title: python strptime format with optional bits
body: <p>Right now I have:</p>
  
  <pre><code>timestamp = datetime.strptime(date_string, '%Y-%m-%d %H:%M:%S.%f')
  </code></pre>
  
  <p>This works great unless I'm converting a string that doesn't have the microseconds.  How can I specify that the microseconds are optional (and should be considered 0 if they aren't in the string)?</p>