When should you choose a list, tuple, set or dict?

Short answer

Use a list for an ordered, changeable sequence, a tuple for a fixed record, a set for fast membership checks and de-duplication, and a dict to map keys to values with fast lookup.

Explanation

The choice of structure expresses the contract you need: ordering, fast lookup, uniqueness, or key-to-value mapping. A membership test x in list is linear O(n), whereas in a set or dict it is O(1) on average. Since Python 3.7 a dict preserves insertion order as part of the language specification, while a set guarantees no order at all. Use a tuple for composite values that do not change, such as coordinates or compound dict keys.

Code example

allowed = {"admin", "editor"}      # set: Fast membership lookup
point = (30.0, 31.2)                  # tuple: Immutable log
prices = {"apple": 3, "milk": 2}   # dict: Key -> value
queue = ["a", "b"]                   # list: Mutable order

if role in allowed:
    ...

Common mistakes

• Using a list for repeated lookups inside a loop, which turns the algorithm into O(n²).
• Relying on the order of set elements when printing or returning them from an API.

Follow-up questions

• How does the choice of data structure change a program's performance?
• What is the difference between mutable and immutable objects in Python?

Sources

• Python tutorial — Data structures