Dict[str, List[str]] = defaultdict(list)

This line of code creates a default dictionary where the keys are strings and the values are lists of strings. The default value for any new key is an empty list.

For example, if we have the following code:

from collections import defaultdict

my_dict: Dict[str, List[str]] = defaultdict(list)

my_dict["key1"].append("value1")
my_dict["key2"].append("value2")
my_dict["key1"].append("value3")

print(my_dict)

The output will be:

defaultdict(<class 'list'>, {'key1': ['value1', 'value3'], 'key2': ['value2']})

Here, we can see that the dictionary has two keys, "key1" and "key2". The values for "key1" are the list ["value1", "value3"], and the value for "key2" is the list ["value2"]. Since we used a default dictionary, we did not need to check if the keys had been initialized with an empty list before appending values to them.