Pydantic，允许属性被解析或传递给构造函数但使其不可变 以下不起作用：

问题描述

我正在尝试制作一个 Pydantic 模型，该模型大部分是可变的，但我希望一个成员是不可变的。

模型

# Built In
from datetime import datetime

# 3rd Party
from pydantic import BaseModel
from pydantic import Field # https://pydantic-docs.helpmanual.io/usage/schema/#field-customisation

class Processor(BaseModel):
    """ Pydantic model for a Processor object within the database. """
    class Config:
        """
        Model Configuration: https://pydantic-docs.helpmanual.io/usage/model_config/
        """
        extra = 'forbid'
        allow_mutation = True # This allows mutation,cool
        validate_assignment = True
        underscore_attrs_are_private = True
    model: str
    created_at: str = Field(default_factory=datetime.utcNow().isoformat) # I want to make sure that this can be passed but not assignable
    updated_at: str
    version: int = 0
    expires: Optional[int] = None

我的目标是允许从对象（或默认）解析 created_at，但防止在创建模型后对其进行分配。

示例

example_object = {
    "model": "foobar_model","created_at": "2020-12-22T15:35:06.262454+00:00","updated_at": "2020-12-22T15:35:06.262454+00:00","version": 2,"expires": None
}
processor = Processor.parse_obj(example_object)
processor.version = 3 # This should be allowed
processor.created_at = datetime.utcNow().isoformat() # This I want to fail

相关但不是我要找的 - GitHub：Is there a way to have a single field be static and immutable with pydantic

---

我最终通过对以下问题的回答解决了这个问题： https://github.com/samuelcolvin/pydantic/issues/2217

解决方法

以下不起作用：

您可以将私有属性与属性装饰器结合使用。私有属性不包含在模型字段中，但可以通过属性访问。

from pydantic import BaseModel,PrivateAttr


class Processor(BaseModel):
    """ Pydantic model for a Processor object within the database. """
    class Config:
        """
        Model Configuration: https://pydantic-docs.helpmanual.io/usage/model_config/
        """
        extra = 'forbid'
        allow_mutation = True # This allows mutation,cool
        validate_assignment = True
        underscore_attrs_are_private = True
    model: str
    _created_at: str = PrivateAttr(default_factory=datetime.utcnow().isoformat)
    _is_date_set: bool = PrivateAttr(default_factory=lambda: False)
    updated_at: str
    version: int = 0
    expires: Optional[int] = None

    @property
    def created_at(self):
        return self._created_at

    @create_at.setter
    def created_at(self,val):
        if self._is_date_set:
            raise AttributeError('The created_at attribute has already been set')
        self._is_date_set = True
        self._created_at = x