~/learn/Python · Collections
Sets and dictionaries
Membership tests and key-value lookups.
why this matters
Dictionaries let you look things up by name instantly — like a price list. Sets remove duplicates in one step.
A set holds unique, unordered values and tests membership in constant time. It also supports mathematical operations.
A dict maps keys to values. Use .get for a safe lookup with a default, and .items() to loop over pairs.
Counting occurrences is the classic dict pattern, and a dict comprehension builds one in a single line.
Pro tip
Use d.get(key, default) instead of d[key] when the key might be missing — it won't crash.
key takeaways
- dict maps keys → values.
- set keeps unique items only.
- Lookups in both are very fast.
check your understanding
prices = {"tea": 2}. What does prices.get("coffee", 0) return?
free practice
Build confidence with a question you can solve without Pro.