Dictionaries
Python Dictionary Interview Questions
Dictionaries are how you count and group by key in Python. Almost every question phrased as "how many X per Y" turns out to be a dictionary question.
What dictionary questions ask you to do
Across the questions here, these are the shapes that come up:
- Count how often each value appears, and take the top few
- Group rows into lists by a key
- Turn a mapping around so the values become the keys
- Merge two mappings with a rule for clashes
- Remember a result so it is not worked out twice
Common mistakes with dictionaries
Asking for a key that is not there raises an error, which is why first attempts end up full of checks for whether a key exists yet. Asking with a default value, or using defaultdict or Counter, removes every one of those checks.
Questions to practice
Questions last reviewed 19 October 2026.