Test your knowledge: Python Dictionaries

Review

1. For a key that is not in the dictionary, what is the difference between prices.get("rye") and prices["rye"]?

   Brackets on a missing key raise KeyError, stopping the program, while get returns a default you may pass, or None, and keeps walking. get is for absent being normal; brackets are for absent being a bug worth stopping for.
2. What does for name, price in menu.items() walk?

   items() yields the dictionary's entries as key and value pairs, which the loop unpacks into two names. Walking keys and indexing back inside the loop works, but it re-asks the dictionary a question it already answered.
3. What does "rye" in menu ask a dictionary?

   Membership on a dictionary asks about its keys, so "rye" in menu is true exactly when rye has an entry. Asking about values is a different query, one the dictionary does not index for.
4. What has Python guaranteed about dictionary order since version 3.7?

   Since 3.7, dictionaries preserve insertion order as a language guarantee, so a menu prints in the order it was written. Code written against older Pythons remembered to sort; code written now can trust it.
5. What is a dictionary whose values are dictionaries for?

   A dictionary of dictionaries models a collection of records, each record itself labeled, so orders keyed by id each carry their own fields. The lookup composes: one bracket for the record, another for the field.