FastAPI modify request model before parsing based on header - fastapi

Question

I have an immutable Pydantic model (Item) that requires a field shape_mode that is schematic or realistic. The default value of the shape_mode should depend on the response class:
for application/json default is schematic
for image/png default is realistic
The question is: can I modify the field shape_mode based on the accept header, before the body is parsed to an Item object?
from enum import Enum
from fastapi import FastAPI, Request
from pydantic import BaseModel
class ShapeModeEnum(str, Enum):
schematic = "schematic"
realistic = "realistic"
class Item(BaseModel):
name: str
shape_mode: ShapeModeEnum
class Config:
allow_mutation = False
extra = "forbid"
strict = True
validate_assignment = True
app = FastAPI()
RESPONSE_CLASSES = {
"application/json": "schematic",
"image/png": "realistic",
}
#app.post("/item")
async def create_item(
request: Request,
item: Item,
):
accept = request.headers["accept"]
return [item, accept]

Answer 1

You could do this by combining a couple of concepts; inheriting BaseModels and dependencies. Below is a working example (although not really robust; it will throw HTTP500 if you try to post a request with a non-valid accept header.
from enum import Enum
from fastapi import Depends, FastAPI, Request
from pydantic import BaseModel
app = FastAPI()
class ShapeModeEnum(str, Enum):
schematic = "schematic"
realistic = "realistic"
class ItemBase(BaseModel):
name: str
class Item(ItemBase):
shape_mode: ShapeModeEnum
class Config:
# allow_mutation = False
extra = "forbid"
strict = True
validate_assignment = True
def get_real_item(request: Request, item: ItemBase) -> Item:
RESPONSE_CLASSES = {"application/json": "schematic", "image/png": "realistic"}
return Item(
name=item.name, shape_mode=RESPONSE_CLASSES[request.headers.get("Accept", None)]
)
#app.post("/item")
async def get_item(item: Item = Depends(get_real_item)):
return item
#app.get("/")
async def root():
return {"hello": "world"}
if __name__ == "__main__":
import uvicorn
uvicorn.run(app, host="0.0.0.0", port=8000)
Calling with curl:
% curl -X 'POST' \
'http://localhost:8000/item' \
-H 'accept: image/png' \
-H 'Content-Type: application/json' \
-d '{
"name": "string"
}'
{"name":"string","shape_mode":"realistic"}%
% curl -X 'POST' \
'http://localhost:8000/item' \
-H 'accept: application/json' \
-H 'Content-Type: application/json' \
-d '{
"name": "string"
}'
{"name":"string","shape_mode":"schematic"}%