How do you work with dictionaries (get, setdefault, items, merging)?

Question presented to candidate: "Walk me through the everyday dictionary toolkit in Python. How do you read a key that might be missing, group items into lists, loop over keys and values, and merge two dictionaries — and which of those have traps?"

What a strong answer should cover:

• d[key] raises KeyError; d.get(key, default) returns a default instead, and d.get(k) or x is wrong when the stored value is falsy. in tests keys, not values.
• setdefault(key, default) returns the value and inserts the default first if the key is missing — the classic grouping idiom — but the default expression is evaluated on every call; collections.defaultdict calls its factory only for missing keys.
• keys(), values() and items() return live views; items() is how you loop over key/value pairs; key views support set operations; deleting while iterating raises RuntimeError.
• Merging: {**a, **b}, a | b (Python 3.9, PEP 584), a |= b and update() — the right-hand side wins, merging is shallow, and | needs a mapping on the right while |=/update also take an iterable of key/value pairs.
• Dicts keep insertion order (a language guarantee since 3.7); overwriting a key keeps its position, popitem removes the last item, dict.fromkeys shares one value object.

Code / implementation expected: Yes — a word-grouping or counting snippet, and a config merge with defaults overridden by user values.