The difference is that a defaultdict will “default” a value if that key has not been set yet. If you didn’t use a defaultdict you’d have to check to see if that key exists, and if it doesn’t, set it to what you want.
The lambda is defining a factory for the default value. That function gets called whenever it needs a default value. You could hypothetically have a more complicated default function.
Help on class defaultdict in module collections:
class defaultdict(__builtin__.dict)
| defaultdict(default_factory) --> dict with default factory
|
| The default factory is called without arguments to produce
| a new value when a key is not present, in __getitem__ only.
| A defaultdict compares equal to a dict with the same items.
|
(from help(type(collections.defaultdict())))
{}.setdefault is similar in nature, but takes in a value instead of a factory function. It’s used to set the value if it doesn’t already exist… which is a bit different, though.