How to handle multiple pydantic response in fast api

[gunjeetgulatigit]

Describe the bug

When using Union on response_model, it doesn't return accurate result
To Reproduce

Create three models

from pydantic import BaseModel

   class parent(BaseModel):
       id: str

   class child1(parent):
      code: str
      user: Any

  class child2(parent):
    `error:` Any

class child3(parent):
 code: str
 user_data: Any

Add it on response_model

     from typing import Union
    from fastapi import APIRouter

    router = APIRouter()

   @router.post('/verify', response_model=Union[child1, child3,child2])
   async def something(some: some):
      dict1 = {
          "id": 1,
          "code" : "213",
          "user":{'id':"123","pass":"pass"}
          }
      dict2 = {
          "id": 1,
          "error" : "213"
         }
       dict3 = {
          "id": 1,
          "code" : "213",
          "user_data":{'id':"123","pass":"pass"}
         }
       
     if cond1 :  
          return child1(**dict1).dict()
    elif cond2 :  
          return child2(**dict2).dict()
     else:
         return child3(**dict3).dict()

output
actual :
On running all the condition one by one output that i got is not correct
when cond 1 gets true then

      {
        "id": 1,
         "code" : "213",
         "user":{'id':"123","pass":"pass"}
        }

when cond 2 gets true then

{
       "id": 1,
        "error" : "213"
       }

when cond 3 gets true then

{
     "id": 1,
      "code" : "213"
     }

Expected behavior

cond 1 and 2 output are correct
but cond3 is incorrect
i want cond3 output to have user_data field as below

{
        "id": 1,
         "code" : "213"
       "user_data":{'id':"123","pass":"pass"}
        }

how can i get the output to have all the fields of dict3

i am new to fast api and can anyone help me with this issue, any resolution to this issue or any other way to get the
expected output with schema in swagger UI

Additional Info:
What i observed here is Field type: Any has issue , as when i specify the field to str or dict , it gives the correct output as expected

[ghost]

When you specify the response model in the decorator, eg: response_model=Union[child1, child2, child3] FastAPI Expects an Instance of that class, See Here from the Documentation.

The line

return child3(**dict3).dict()

is what is causing the problem here. Do not convert it to a dict, FastAPI Will take care of that for you,

What you need to be doing:

return child3(**dict3)

[gunjeetgulatigit]

Hi @Rushyanth111

return child3(**dict3)
Tried above statement to return the response but still getting the same output .

[ghost]

Ok, I tested this once again, and it seems to be true, the output @gunjeetgulatigit is getting is valid:

{
  "id": "1",
  "code": "213",
  "user": null
}

When return child3(**dict3) is executed.

I solved the problem by Introducing type-annotations into the pydantic Models.

So I converted your models a little:

from typing import Dict

class parent(BaseModel):
    id: str


class child1(parent):
    code: str
    user: Dict[str, str] #Note here the Use of Dict[str, str]


class child2(parent):
    error: Any #Should be str, really, Using Any is not advisable. 


class child3(parent):
    code: str
    user_data: Dict[str, str] #Note here the Use of Dict[str, str]

Changing to the appropriate types ensured the output is what you expect.

I'm a little confused as to how that happened, since the keys are obviously different, if anyone else could elucidate on that, I'd be happy.

[gunjeetgulatigit]

thanks @Rushyanth111
Any idea to show default scheme in fast api doc and redoc for same part .
currently below response is displayed over fast-api doc

    Example Value
    Schema
  (no example available)

[ghost]

Currently, it is a known missing feature within the Swagger UI (See: #1083 ) and the generation of the example cannot be automatically done by Swagger UI, however it is possible to provide a manual example, which would mean that you have to make an example to be displayed.

Link to the Relevant Issue Solution from #1083

Link to Another Solution from #86

The Example Shows you that you need to provide a manual example like so:

Create A Class that holds the Union of any of the three Classes:

class parent(BaseModel):
    id: str


class child1(parent):
    code: str
    user: Dict[str, str]


class child2(parent):
    error: Any


class child3(parent):
    code: str
    user_data: Dict[str, str]


class UnionChild(BaseModel): # This will Hold the Union
    Child: Union[child1, child2, child3] #The Union Required

    class Config: #This is inside UnionChild, do not place it outside. 
        schema_extra = {
            "example": {                    #Mandatory field, this holds your example. Define Your Field from here. 
                "id": "1",
                "error": "Some Random String",
                "user": "OSAS UVUWE",
                "user_data": {"oh": "oh_no"},
            }
        }


app = FastAPI()


@app.get("/verify", response_model=UnionChild)
async def something(some: int):
    pass

[gunjeetgulatigit]

thanks @Rushyanth111
This way of displaying scheme is working

[gunjeetgulatigit]

Hi @Rushyanth111

Is there any way to show multiple response scheme at swagger UI through this manual example step .. ?

[ghost]

@gunjeetgulatigit Yes, the example is indicative of your needs, you should edit the schema, according to the Open API Documentations, as it is manually set.

As a small side note, please close the issue if the help required is satisfactory. Thanks!

[gunjeetgulatigit]

Thanks @Rushyanth111

[tiangolo]

Thanks for the help here @Rushyanth111 ! 👏 🙇

Thanks for reporting back and closing the issue @gunjeetgulatigit 👍

[singhyadvendra]

try this it works
responses={status.HTTP_200_OK: {"model": Union[child1, child2, child3]}, status.HTTP_404_NOT_FOUND: {"model": NotFoundMessage}}