What are the differences between a list, a set, and a dictionary?

A list is an ordered, mutable sequence that allows duplicate elements and supports indexing and slicing. A set is an unordered collection of unique elements; it is mutable but does not support indexing, and membership tests are O(1). A dictionary stores key‑value pairs; keys are unique and hashable, values can be any type. Dictionaries are unordered prior to Python 3.7, but maintain insertion order from 3.7 onward. Use lists for ordered data, sets for membership tests and deduplication, and dictionaries for mapping keys to values. Complexity: list membership O(n), set and dict membership O(1).