title: Replacing present values (all except NA/NaN) in pandas data frame column
body: <p>There is a <code>fillna</code> method for filling missing values, but is there a method for replacing all the actual values with a given value.</p>
  
  <p>For example:</p>
  
  <pre><code>      c
    0  NA/NaN
    1  2.0
    2  NA/NaN
    3  6.0
    4  8.0
    5  NA/NaN
    6  12.0
  </code></pre>
  
  <p>For every data point, I would like to mark it with 'v' meaning that it contains a valid value:</p>
  
  <pre><code>      c
    0   NA/NaN
    1  'v'
    2   NA/NaN
    3  'v'
    4  'v'
    5   NA/NaN
    6  'v'
  </code></pre>