Pydantic v2：如何让带默认值的type字段在OpenAPI Schema中标记为必填？

要实现让type字段在OpenAPI规范中被标记为必填，同时实例化Dog时无需传入该参数，可采用以下两种方法：

方法一：通过Field的json_schema_extra强制标记必填 #

直接在type字段的Field配置中添加json_schema_extra，手动指定该字段为必填：

from pydantic import BaseModel, Field, Annotated, Literal
from enum import Enum

class PetType(str, Enum):
    DOG = "Dog"
    CAT = "Cat"

class Pet(BaseModel):
    name: Annotated[str, Field(..., examples=["Unnamed pet"])]

class Dog(Pet):
    type: Annotated[Literal[PetType.DOG], Field(default=PetType.DOG, json_schema_extra={"required": True})]

方法二：通过模型配置修改schema #

在Dog类的model_config中，通过schema_extra覆盖OpenAPI schema的required字段列表：

from pydantic import BaseModel, Field, Annotated, Literal
from enum import Enum

class PetType(str, Enum):
    DOG = "Dog"
    CAT = "Cat"

class Pet(BaseModel):
    name: Annotated[str, Field(..., examples=["Unnamed pet"])]

class Dog(Pet):
    type: Literal[PetType.DOG] = PetType.DOG

    model_config = {
        "schema_extra": {
            "required": ["name", "type"]
        }
    }

效果验证 #

修改后生成的OpenAPI规范中，type会被加入required列表，同时保留默认值：

{
    "title": "Dog",
    "type": "object",
    "properties": {
        "name": {
            "examples": [
                "Unnamed pet"
            ],
            "title": "Name",
            "type": "string"
        },
        "type": {
            "const": "Dog",
            "default": "Dog",
            "title": "Type"
        }
    },
    "required": [
        "name",
        "type"
    ]
}

此时实例化Dog(name="scooby")时，无需传入type参数，模型会自动使用默认值。

内容的提问来源于stack exchange，提问作者helt