Python raises a KeyError whenever a dict() object is requested (using the format a = adict[key]) and the key is not in the dictionary.
If you don't want to have an exception but would rather a default value used instead, you can use the get() method:
Even more handy is somewhat controversially-named setdefault(key, val) which sets the value of the key only if it is not already in the dict, and returns that value in any case: