How to use discriminated union types in FastAPI body validation? (Union on models)

user_5334773 0
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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()
python-typing pydantic fastapi validation python
2 Answers

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

user_31343330

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

user_142527960
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