title: Try, Except ValueError replaces with None
body: <p>I have a function to covert string dates to date times for values in a list of list. Not all values are string dates so I am using a <code>try</code> and <code>except ValueError</code> function. For things that are not string dates, I would like to just do nothing, but the code is overwriting the values with <code>None</code>. </p>
  
  <pre><code>import datetime
  
  def parse_date(datestamp):
        try:
            return datetime.datetime.strptime(str(datestamp)[:10], '%Y-%m-%d')
        except ValueError:
            pass
  
  My_List = [['ID1', '2015-1-12'], ['ID2', '2015-12-12']]
  
  My_List =  [[parse_date(x) for x in i] for i in My_List]
  
  print My_List
  </code></pre>
  
  <p>Desired Output:</p>
  
  <pre><code>[[ID1, datetime.datetime(2015, 1, 12, 0, 0)], [ID2, datetime.datetime(2015, 12, 12, 0, 0)]]
  </code></pre>