FastAPI プロジェクトのセットアップと最初のエンドポイント

FastAPI とは

# pip install fastapi uvicorn
from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def root():
    return {"message": "Hello, FastAPI"}

サーバーの実行

# main.py
from fastapi import FastAPI
app = FastAPI()

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

# Terminal:
# uvicorn main:app --reload
# → http://127.0.0.1:8000
# → http://127.0.0.1:8000/docs  (Swagger UI)

HTTP メソッド

from fastapi import FastAPI
app = FastAPI()

@app.get("/items")
def list_items(): return []

@app.post("/items")
def create_item(): return {"id": 1}

@app.delete("/items/{item_id}")
def delete_item(item_id: int): return {"deleted": item_id}

パスパラメータ

from fastapi import FastAPI
app = FastAPI()

@app.get("/users/{user_id}")
def get_user(user_id: int):
    return {"user_id": user_id, "name": "Alice"}

# GET /users/42  → {"user_id": 42, "name": "Alice"}

クエリパラメータ

from fastapi import FastAPI
app = FastAPI()

@app.get("/items")
def list_items(skip: int = 0, limit: int = 10, q: str | None = None):
    return {"skip": skip, "limit": limit, "q": q}

# GET /items?skip=5&limit=20&q=search

Pydantic のリクエストボディ

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel):
    name: str
    price: float
    in_stock: bool = True

@app.post("/items")
def create_item(item: Item):
    return {"received": item.model_dump()}

Pydantic の検証エラー

# POST /items with body {"name": 123}
# → HTTP 422 Unprocessable Entity
# {
#   "detail": [
#     {
#       "type": "string_type",
#       "loc": ["body","name"],
#       "msg": "Input should be a valid string"
#     }
#   ]
# }

レスポンスモデル

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class UserOut(BaseModel):
    id: int
    name: str

@app.get("/users/{uid}", response_model=UserOut)
def get_user(uid: int):
    return {"id": uid, "name": "Alice", "password": "secret"}  # password filtered out

ステータスコード

from fastapi import FastAPI, status
app = FastAPI()

@app.post("/items", status_code=status.HTTP_201_CREATED)
def create_item():
    return {"id": 1}

# HTTP 201 Created is returned on success

HTTPException

from fastapi import FastAPI, HTTPException
app = FastAPI()

fake_db = {1: "Alice"}

@app.get("/users/{uid}")
def get_user(uid: int):
    if uid not in fake_db:
        raise HTTPException(status_code=404, detail="User not found")
    return {"name": fake_db[uid]}

自動ドキュメント生成

# After starting with uvicorn:
# http://localhost:8000/docs     — Swagger UI (try the API live)
# http://localhost:8000/redoc    — ReDoc (clean docs view)
# http://localhost:8000/openapi.json  — raw OpenAPI schema

確認問題

復習