FastAPI vs Flask（2026年）：どちらのPython Webフレームワークを選ぶべきか？

from flask import Flask
 
app = Flask(__name__)
 
@app.route('/')
def hello():
    return {'message': 'Hello, World!'}
 
if __name__ == '__main__':
    app.run(debug=True)

from fastapi import FastAPI
 
app = FastAPI()
 
@app.get('/')
def hello():
    return {'message': 'Hello, World!'}
 
# 実行: uvicorn main:app --reload

from flask import Flask, request, jsonify
 
app = Flask(__name__)
 
@app.route('/users', methods=['POST'])
def create_user():
    data = request.get_json()
 
    # 手動バリデーション
    if not data or 'name' not in data:
        return jsonify({'error': 'name is required'}), 400
    if not isinstance(data.get('age'), int) or data['age'] < 0:
        return jsonify({'error': 'age must be a positive integer'}), 400
    if 'email' not in data or '@' not in data['email']:
        return jsonify({'error': 'valid email is required'}), 400
 
    # 有効なデータを処理
    return jsonify({'id': 1, **data}), 201

from fastapi import FastAPI
from pydantic import BaseModel, EmailStr
 
app = FastAPI()
 
class UserCreate(BaseModel):
    name: str
    age: int
    email: EmailStr
 
@app.post('/users')
def create_user(user: UserCreate):
    # バリデーションはこの関数が実行される前に自動的に行われます
    # 無効なリクエストは詳細な422エラーレスポンスを受け取ります
    return {'id': 1, **user.model_dump()}

from fastapi import FastAPI
import httpx
 
app = FastAPI()
 
@app.get('/data')
async def get_data():
    async with httpx.AsyncClient() as client:
        # ノンブロッキング：待機中に他のリクエストを処理可能
        response = await client.get('https://api.example.com/data')
        return response.json()

from flask import Flask
import requests
 
app = Flask(__name__)
 
@app.route('/data')
def get_data():
    # ブロッキング：レスポンスが届くまでスレッドが占有される
    response = requests.get('https://api.example.com/data')
    return response.json()

from fastapi import FastAPI
from pydantic import BaseModel
 
app = FastAPI(title="My API", version="1.0.0")
 
class Item(BaseModel):
    name: str
    price: float
    in_stock: bool = True
 
@app.get('/items/{item_id}')
def get_item(item_id: int):
    """IDでアイテムを取得する。"""
    return {'item_id': item_id, 'name': 'Widget'}
 
@app.post('/items')
def create_item(item: Item):
    """在庫に新しいアイテムを作成する。"""
    return {'id': 1, **item.model_dump()}
 
# http://localhost:8000/docs にアクセスしてSwagger UIを表示
# http://localhost:8000/redoc にアクセスしてReDocを表示

from fastapi import FastAPI
from pydantic import BaseModel
import numpy as np
 
app = FastAPI()
 
class PredictionRequest(BaseModel):
    features: list[float]
    model_name: str = "default"
 
class PredictionResponse(BaseModel):
    prediction: float
    confidence: float
 
@app.post('/predict', response_model=PredictionResponse)
def predict(req: PredictionRequest):
    # モデル推論
    features = np.array(req.features)
    prediction = float(features.mean())  # プレースホルダー
    return PredictionResponse(
        prediction=prediction,
        confidence=0.95
    )