title: Python pandas: selecting rows whose column value is null / None / nan
body: <p>How do I select those rows of a DataFrame whose value in a column is none?</p>
  
  <p>I've coded these to <code>np.nan</code> and can't match against this type.</p>
  
  <pre><code>In [1]: import numpy as np
  
  In [2]: import pandas as pd
  
  In [3]: df = pd.DataFrame([[1, 2, 3], [3, 4, None]])
  
  In [4]: df
  Out[4]: 
     0  1    2
  0  1  2  3.0
  1  3  4  NaN
  
  In [5]: df = df.fillna(np.nan)
  
  In [6]: df
  Out[6]: 
     0  1    2
  0  1  2  3.0
  1  3  4  NaN
  
  In [7]: df.iloc[1][2]
  Out[7]: nan
  
  In [8]: df.iloc[1][2] == np.nan
  Out[8]: False
  
  In [9]: df[df[2] == None]
  Out[9]: 
  Empty DataFrame
  Columns: [0, 1, 2]
  Index: []
  </code></pre>