title: Pandas KeyError when working on split data frame
body: <p>I want to perform some operations on a pandas data frame that is split into chunks. After splitting the data frame, I then try to iterate over the chunks, but after the first iterations runs well, I get an error (see below). I have gone through some questions like these: <a href="https://stackoverflow.com/questions/22757906/keyerror-0l-when-running-packaged-code">1</a> and <a href="https://stackoverflow.com/questions/12406162/keyerror-when-plotting-a-sliced-pandas-dataframe-with-datetimes">2</a> but they don't quite address my issue. Kindly help me resolve this as I don't fully understand it.</p>
  
  <pre><code>import pandas as pd
  
  tupList = [('Eisenstadt', 'Paris','1', '2'), ('London', 'Berlin','1','3'), ('Berlin', 'stuttgat','1', '4'),
             ('Liverpool', 'Southampton','1', '5'),('Tirana', 'Blackpool', '1', '6'),('blackpool', 'tirana','1','7'),
             ('Paris', 'Lyon','1','8'), ('Manchester', 'Nice','1','10'),('Orleans', 'Madrid','1', '12'),
             ('Lisbon','Stockholm','1','12')]
  
  
  cities = pd.DataFrame(tupList, columns=['Origin', 'Destination', 'O_Code', 'D_code'])
  
  
  # purpose - splits the DataFrame into smaller of max size chunkSize (last is smaller)
  def splitDataFrameIntoSmaller(df, chunkSize = 3):
      listOfDf = list()
      numberChunks = len(df) // chunkSize + 1
      for i in range(numberChunks):
          listOfDf.append(df[i*chunkSize:(i+1)*chunkSize])
      return listOfDf
  
  citiesChunks = splitDataFrameIntoSmaller(cities)
  
  for ind, cc in enumerate(citiesChunks):
      cc["distance"] = 0
      cc["time"] = 0
  
      for i in xrange(len(cc)):
          al = cc['Origin'][i]
          bl = cc['Destination'][i]
          '...' #trucating to make it readable
  
      cc.to_csv('out.csv', sep=',', encoding='utf-8')
  
  
  Traceback (most recent call last):
    File ..., line 39, in &lt;module&gt;
      al = cc['Origin'][i]
    File ..., line 603, in __getitem__
      result = self.index.get_value(self, key)
    File ..., line 2169, in get_value
      tz=getattr(series.dtype, 'tz', None))
    File "pandas\index.pyx", line 98, in pandas.index.IndexEngine.get_value (pandas\index.c:3557)
    File "pandas\index.pyx", line 106, in pandas.index.IndexEngine.get_value (pandas\index.c:3240)
    File "pandas\index.pyx", line 154, in pandas.index.IndexEngine.get_loc (pandas\index.c:4279)
    File "pandas\src\hashtable_class_helper.pxi", line 404, in pandas.hashtable.Int64HashTable.get_item (pandas\hashtable.c:8564)
    File "pandas\src\hashtable_class_helper.pxi", line 410, in pandas.hashtable.Int64HashTable.get_item (pandas\hashtable.c:8508)
  KeyError: 0L
  </code></pre>