None is the default, so it won't fail.
http://docs.python.org/2/library/stdtypes.html#dict.get
<pre>
$ python
Python 2.7.3 (default, Aug  1 2012, 05:16:07) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> d = {'item1': 1, 'item2': 2}
>>> d.get('item3')
>>> d.get('item3') is None
True
</pre>