[QUESTION] One or more objects in request body?

Description

I’d like to have an endpoint in my API that accepts one or more objects in the request body. For example, I would like both of these to be valid request bodies:

{
  "name": "foo"
}

[
  {"name": "foo"},
  {"name": "bar"}
]

I’m not sure how to do this in fastapi/pydantic. Is it even possible? Any advice?

[dmontagu]

@kevlar1818

Union is currently somewhat finicky in pydantic due to the way it is implemented – the order matters because it will try to parse types in the provided order. For what it’s worth, if you have issues with pydantic Union parsing, I would encourage you to open a github issue over on the pydantic repository – if the problem isn’t a bug (which it might be), you’ll probably get a suggestion for how to accomplish your goal.

---

For what it’s worth, the following script does work for me:

from typing import List

from pydantic import BaseModel, Union


class Name(BaseModel):
    name: str


class NameData(BaseModel):
    name_data: Union[Name, List[Name]]


data1 = {
    "name_data": {
        "name": "foo"
    }
}

data2 = {
    "name_data": [
        {"name": "foo"},
        {"name": "bar"}
    ]
}

print(NameData(**data1))
# NameData name_data=<Name name='foo'>

print(NameData(**data2))
# NameData name_data=[<Name name='foo'>, <Name name='bar'>]

---

If your issue with the container approach is that you don’t want to write a container model for every list-or-object type you want to have, you can leverage pydantic.generics.GenericModel:

from typing import List, TypeVar, Generic

from pydantic import BaseModel, Union
from pydantic.generics import GenericModel

T = TypeVar("T")


class Name(BaseModel):
    name: str


class ListOrObject(GenericModel, Generic[T]):
    contained: Union[T, List[T]]


data1 = {
    "contained": {
        "name": "foo"
    }
}

data2 = {
    "contained": [
        {"name": "foo"},
        {"name": "bar"}
    ]
}

data3 = {
    "contained": "string1"
}

data4 = {
    "contained": ["string1", "string2"]
}

print(ListOrObject[Name](**data1).contained)
# Name name='foo'

print(ListOrObject[Name](**data2).contained)
# [<Name name='foo'>, <Name name='bar'>]

print(ListOrObject[str](**data3).contained)
# string1

print(ListOrObject[str](**data4).contained)
# ['string1', 'string2']

(The above script runs for me as is.)

I believe this plays nicely with FastAPI as well.

[tiangolo]

Thanks for the help @dmontagu 🌮!

Thanks @kevlar1818 for reporting back and closing the issue.