Test your knowledge: collections.defaultdict

Review

1. When a missing key is accessed using groups[key], what does defaultdict(list) do?

   The factory list() is called without arguments; the returned list is stored and returned.
2. Does groups.get("new") call the default factory for an absent key?

   Only missing-key square-bracket lookup invokes __missing__; get() does not.
3. Which default factory is appropriate when each key collects distinct tags?

   A set keeps one copy of each unique tag per key.
4. How is a custom factory called for a missing key?

   default_factory is called without arguments and its returned value becomes that key’s value.
5. Which type is designed specifically for counting occurrences of hashable items?

   Counter communicates that the values are frequency counts and provides methods tailored to that job.