Regression since v0.44.1 - Error 422 if `Form` is declared before `File`

[thomasleveil]

Here's a branch with my use case:

from starlette.testclient import TestClient

from fastapi import FastAPI, File, Form

app = FastAPI()


@app.post("/file_before_form")
def file_before_form(
    file: bytes = File(...), city: str = Form(...),
):
    return {"file_content": file, "city": city}


@app.post("/file_after_form")
def file_after_form(
    city: str = Form(...), file: bytes = File(...),
):
    return {"file_content": file, "city": city}


client = TestClient(app)


def test_file_before_form():
    response = client.post(
        "/file_before_form", data={"city": "Thimphou"}, files={"file": "<file content>"}
    )
    assert response.status_code == 200, response.text
    assert response.json() == {"file_content": "<file content>", "city": "Thimphou"}


def test_file_after_form():
    response = client.post(
        "/file_after_form", data={"city": "Thimphou"}, files={"file": "<file content>"}
    )
    assert response.status_code == 200, response.text  # <-------------- this fails since ab2b86f
    assert response.json() == {"file_content": "<file content>", "city": "Thimphou"}

Description

Upgrading FastApi from 0.42.0 to 0.61.0 on one of my project, I stumbled upon a regression in which FastApi will respond with HTTP Error 422 when ever posting a form-data request to an endpoint where a File is declared after a Form.

Searching the Internet, I found another user suffering from this issue : https://stackoverflow.com/questions/61376754/fastapi-files-must-be-loaded-before-form-in-function-parameters

I've created a branch with tests to showcase the issue : https://github.com/thomasleveil/fastapi/tree/file_and_form_together

Environment

• OS: Linux
• FastAPI Version 0.44.1 and later
• Python version: 3.8.5

Additional context

I've bisected the repo and figured out the regression was introduced in commit ab2b86f

[HacKanCuBa]

[EDIT: dismiss this comment]

After almost 2 hours of banging my head against the desktop, I end up concluding that the problem I was facing had to be a FastAPI bug. So I came here and found this issue and voilá! It was a bug.

But that's not all! Because my problem wasn't exactly the same. Take a look at this sample code:

router = APIRouter()

router.post('/{id}/')
async def update_blah(request: Request, id: int, something: str = Form(...)):
   ...

router.post('/other/')
async def other_blah(request: Request, file: UploadFile = File(...)):
   ...

And that was failing when POSTing to /other/ with 422 reporting errors as if it were posted to the update_blah endpoint. The solution? As @thomasleveil said: file before form.

router = APIRouter()

router.post('/other/')
async def other_blah(request: Request, file: UploadFile = File(...)):
   ...

router.post('/{id}/')
async def update_blah(request: Request, id: int, something: str = Form(...)):
   ...

Can't believe it? I couldn't either. This is some really crazy bug, I'm gonna take a look into it later today :)

FastAPI version: 0.61.1

[thomasleveil]

@HacKanCuBa what you describe is not a bug and the behavior is clearly described in the documentation at https://fastapi.tiangolo.com/tutorial/path-params/#order-matters

[HacKanCuBa]

Damn, you are totally right!!! it's because /other/ could be an id for the other route! I need to sleep more xD
Thanks for the quick reply!

[perkfly]

This is still not fixed as of May 2021(0.65.1), using UploadFile as the type seems to be ok, though. Any updates on this issue?

[AKlaus]

Confirm that the issue still exists in FastAPI v0.70.1 (used Python v3.7.10).

Tested by running cURL (to avoid accusations towards Swagger):

curl --location --request POST 'http://127.0.0.1:5001/file_after_form' --form 'city="fff"' --form 'file=@"file.png"'

that gets HTTP code 422 with message:

{"detail":[{"loc":["body","file"],"msg":"byte type expected","type":"type_error.bytes"}]}

(I raised #4384 initially, but closing it as a duplicate).