defaultdict — missing keys return a default value

The standard dictionary includes the method setdefault() for retrieving a value and establishing a default if the value does not exist. By contrast, defaultdict lets the caller specify the default up front when the container is initialized.
import collections

def default_factory():
    return 'default value'

d = collections.defaultdict(default_factory, foo='bar')
print('d:', d)
print('foo =>', d['foo'])
print('bar =>', d['bar'])

This method works well as long as it is appropriate for all keys to have the same default. It can be especially useful if the default is a type used for aggregating or accumulating values, such as a list, set, or even int. The standard library documentation includes several examples of using defaultdict this way.

$ python3

d: defaultdict(<function default_factory at 0x101921950>,
{'foo': 'bar'})
foo => bar
bar => default value

See also