title: Python class with datetime.now() when instantiated
body: <p>I want a Class attribute which sets <code>datetime.now()</code> when the a new instance of the class is instantiated.  With this code, <code>MyThing.created</code> seems to always be when <code>MyThing</code> is imported, as opposed to when <code>mt</code> is instantiated.</p>
  
  <pre><code>from datetime import datetime
  
  class MyThing:
      __init__(self, created=datetime.now()):
          self.created = created
  
  MyThing.created
  datetime.datetime(2012, 7, 5, 10, 54, 24, 865791)
  
  mt = MyThing()
  mt.created
  datetime.datetime(2012, 7, 5, 10, 54, 24, 865791)
  </code></pre>
  
  <p>How can I do it so that <code>created</code> is when <code>mt</code> is instantiated, as opposed to <code>MyThing</code>?</p>