title: Can't drop NAN with dropna in pandas
body: <p>I import pandas as pd and run the code below and get the following result</p>
  
  <p>Code:</p>
  
  <pre><code>traindataset = pd.read_csv('/Users/train.csv')
  print traindataset.dtypes
  print traindataset.shape
  print traindataset.iloc[25,3]
  traindataset.dropna(how='any')
  print traindataset.iloc[25,3]
  print traindataset.shape
  </code></pre>
  
  <p>Output</p>
  
  <pre><code>TripType                   int64  
  VisitNumber                int64  
  Weekday                   object  
  Upc                      float64  
  ScanCount                  int64  
  DepartmentDescription     object  
  FinelineNumber           float64  
  dtype: object
  
  (647054, 7)
  
  nan  
  nan
  
  (647054, 7) 
  [Finished in 2.2s]
  </code></pre>
  
  <p>From the result, the dropna line doesn't work because the row number doesn't change and there is still NAN in the dataframe. How that comes? I am craaaazy right now.</p>