occur as <code>pandas.io.common.CParserError: Error tokenizing data. C error: Expected 4 fields in line 242, saw 5</code>. How can I do with <code>pandas</code> to avoid the error??</p>
  
  <p>I have figure out one naive approach: just to process the original data, to make the numbers of columns equally.</p>
  
  <p>Can it be more elegant?? I think the missing columns can be filled with <code>np.nan</code> after <code>pd.append</code>.</p>