title: Using pandas .append within for loop
body: <p>I am appending rows to a pandas DataFrame within a for loop, but at the end the dataframe is always empty. I don't want to add the rows to an array and then call the DataFrame constructer, because my actual for loop handles lots of data. I also tried <code>pd.concat</code> without success. Could anyone highlight what I am missing to make the append statement work? Here's a dummy example:</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  
  data = pd.DataFrame([])
  
  for i in np.arange(0, 4):
      if i % 2 == 0:
          data.append(pd.DataFrame({'A': i, 'B': i + 1}, index=[0]), ignore_index=True)
      else:
          data.append(pd.DataFrame({'A': i}, index=[0]), ignore_index=True)
  
  print data.head()
  
  Empty DataFrame
  Columns: []
  Index: []
  [Finished in 0.676s]
  </code></pre>