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

I know a concept from Typescript called Discriminated unions . That's a thing where you put 2 struct (classes, etc) and the type is decided depending on a struct's values. I'm trying to achieve similar thing in FastAPI with Pydantic validation. There are two different request payloads that I can recieve. Whether it's one or another depends on accountType variable. If it's creative it should be validated by RegistrationPayloadCreative and if it's brand it should validate by RegistrationPayloadBrand . How do I achieve this? Couldn't find any other solution.

The problem is that it either returns

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

Or it doesn't work at all.

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

You should use __root__ and parse_obj instead.

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

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

Answer 2

The error message is a bit misleading since the problem is that company/phone/vat fields are mandatory in RegistrationPayloadBrand.

So:

 >>> 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'))

Or making them Optional (if payload not necessarily contains those fields)

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'))

will solve the problem

Answer 3

I know a concept from Typescript called Discriminated unions . That's a thing where you put 2 struct (classes, etc) and the type is decided depending on a struct's values. I'm trying to achieve similar thing in FastAPI with Pydantic validation. There are two different request payloads that I can recieve. Whether it's one or another depends on accountType variable. If it's creative it should be validated by RegistrationPayloadCreative and if it's brand it should validate by RegistrationPayloadBrand . How do I achieve this? Couldn't find any other solution.

The problem is that it either returns

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

Or it doesn't work at all.

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()