Body parameter cannot be a pydantic union anymore since v0.95

[morian]

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Example Code

from typing import Annotated, Literal

from fastapi import FastAPI
from pydantic import BaseModel, Field


class ItemV1(BaseModel):
    version: Literal[1] = 1

class ItemV2(BaseModel):
    version: Literal[2] = 2

Item = Annotated[
    ItemV1 | ItemV2,
    Field(discriminator='version'),
]

app = FastAPI()

@app.post('/')
async def create(body: Item) -> None:
    pass

Description

• Run this code with uvicorn 'main:app'

It raises the following backtrace:

Traceback (most recent call last):
[...]
  File "/tmp/lab/main.py", line 20, in <module>
    @app.post('/')
     ^^^^^^^^^^^^^
  File "/tmp/lab/venv/lib/python3.11/site-packages/fastapi/routing.py", line 661, in decorator
    self.add_api_route(
  File "/tmp/lab/venv/lib/python3.11/site-packages/fastapi/routing.py", line 600, in add_api_route
    route = route_class(
            ^^^^^^^^^^^^
  File "/tmp/lab/venv/lib/python3.11/site-packages/fastapi/routing.py", line 446, in __init__
    self.dependant = get_dependant(path=self.path_format, call=self.endpoint)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/lab/venv/lib/python3.11/site-packages/fastapi/dependencies/utils.py", line 328, in get_dependant
    if is_body_param(param_field=param_field, is_path_param=is_path_param):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/lab/venv/lib/python3.11/site-packages/fastapi/dependencies/utils.py", line 491, in is_body_param
    assert isinstance(
AssertionError: Param: body can only be a request body, using Body()

The same code used to work well with FastAPI v0.94.1 (and priors).
I tried to annotate using Annotated[Item, Body()] but it did not fix anything:

Traceback (most recent call last):
[...]
  File "/tmp/lab/main.py", line 20, in <module>
    @app.post('/')
     ^^^^^^^^^^^^^
  File "/tmp/lab/venv/lib/python3.11/site-packages/fastapi/routing.py", line 661, in decorator
    self.add_api_route(
  File "/tmp/lab/venv/lib/python3.11/site-packages/fastapi/routing.py", line 600, in add_api_route
    route = route_class(
            ^^^^^^^^^^^^
  File "/tmp/lab/venv/lib/python3.11/site-packages/fastapi/routing.py", line 446, in __init__
    self.dependant = get_dependant(path=self.path_format, call=self.endpoint)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/lab/venv/lib/python3.11/site-packages/fastapi/dependencies/utils.py", line 303, in get_dependant
    type_annotation, depends, param_field = analyze_param(
                                            ^^^^^^^^^^^^^^
  File "/tmp/lab/venv/lib/python3.11/site-packages/fastapi/dependencies/utils.py", line 382, in analyze_param
    len(fastapi_annotations) <= 1
AssertionError: Cannot specify multiple `Annotated` FastAPI arguments for 'body'

Please let me know if there is any additional inputs I can provide or whether this is now a desired behavior.
Thanks!

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.95.0

Python Version

Python 3.11.2

Additional Context

Pip freeze:

anyio==3.6.2
click==8.1.3
fastapi==0.95.0
h11==0.14.0
httptools==0.5.0
idna==3.4
pydantic==1.10.6
python-dotenv==1.0.0
PyYAML==6.0
sniffio==1.3.0
starlette==0.26.1
typing_extensions==4.5.0
uvicorn==0.21.1
uvloop==0.17.0
watchfiles==0.18.1
websockets==10.4

[phillipuniverse]

Same issue here, certainly seems like a bug in the recent 0.95.0.

[phillipuniverse]

Appears to be caused by the changes at #4871

[stephen-christie-ss]

I also experienced this issue, updating to 0.95.0.

[Lawouach]

I got bitten today as well.

[leokhachatorians]

So uh, when does a 'discussion' become an 'issue'? Or are we just yelling into the void

[nzig]

This was indeed caused by the changes in #4871. Apologies for the inconvenience.

The quick solution to this problem is to replace

Item = Annotated[
    ItemV1 | ItemV2,
    Field(discriminator='version'),
]

with

Item = Annotated[
    ItemV1 | ItemV2,
    Body(discriminator='version'),
]

I'm not really sure whether the original code was supposed to work before, or if it just worked incidentally.

@tiangolo what do you think? If you think it should work, I can try and figure out how to make it work.

[nzig]

I think either will work, because Body is a subclass of the FieldInfo type that Field() returns.

I would use Field() unless the type definition is shared with something else that requires Body.

[karolpawlowski]

Thanks @nzig !

[stdkoehler]

This also bit me. Thanks for providing a solution.

If I might add my view on the matter: Personally, I would prefer if we could use Field as was the case in the previous FastAPI versions.

1. Separation of responsibility: When defining the models, we are in the "pydantic world" (until now I did not have any FastAPI dependency in the file where I defined my models). Using those models would be the responsibility of FastAPI, mixing FastAPI into the models themselves would be no good practice (imho)
2. pydantic standard A Discriminated Union is defined by pydantic to create an OpenAPI compliant structure (https://docs.pydantic.dev/latest/usage/types/#discriminated-unions-aka-tagged-unions). Here explicitly the use of Field is shown (and still works when nested into models. Only when providing the Discriminated Union directly as Request Body do we trigger the "bug")
3. Ease of Use / Documentation The pydantic doc is where probably most people will land, when they are looking on how to get Discriminators into FastAPI. There is no according documentation (at least I found none) how to do it in FastAPI. (But mostly you just need to know pydantic so that's fine - except now: this case). From my research, this discussion thread is the only place where you could find that you need to use Body now. (And I didn't find in via search engine - I had to click through 10 pages of discussion looking into every thread whose description could've been related to my problem)

[fortzi]

Thanks for your support @nzig, really appreciated!

Unfortunately the suggested solution doesn't work for me, as I'm using an external package and cannot control its typing - I cannot change the Annotated type from Field to Body. Hence I cannot update FastAPI, which is also blocking me from using pydantic2...

Is this issue expected to be addressed? I'm sure that many more can't use the suggested workaround (or won't due to the reasons that @stdkoehler laid out)

[nzig]

If you're migrating to pydantic v2, could wrapping the third-party type in a RootModel solve your problem?

[pr0gg3d]

Hi, first of all thanks to FastAPI developers for such a great project!

Just wanted to know if there's any progress on this, because it seems that it's a showstopper for fully migrating to pydantic V2.

I also agree with a solution that allows the use of Field(discriminator=....), for the reasons that @stdkoehler pointed out very well.

For instance, in our projects we are using these Discriminated Unions throughout the application logic, where FastAPI it's only the final API part in order to interact with outside world.

Having to mantain 2 identical sets of Discriminated Unions, only by changing Field() into Body() if these are going to be used in FastAPI is not efficient at all.

Thanks again!

[psychedelicious]

@pr0gg3d This issue is resolved for us by #10773

[morian]

Indeed, it also now works for me as well. Closing this discussion.