title: In python pandas, how to fill NaN value to nearest past Value?
body: <p>Filling NaN value in Pandas Dataframe is easy when the replacing value is static. </p>
  
  <p>But what I need to do is iterating through column from index 0 and remember the last observed non-NaN value and replace NaN value with it. </p>
  
  <pre>
  index       cur_price 
  2017-07-01  1000
  2017-07-02  NaN
  2017-07-03  12000
  2017-07-04  13000
  2017-07-05  NaN
  2017-07-06  9000
  2017-07-07  9500
  2017-07-08  NaN
  2017-07-09  NaN
  </pre>
  
  <p>it should be become </p>
  
  <pre>
  index       cur_price
  2017-07-01  1000
  2017-07-02  1000
  2017-07-03  12000
  2017-07-04  13000
  2017-07-05  13000
  2017-07-06  9000
  2017-07-07  9500
  2017-07-08  9500
  2017-07-09  9500
  </pre>
  
  <p>Is there good functionality in pandas for this? </p>