如何在FastAPI主体验证中使用可区分的联合类型？ （模型上的联盟）

Question

我从 Typescript 中知道一个概念，称为“歧视工会”。这是你放置 2 个结构体（类等）的地方，并且类型是根据结构体的值决定的。我正在尝试通过 Pydantic 验证在 FastAPI 中实现类似的事情。我可以收到两种不同的请求负载。是其中之一取决于 accountType 变量。如果是

creative

，则应通过

RegistrationPayloadCreative

进行验证，如果是

brand

，则应通过

RegistrationPayloadBrand

进行验证。我该如何实现这一目标？找不到任何其他解决方案。问题是它要么返回

unexpected value; permitted: 'creative' (type=value_error.const; given=brand; permitted=('creative',))

或者根本不起作用。

class RegistrationPayloadBase(BaseModel):
    first_name: str
    last_name: str
    email: str
    password: str


class RegistrationPayloadCreative(RegistrationPayloadBase):
    accountType: Literal['creative']


class RegistrationPayloadBrand(RegistrationPayloadBase):
    company: str
    phone: str
    vat: str
    accountType: Literal['brand']

class A(BaseModel):
    b: Union[RegistrationPayloadBrand, RegistrationPayloadCreative]

def main():
    A(b={'first_name': 'sdf', 'last_name': 'sdf', 'email': 'sdf', 'password': 'sdfds', 'accountType': 'brand'})

if __name__ == '__main__':
    main()

Answer 1

__root__

和

parse_obj

来代替。

from typing import Union

from pydantic import BaseModel


class PlanetItem(BaseModel):
    id: str
    planet_name: str 
    # ...

class CarItem(BaseModel):
    id: str
    name: str
    # ...

class EitherItem(BaseModel):
    __root__: Union[PlanetItem, CarItem]



@app.get("/items/{item_id}", response_model=EitherItem)
def get_items(item_id):
    return EitherItem.parse_obj(response) # Now you get either PlanetItem or CarItem

信用：

https://github.com/tiangolo/fastapi/issues/2279#issuecomment-787517707

Answer 2

所以：

 >>> A(b={'first_name': 'sdf', 'last_name': 'sdf', 'email': 'sdf', 'password': 'sdfds', 'accountType': 'brand', 'company':'foo','vat':'bar', 'phone':'baz'}) 
A(b=RegistrationPayloadBrand(first_name='sdf', last_name='sdf',
email='sdf', password='sdfds', company='foo', phone='baz', vat='bar', accountType='brand'))

>>> A(b={'first_name': 'sdf', 'last_name': 'sdf', 'email': 'sdf', 'password': 'sdfds', 'accountType': 'creative'})
A(b=RegistrationPayloadCreative(first_name='sdf', last_name='sdf', email='sdf', password='sdfds', accountType='creative'))

或者将它们设置为可选（如果有效负载不一定包含这些字段）

class RegistrationPayloadBrand(RegistrationPayloadBase):
    company: Optional[str]
    phone:   Optional[str]
    vat:     Optional[str]
    accountType: Literal['brand']

>>> A(b={'first_name': 'sdf', 'last_name': 'sdf', 'email': 'sdf', 'password': 'sdfds', 'accountType': 'brand'})
A(b=RegistrationPayloadBrand(first_name='sdf', last_name='sdf', email='sdf', password='sdfds', company=None, phone=None, vat=None, accountType='brand'))

>>> A(b={'first_name': 'sdf', 'last_name': 'sdf', 'email': 'sdf', 'password': 'sdfds', 'accountType': 'creative'})
A(b=RegistrationPayloadCreative(first_name='sdf', last_name='sdf', email='sdf', password='sdfds', accountType='creative'))

就能解决问题

Answer 3

Field(discriminator='pipeline_name')

指定鉴别器来明确这一点。它为 FastAPI（或者我猜是 Pydantic）提供了一个明确的标准，用于决定如何确定有效负载是哪种联合类型。

class A(BaseModel):
    b: Union[RegistrationPayloadBrand, RegistrationPayloadCreative] = Field(discriminator='pipeline_name')

https://docs.pydantic.dev/latest/concepts/unions/#discriminated-unions