FastAPI: your first endpoint

Part 1 faked the server. FastAPI builds a real one: you write ordinary Python functions and say which URL each one answers.

from fastapi import FastAPI

app = FastAPI()


@app.get("/health")
def health():
    return {"status": "ok"}

app = FastAPI() makes the application. @app.get("/health") is a decorator saying: when a GET request arrives for /health, run the function below. A function connected to a URL like this is an endpoint.

The function returns a dictionary and FastAPI sends it as JSON. /health is a common first endpoint: other systems call it to check the service is up.

Calling it

from fastapi.testclient import TestClient
from health import app

client = TestClient(app)
response = client.get("/health")
print(response.status_code)
print(response.json())

TestClient is an httpx2.Client whose transport hands each request straight to your app, the way MockTransport handed it to a function in lesson 3. Your app does not know the difference, and no server has to be started. Lesson 8 runs the same app on a real server.

A URL nobody wrote

print(client.get("/status").status_code)
print(client.get("/status").json())
print(client.post("/health").status_code)

A path with no endpoint is 404. A path that exists, called with the wrong method, is 405, method not allowed. FastAPI sends both for you.