如何基于其他字段设置Pydantic属性值？解决未绑定变量报错

from pydantic import BaseModel


class Grafana(BaseModel):
    user: str
    password: str
    host: str
    port: str
    api_key: str | None = None
    GRAFANA_URL = f"http://{user}:{password}@{host}:{port}"
    API_DATASOURCES = "/api/datasources"
    API_KEYS = "/api/auth/keys"

上述代码中GRAFANA_URL的定义会报错，因为类定义阶段无法直接引用实例字段user、password等。下面是几种可行的解决方法：

方法一：使用Pydantic v2的model_post_init #

这是Pydantic v2推荐的实例初始化后执行逻辑的方式，类似普通类的__init__后续操作：

from pydantic import BaseModel

class Grafana(BaseModel):
    user: str
    password: str
    host: str
    port: str
    api_key: str | None = None
    API_DATASOURCES = "/api/datasources"
    API_KEYS = "/api/auth/keys"
    
    def model_post_init(self, __context) -> None:
        self.GRAFANA_URL = f"http://{self.user}:{self.password}@{self.host}:{self.port}"

方法二：使用@property装饰器 #

把GRAFANA_URL定义为动态属性，每次访问时自动计算，适合字段可能被修改的场景：

from pydantic import BaseModel

class Grafana(BaseModel):
    user: str
    password: str
    host: str
    port: str
    api_key: str | None = None
    API_DATASOURCES = "/api/datasources"
    API_KEYS = "/api/auth/keys"
    
    @property
    def GRAFANA_URL(self) -> str:
        return f"http://{self.user}:{self.password}@{self.host}:{self.port}"

方法三：Pydantic v1版本使用root_validator #

如果还在使用Pydantic v1，可以通过根验证器来设置字段值：

from pydantic import BaseModel, root_validator

class Grafana(BaseModel):
    user: str
    password: str
    host: str
    port: str
    api_key: str | None = None
    GRAFANA_URL: str
    API_DATASOURCES = "/api/datasources"
    API_KEYS = "/api/auth/keys"
    
    @root_validator(pre=False)
    def set_grafana_url(cls, values):
        values["GRAFANA_URL"] = f"http://{values['user']}:{values['password']}@{values['host']}:{values['port']}"
        return values

内容的提问来源于stack exchange，提问作者An old man in the sea.