Web API：用路由、参数校验和异常响应建立服务边界

python -m pip install fastapi uvicorn

from typing import Literal

from fastapi import FastAPI, HTTPException, Query, status
from pydantic import BaseModel, Field

app = FastAPI(title="Task API")


class TaskCreate(BaseModel):
    title: str = Field(min_length=1, max_length=200)


class Task(BaseModel):
    id: int
    title: str
    status: Literal["todo", "doing", "done"]


tasks: dict[int, Task] = {}


@app.post("/tasks", response_model=Task, status_code=status.HTTP_201_CREATED)
def create_task(payload: TaskCreate) -> Task:
    task_id = max(tasks, default=0) + 1
    task = Task(id=task_id, title=payload.title, status="todo")
    tasks[task_id] = task
    return task


@app.get("/tasks", response_model=list[Task])
def list_tasks(limit: int = Query(default=20, ge=1, le=100)) -> list[Task]:
    return list(tasks.values())[:limit]


@app.get("/tasks/{task_id}", response_model=Task)
def get_task(task_id: int) -> Task:
    task = tasks.get(task_id)
    if task is None:
        raise HTTPException(status_code=404, detail="task not found")
    return task

uvicorn api:app --reload --host 127.0.0.1 --port 8000
curl -i -X POST http://127.0.0.1:8000/tasks \
  -H 'Content-Type: application/json' \
  -d '{"title":"build API boundary"}'
curl -i http://127.0.0.1:8000/tasks/1
curl -i http://127.0.0.1:8000/tasks/999

from fastapi.testclient import TestClient

from api import app

client = TestClient(app)


def test_create_and_get_task() -> None:
    created = client.post("/tasks", json={"title": "test contract"})
    assert created.status_code == 201
    task_id = created.json()["id"]

    fetched = client.get(f"/tasks/{task_id}")
    assert fetched.status_code == 200
    assert fetched.json()["title"] == "test contract"


def test_reject_empty_title() -> None:
    response = client.post("/tasks", json={"title": ""})
    assert response.status_code == 422