title: How To replace multiple unknown values with default value except nan in python dataframe column
body: <p>I have  a dataframe. How can I replace  multiple unknown values with default value except nan in python dataframe column.</p>
  <pre><code>df = S.No.  Columns_A
  
        1      python
        2       java
        3        NAN
        4       C++
        5       python , java
  </code></pre>
  <p>How to get updated data frame</p>
  <pre><code>df_updated = S.No.  Columns_A
                 1      Good
                 2      Good
                 3      NAN
                 4      Good
                 5      Good
  </code></pre>