Is the Type Hints lesson free?

Yes. The first 5 lessons on PyRun are free forever — no signup, no card. Type Hints runs real Python in your browser via Pyodide.

How long does the Type Hints lesson take?

About 7 minutes end-to-end. It's a hands-on lesson — you read a paragraph, edit the code, and run it in the same tab.

Do I need to install Python to try the Type Hints lesson?

No install required. PyRun runs real CPython in your browser via Pyodide (Python compiled to WebAssembly). Works on Chrome, Firefox, Safari, and Edge — desktop or mobile.

What level is the Type Hints lesson?

Advanced level. Assumes comfort with classes, functions, and standard library. Not for first-time Python learners.

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Advanced·7 min·advanced · typing

Type Hints

You’ll be able to

Annotate function signatures with parameter and return types
Use generic types from typing: List, Dict, Optional
Validate hints at development time with mypy or Pyright
Recognize that type hints do not enforce types at runtime

Why this matters

Pydantic, FastAPI, and SQLModel do runtime schema generation directly from typing annotations — a wrong Optional[int] versus int | None can silently change API validation. Staff engineers run mypy --strict in CI and use TypeVar, Protocol, and ParamSpec to type generic decorators without losing signatures.

Common pitfalls

Writing List[int] = [] as a default — mutable default trap; use Optional[List[int]] = None and init inside.
Confusing Optional[X] with X = None — Optional only marks nullability, it doesn't set the default.
Skipping from __future__ import annotations on Python <3.10 — forward references and X | Y syntax break at runtime.

Hints describe what types your functions expect and return. Python doesn't enforce them at runtime — tools like mypy, pyright, and your editor do.

Cheat sheet

x: int, name: str, flag: bool
list[int], dict[str, int], tuple[int, str] (Python 3.9+)
Optional[X] / X | None — nullable
Callable[[int, int], int] — function that takes two ints, returns int
Iterable[X] — anything you can for ... in

Why bother

Editor autocomplete catches typos and wrong-type calls
Self-documenting — type hints often replace half your docstring
Forces you to think about your interfaces

Try it

Add type hints to a function you've written.
What's the difference between list and list[int]?

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Write a typed function add(a: int, b: int) -> int that returns their sum. Then print add(3, 4) — expected: 7. The checker will verify the annotations are set correctly.
Exercise 2
Write a typed find_user(users: list[dict], name: str) -> dict | None that returns the first dict where d["name"] == name, or None if not found. Test with users = [{"name": "Ada"}, {"name": "Bob"}] — print find_user(users, "Bob") (dict) then find_user(users, "Zed") (None).
Exercise 3
Use TypedDict from typing to declare Session with fields user_id: int and token: str. Then write a function is_valid(s: Session) -> bool that returns True when token is non-empty. Test with a real dict — print is_valid({"user_id": 1, "token": "abc"}) and is_valid({"user_id": 2, "token": ""}).

← PreviousClosures & nonlocal Next →Dataclasses

Type Hints

You’ll be able to

Annotate function signatures with parameter and return types
Use generic types from typing: List, Dict, Optional
Validate hints at development time with mypy or Pyright
Recognize that type hints do not enforce types at runtime

Why this matters

Common pitfalls

Writing List[int] = [] as a default — mutable default trap; use Optional[List[int]] = None and init inside.
Confusing Optional[X] with X = None — Optional only marks nullability, it doesn't set the default.
Skipping from __future__ import annotations on Python <3.10 — forward references and X | Y syntax break at runtime.

Hints describe what types your functions expect and return. Python doesn't enforce them at runtime — tools like mypy, pyright, and your editor do.

Cheat sheet

x: int, name: str, flag: bool
list[int], dict[str, int], tuple[int, str] (Python 3.9+)
Optional[X] / X | None — nullable
Callable[[int, int], int] — function that takes two ints, returns int
Iterable[X] — anything you can for ... in

Why bother

Editor autocomplete catches typos and wrong-type calls
Self-documenting — type hints often replace half your docstring
Forces you to think about your interfaces

Try it

Add type hints to a function you've written.
What's the difference between list and list[int]?

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Write a typed function add(a: int, b: int) -> int that returns their sum. Then print add(3, 4) — expected: 7. The checker will verify the annotations are set correctly.
Exercise 2
Write a typed find_user(users: list[dict], name: str) -> dict | None that returns the first dict where d["name"] == name, or None if not found. Test with users = [{"name": "Ada"}, {"name": "Bob"}] — print find_user(users, "Bob") (dict) then find_user(users, "Zed") (None).
Exercise 3
Use TypedDict from typing to declare Session with fields user_id: int and token: str. Then write a function is_valid(s: Session) -> bool that returns True when token is non-empty. Test with a real dict — print is_valid({"user_id": 1, "token": "abc"}) and is_valid({"user_id": 2, "token": ""}).