Data Structures — Overview

Python ships with a rich set of data structures. Knowing which one to reach for — and why — is one of the clearest markers of Python fluency.

The landscape

Structure	Ordered	Mutable	Duplicates	Import needed
list	✓	✓	✓	—
tuple	✓	✗	✓	—
dict	✓ (3.7+)	✓	keys: ✗	—
set	✗	✓	✗	—
frozenset	✗	✗	✗	—
deque	✓	✓	✓	collections
defaultdict	✓	✓	keys: ✗	collections
Counter	✓	✓	keys: ✗	collections
namedtuple	✓	✗	✓	collections

Quick mental model

Need a sequence you'll change?          → list
Need a sequence that must not change?   → tuple
Need named fields on a tuple?           → namedtuple
Need key → value lookup?                → dict
Need key → value with a safe default?   → defaultdict
Need to count things?                   → Counter
Need fast append/pop from both ends?    → deque
Need membership tests, no duplicates?   → set
Need an immutable set (e.g. dict key)?  → frozenset

The rest of this chapter covers each structure in depth, with exercises after every section.