Default value for non-existent object keys

[linkyndy]

It should be a way to silently retrieve a non-existent object key, especially since remodel's interface wants to be dict-like (which has get(key[, default])).

In the end, we'll be able to do the following:

user = User(name='Andrei')

print user.get('name') # prints 'Andrei'
print user.get('address') # prints None
print user.get('address', 'Example Street') # prints 'Example Street'