如何用 Pydantic 验证接口参数和返回值，避免 90% 的参数错误

from fastapi import FastAPI
from pydantic import BaseModel, EmailStr

app = FastAPI()

# 定义用户注册的数据结构
class UserRegister(BaseModel):
    username: str
    email: EmailStr
    password: str
    age: int = None

@app.post("/register")
def user_register(user: UserRegister):
    """
    用户注册的接口
    :param user: 用户注册信息（请求体参数，类型为 UserRegister）
    :return: 注册成功的响应
    """
    return {"message": "用户注册成功", "data": user}

{
  "username": "testuser",
  "password": "Test1234"
}

{
  "detail": [
    {
      "loc": [
        "body",
        "email"
      ],
      "msg": "field required",
      "type": "value_error.missing"
    }
  ]
}

{
  "username": "testuser",
  "email": "testuser@example.com",
  "password": "Test1234",
  "age": "abc"
}

{
  "detail": [
    {
      "loc": [
        "body",
        "age"
      ],
      "msg": "value is not a valid integer",
      "type": "type_error.integer"
    }
  ]
}

{
  "username": "testuser",
  "email": "testuserexample.com",
  "password": "Test1234",
  "age": 18
}

{
  "detail": [
    {
      "loc": [
        "body",
        "email"
      ],
      "msg": "value is not a valid email address",
      "type": "value_error.email"
    }
  ]
}

from fastapi import FastAPI
from pydantic import BaseModel, EmailStr, Field
import re

app = FastAPI()

class UserRegister(BaseModel):
    # 用户名：长度在 3-20 个字符之间，只能包含字母、数字和下划线
    username: str = Field(..., min_length=3, max_length=20, pattern=r"^[a-zA-Z0-9_]+$")
    # 邮箱：必填，必须是有效的邮箱格式
    email: EmailStr
    # 密码：长度在 8-20 个字符之间，必须包含至少一个大写字母、一个小写字母和一个数字
    password: str = Field(..., min_length=8, max_length=20, 
                          regex=r"^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).*$")
    # 年龄：可选，必须在 1-120 岁之间
    age: int = Field(None, ge=1, le=120)

from fastapi import FastAPI
from pydantic import BaseModel, EmailStr, Field, validator
import re

app = FastAPI()

class UserRegister(BaseModel):
    username: str = Field(..., min_length=3, max_length=20, pattern=r"^[a-zA-Z0-9_]+$")
    email: EmailStr
    password: str = Field(..., min_length=8, max_length=20, 
                          regex=r"^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).*$")
    age: int = Field(None, ge=1, le=120)

    # 自定义验证函数，验证密码不能包含用户名
    @validator("password")
    def password_not_contain_username(cls, v, values):
        """
        验证密码不能包含用户名的自定义函数
        :param cls: 类本身（Pydantic 验证时会自动传递）
        :param v: 密码字段的值
        :param values: 其他已验证的字段值（如果有）
        :return: 验证通过的密码值
        :raises ValueError: 验证失败时抛出的异常
        """
        if "username" in values and v.lower().find(values["username"].lower()) != -1:
            raise ValueError("密码不能包含用户名")
        return v

# 定义用户信息的数据结构
class UserInfo(BaseModel):
    user_id: int
    username: str
    email: EmailStr
    age: int = None
    create_time: str

# 修改用户注册接口的返回值类型
@app.post("/register", response_model=UserInfo)
def user_register(user: UserRegister):
    """
    用户注册的接口
    :param user: 用户注册信息（请求体参数，类型为 UserRegister）
    :return: 注册成功的响应（类型为 UserInfo）
    """
    # 模拟生成用户 ID 和创建时间
    user_id = 123
    create_time = "2025-01-01 10:00:00"
    # 构造返回值
    return {
        "user_id": user_id,
        "username": user.username,
        "email": user.email,
        "age": user.age,
        "create_time": create_time,
        # 额外的字段（不会返回给前端）
        "password": user.password
    }