title: Replacing the missing values in pandas
body: <p>I have a pandas dataframe where missing values are indicated as -999. </p>
  
  <pre><code>In [58]: df.head()
  Out[58]: 
  
  EventId                    A                  B                    C
  100000                   0.91           124.711             2.666000   
  100001                -999.00          -999.000            -0.202838   
  100002                -999.00          -999.000            -0.202838   
  100003                -999.00          -999.000            -0.202838   
  </code></pre>
  
  <p>I want to replace the missing values (indicated by -999) with the mean of that column taken over non-missing values.
  Which is the best way to do this? Is there any pandas function which can be used to achieve this easily?</p>