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:
1 default = 'Scruffy'
2 dog_owned_by = {'Peter': 'Furry', 'Sally': 'Fluffy'}
3
4 dogs = []
5 for owner in ('Peter', 'Sally', 'Tim'):
6 dogs.append(dog_owned_by.setdefault(owner, default))
7
8 # dogs == ['Furry', 'Fluffy', 'Scruffy']
9 # dog_owned_by == {'Tim': 'Scruffy', 'Peter': 'Furry', 'Sally': 'Fluffy'}