Is the REST APIs with FastAPI lesson free?

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

How long does the REST APIs with FastAPI lesson take?

About 18 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 REST APIs with FastAPI 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 REST APIs with FastAPI lesson?

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

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Advanced·18 min·web · fastapi · rest · api

REST APIs with FastAPI

FastAPI is Flask's modern successor — same simplicity, but type hints do triple duty:

Editor autocomplete — IDEs know the shape of your data.
Runtime validation — Pydantic rejects bad payloads with a clear 422 error.
Auto-generated OpenAPI docs — visit /docs for a live Swagger UI, /redoc for ReDoc.

Core concepts

Pydantic models = your API contract

class TodoIn(BaseModel):
    title: str = Field(min_length=1, max_length=200)
    done: bool = False

If a client POSTs {"title": ""}, FastAPI auto-rejects with 422. You don't write validation code.

Response models

response_model=Todo filters output — you can't leak fields you didn't declare.

HTTPException

Raise HTTPException(404, "Not found") for typed error responses.

Async when you need it

Any handler can be async def — great for calling other APIs or an async DB driver like asyncpg.

Run it

uvicorn main:app --reload

Then open http://localhost:8000/docs.

Try it (mentally, before running locally)

Add a DELETE /todos/{todo_id} endpoint.
Add a query param ?done=true to GET /todos that filters.

Practice

3 graded exercises

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

Progress0 / 3

Exercise 1
Create a FastAPI app with a GET / route that returns {"ok": True}. Wrap it in a TestClient, call /, and print the value of the ok field from the JSON response.
Exercise 2
Define a Pydantic model TodoIn with title: str and done: bool = False. Add a POST /todos route that accepts a TodoIn payload and returns the dict plus an id: 1. POST {"title": "ship"} and print the returned title.
Exercise 3
Add a GET /todos/{todo_id} route. If todo_id > 100, raise HTTPException(status_code=404). Otherwise return {"id": todo_id, "title": f"todo #{todo_id}"}. Call /todos/5 and /todos/999, then print each response's status code on its own line.

← PreviousYour first Flask web server Next →Auth: password hashing + JWTs

REST APIs with FastAPI

FastAPI is Flask's modern successor — same simplicity, but type hints do triple duty:

Editor autocomplete — IDEs know the shape of your data.
Runtime validation — Pydantic rejects bad payloads with a clear 422 error.
Auto-generated OpenAPI docs — visit /docs for a live Swagger UI, /redoc for ReDoc.

Core concepts

Pydantic models = your API contract

class TodoIn(BaseModel):
    title: str = Field(min_length=1, max_length=200)
    done: bool = False

If a client POSTs {"title": ""}, FastAPI auto-rejects with 422. You don't write validation code.

Response models

response_model=Todo filters output — you can't leak fields you didn't declare.

HTTPException

Raise HTTPException(404, "Not found") for typed error responses.

Async when you need it

Any handler can be async def — great for calling other APIs or an async DB driver like asyncpg.

Run it

uvicorn main:app --reload

Then open http://localhost:8000/docs.

Try it (mentally, before running locally)

Add a DELETE /todos/{todo_id} endpoint.
Add a query param ?done=true to GET /todos that filters.

Practice

3 graded exercises

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

Progress0 / 3

Exercise 1
Create a FastAPI app with a GET / route that returns {"ok": True}. Wrap it in a TestClient, call /, and print the value of the ok field from the JSON response.
Exercise 2
Define a Pydantic model TodoIn with title: str and done: bool = False. Add a POST /todos route that accepts a TodoIn payload and returns the dict plus an id: 1. POST {"title": "ship"} and print the returned title.
Exercise 3
Add a GET /todos/{todo_id} route. If todo_id > 100, raise HTTPException(status_code=404). Otherwise return {"id": todo_id, "title": f"todo #{todo_id}"}. Call /todos/5 and /todos/999, then print each response's status code on its own line.