Conditional call of a FastAPI Model

Option 1

A solution would be the following. Define lang as Query paramter and add a regular expression that the parameter should match. In your case, that would be ^(fr|en)$, meaning that only fr or en would be valid inputs. Thus, if no match was found, the request would stop there and the client would receive a "string does not match regex…" error.

Next, define the body parameter as a generic type of dict and declare it as Body field; thus, instructing FastAPI to expect a JSON body.

Following, create a dictionary of your models that you can use to look up for a model using the lang attribute. Once you find the corresponding model, try to parse the JSON body using models[lang].parse_obj(body) (equivalent to using models[lang](**body)). If no ValidationError is raised, you know the resulting model instance is valid. Otherwise, return an HTTP_422_UNPROCESSABLE_ENTITY error, including the errors, which you can handle as desired.

If you would also like FR and EN being valid lang values, adjust the regex to ignore case using ^(?i)(fr|en)$ instead, and make sure to convert lang to lower case when looking up for a model (i.e., models[lang.lower()].parse_obj(body)).

import  pydantic 
from fastapi import FastAPI, Response, status, Body, Query
from fastapi.responses import JSONResponse
from fastapi.encoders import jsonable_encoder

models = {"fr": DatasetFR, "en": DatasetEN}

@router.post("/", response_description="Add a dataset")
async def create_dataset(body: dict = Body(...), lang: str = Query(..., regex="^(fr|en)$")):
    try:
        model = models[lang].parse_obj(body)
    except pydantic.ValidationError as e:
        return Response(content=e.json(), status_code=status.HTTP_422_UNPROCESSABLE_ENTITY, media_type="application/json")

    return JSONResponse(content=jsonable_encoder(model.dict()), status_code=status.HTTP_201_CREATED)

Update

Since the two models have identical attributes (i.e., title and description), you could define a parent model (e.g., Dataset) with those two attributes, and have DatasetFR and DatasetEN models inherit those.

class Dataset(BaseModel):
    title:str
    description: str
    
class DatasetFR(Dataset):
    category: CategoryFR
    tags: Optional[List[TagsFR]]
    
class DatasetEN(Dataset):
    category: CategoryEN
    tags: Optional[List[TagsEN]]

Additionally, it might be a better approach to move the logic from inside the route to a dependecy function and have it return the model, if it passes the validation; otherwise, raise an HTTPException, as also demonstrated by @tiangolo. You can use jsonable_encoder, which is internally used by FastAPI, to encode the validation errors() (the same function can also be used when returning the JSONResponse).

from fastapi.exceptions import HTTPException
from fastapi import Depends

models = {"fr": DatasetFR, "en": DatasetEN}

async def checker(body: dict = Body(...), lang: str = Query(..., regex="^(fr|en)$")):
    try:
        model = models[lang].parse_obj(body)
    except pydantic.ValidationError as e:
        raise HTTPException(detail=jsonable_encoder(e.errors()), status_code=status.HTTP_422_UNPROCESSABLE_ENTITY)

    return model
        
@router.post("/", response_description="Add a dataset")
async def create_dataset(model: Dataset = Depends(checker)):    
    return JSONResponse(content=jsonable_encoder(model.dict()), status_code=status.HTTP_201_CREATED)

Option 2

A further approach would be to have a single Pydantic model (let’s say Dataset) and customise the validators for category and tags fields. You can also define lang as part of Dataset, thus, no need to have it as query parameter. You can use a set, as described here, to keep the values of each Enum class, so that you can efficiently check if a value exists in the Enum; and have dictionaries to quickly look up for a set using the lang attribute. In the case of tags, to verify that every element in the list is valid, use set.issubset, as described here. If an attribute is not valid, you can raise ValueError, as shown in the documentation, "which will be caught and used to populate ValidationError" (see "Note" section here). Again, if you need the lang codes written in uppercase being valid inputs, adjust the regex pattern, as described earlier.

P.S. You don’t even need to use Enum with this approach. Instead, populate each set below with the permitted values. For instance,
categories_FR = {"Eau"} categories_EN = {"Water"} tags_FR = {"eau", "pesticides"} tags_EN = {"water", "pesticides"}. Additionally, if you would like not to use regex, but rather have a custom validation error for lang attribute as well, you could add it in the same validator decorator and perform validation similar (and previous) to the other two fields.

from pydantic import validator

categories_FR = set(item.value for item in CategoryFR) 
categories_EN = set(item.value for item in CategoryEN) 
tags_FR = set(item.value for item in TagsFR) 
tags_EN = set(item.value for item in TagsEN) 
cats = {"fr": categories_FR, "en": categories_EN}
tags = {"fr": tags_FR, "en": tags_EN}

def raise_error(values):
    raise ValueError(f'value is not a valid enumeration member; permitted: {values}')

class Dataset(BaseModel):
    lang: str = Body(..., regex="^(fr|en)$")
    title: str
    description: str
    category: str
    tags: List[str]

    @validator("category", "tags")
    def validate_atts(cls, v, values, field):
        lang = values.get('lang')
        if lang:
            if field.name == "category":
                if v not in cats[lang]: raise_error(cats[lang])
            elif field.name == "tags":
                if not set(v).issubset(tags[lang]): raise_error(tags[lang])
        return v

        
@router.post("/", response_description="Add a dataset")
async def create_dataset(model: Dataset): 
    return JSONResponse(content=jsonable_encoder(model.dict()), status_code=status.HTTP_201_CREATED)

Option 3

Another approach would be to use Discriminated Unions, as described in this answer.

As per the documentation:

When Union is used with multiple submodels, you sometimes know
exactly which submodel needs to be checked and validated and want to
enforce this. To do that you can set the same field – let’s call it
my_discriminator – in each of the submodels with a discriminated
value, which is one (or many) Literal value(s). For your Union,
you can set the discriminator in its value:
Field(discriminator='my_discriminator').

Setting a discriminated union has many benefits:

• validation is faster since it is only attempted against one model
• only one explicit error is raised in case of failure
• the generated JSON schema implements the associated OpenAPI specification