multipart/form-data: Object parsing error when using files and object in request form

First check

• I added a very descriptive title to this issue.
• I used the GitHub search to find a similar issue and didn’t find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google “How to X in FastAPI” and didn’t find any information.
• I already read and followed all the tutorial in the docs and didn’t find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.
• After submitting this, I commit to one of:
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  • Implement a Pull Request for a confirmed bug.

Example

Here’s a self-contained, minimal, reproducible, example with my use case:

from fastapi import FastAPI, File, Form
from pydantic import BaseModel

app = FastAPI()


class Address(BaseModel):
    first_line: str
    post_code: str


class User(BaseModel):
    first_name: str
    last_name: str
    address: Address


@app.post("/users/")
async def create_user(avatar: bytes = File(...), user: User = Form(...)):
    return {
        "file_size": len(avatar),
        "user": user,
    }


if __name__ == '__main__':
    import uvicorn

    uvicorn.run(app, host='0.0.0.0', port=8000, debug=True)

Description

• Open the browser and call the endpoint /docs. Click try it out on the API
• upload a file, click execute
• It returns error with status 422 Unprocessable Entity

{
  "detail": [
    {
      "loc": [
        "body",
        "user"
      ],
      "msg": "value is not a valid dict",
      "type": "type_error.dict"
    }
  ]

.

Environment

• OS: “Ubuntu 18.04.5 LTS”

• pip fastapi==0.61.2 python-multipart==0.0.5 requests==2.24.0 starlette==0.13.6 uvicorn==0.12.1 …

• Python version: 3.8.6

Additional context

I found #842 has similar issue, this one is more a mixed use of files and object in a form data.

Not sure if I messed something up, but thanks for any advice.

[Mause]

I don’t believe my method would work for nested fields I’m afraid.

Perhaps you could provide a sample of the form body you are posting and the source of your as_form function in a new issue?

[Kludex]

The problem is using a pydantic model with form. I recommend what I call Mause Form Solution: https://github.com/tiangolo/fastapi/issues/1989#issuecomment-684170738