title: Pandas Fill NaN with Column Values
body: <p>Given the following data frame:</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  df = pd.DataFrame({'A':[1,1,np.nan],
                     'B':[2.2,np.nan,2.2]})           
  df
  
      A       B
  0   1.0     2.2
  1   1.0     NaN
  2   NaN     2.2
  </code></pre>
  
  <p>If I want to replace the NaN value in column A with the value that repeats in that column (1) and do the same for column B, what sort of fillna() do I need to use?</p>
  
  <pre><code>    A       B
  0   1.0     2.2
  1   1.0     NaN
  2   NaN     2.2
  </code></pre>
  
  <p>Looking for a generic solution as I really have thousands of rows.
  Thanks in advance!</p>