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FastAPI 快速入门

一、安装

pip install fastapi uvicorn

二、基础 GET 接口

新建 main.py：

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def hello(name: str = "world"):
    return {"message": f"hello {name}"}

三、POST 接口实战

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

# 定义请求体模型，自动校验类型
class Item(BaseModel):
    name: str
    price: float
    is_offer: bool = False  # 可选字段，默认False

@app.post("/items/")
def create_item(item: Item):
    final_price = item.price * 0.9 if item.is_offer else item.price
    return {"name": item.name, "final_price": final_price}

请求自动映射流程

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自动映射 + 类型校验
客户端 POST JSON

{name: '可乐', price: 5}
FastAPI 接收请求
Pydantic Item 实例
业务逻辑处理
自动序列化返回 JSON
客户端收到响应

核心特性：请求体自动映射与校验

这是 FastAPI + Pydantic 最核心的魔力：

• 只要 POST 请求的 JSON 字段名与 Item 类字段名一一对应，就会自动映射 成 Item 实例，无需手动解析 JSON
• 自动完成参数校验：缺必填字段、类型不匹配时，直接返回标准 422 错误，无需手写判断逻辑
• 自动类型转换：例如传字符串 "5" 给 price: float，会自动转为数字 5.0
• 自动补全默认值：不传 is_offer 时，自动填充默认值 False
• 

四、启动与测试

启动服务

uvicorn main:app --reload

两种测试方式

1. 自动交互式文档 ：浏览器打开 http://127.0.0.1:8000/docs，可视化调试接口
2. 命令行调用

# GET 测试
curl http://127.0.0.1:8000/?name=fastapi

# POST 测试
curl -X POST http://127.0.0.1:8000/items/ \
  -H "Content-Type: application/json" \
  -d '{"name":"可乐","price":5,"is_offer":true}'

五、为什么 FastAPI 这么火

1. 性能强：基于 Starlette + Pydantic，是 Python 最快的 Web 框架之一，性能接近 Go/Node.js
2. 效率高：靠类型注解自动完成参数校验、类型转换，零配置生成 OpenAPI 文档
3. 标准兼容：原生支持 OpenAPI、JSON Schema 规范，适配前后端分离、微服务、AI 接口等场景
4. 门槛低：语法贴近原生 Python，兼容现有 Python 生态，学习成本极低