Make exclude_unset work in pydantic models dependencies in form body

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.

Commit to Help

• I commit to help with one of those options 👆

Example Code

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

app = FastAPI()


class Item(BaseModel):
    x: str = Form()


@app.get("/")
async def foo(item: Item = Depends()):
    print(item.dict(exclude_unset=True))

Description

Open the browser and call endpoint “/” It prints {}

Wanted Solution

I would like it not to fill dependency model with nulls, but leave it unset, with a parameter in Depends for backward compatibilty

Wanted Code

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

app = FastAPI()


class Item(BaseModel):
    x: str = Form()


@app.get("/")
async def foo(item: Item = Depends(leave_unset=True)):
    print(item.dict(exclude_unset=True))

Alternatives

Make Form() in fields in models work without Depends() in handler function. Right now it is considered as application/json

Operating System

Windows

Operating System Details

Windows 11

FastAPI Version

0.78.0

Python Version

3.10.6

Additional Context

No response

[drforse]

I see. You could use exclude_none=True like so:

class Item(BaseModel):
    x: str | None = Form()


@app.get("/")
async def foo(item: Item = Depends()):
    print(item.dict(exclude_unset=True, exclude_none=True))

This would print {} as the default value is None. Not exactly the same you were trying to achieve but maybe might work in your specific use case? It would mean that you would not allow null values as a valid input in certain Form() fields.

I know about exclude_none, but that’s not exactly what I wanted. That’s different things

[JarroVGIT]

I see. You could use exclude_none=True like so:

class Item(BaseModel):
    x: str | None = Form()


@app.get("/")
async def foo(item: Item = Depends()):
    print(item.dict(exclude_unset=True, exclude_none=True))

This would print {} as the default value is None. Not exactly the same you were trying to achieve but maybe might work in your specific use case? It would mean that you would not allow null values as a valid input in certain Form() fields.