title: Is not NaN conditional statement for python 3 and pandas
body: <p>I am trying to create a new column in a pandas data frame by and calculating the value from existing columns.  </p>
  
  <p>I have 3 existing columns ("launched_date", "item_published_at", "item_created_at")</p>
  
  <p>However, my "if row[column_name] is not None:" statement is allowing columns with NaN value and not skipping to the next statement.  </p>
  
  <p>In the code below, I would not expect the value of "nan" to be printed after the first conditional, I would expect something like "2018-08-17"</p>
  
  <pre><code>df['adjusted_date'] = df.apply(lambda row: adjusted_date(row), axis=1)
  
  
  
  def adjusted_launch(row): 
      if row['launched_date']is not None:
          print(row['launched_date'])
          exit()
          adjusted_date = date_to_time_in_timezone(row['launched_date']) 
  
      elif row['item_published_at'] is not None:
          adjusted_date = row['item_published_at']#make datetime in PST
      else:
          adjusted_date = row['item_created_at'] #make datetime in PST
      return adjusted_date
  </code></pre>
  
  <p>How can I structure this conditional statement correctly? </p>