Above details are not sufficient to define the issue(follow issue template).
Can you share example code?
If you can or your project supports, update fastapi to latest version.

Sorry I should have given more details.

On latest version of fastapi (0.63.0):

from typing import Optional

from fastapi import FastAPI, Form

app = FastAPI()

@app.post("/items")
async def create_item(
        item_description: Optional[str] = Form(None),
):
    print(item_description)
    print(item_description is None)
    print(item_description == '')

If I post on the endpoint:

curl -X POST "http://127.0.0.1:8000/items" -H  "accept: application/json" -H  "Content-Type: application/x-www-form-urlencoded" -d "item_description="

as a result I get:

None
True
False

I expect item_description to be an empty string, not None.

I want to be able to differentiate those 2 calls.

No data:

curl -X POST "http://127.0.0.1:8001/items" -H  "accept: application/json" -H  "Content-Type: application/x-www-form-urlencoded" -d ""

Data empty:

curl -X POST "http://127.0.0.1:8001/items" -H  "accept: application/json" -H  "Content-Type: application/x-www-form-urlencoded" -d "item_description="

Have you tried this one?

curl -X POST "http://127.0.0.1:8001/items" -H "accept: application/json" -H "Content-Type: application/x-www-form-urlencoded" -d "item_description='' "
There is a difference between without passing any params value or passing blank param values.

Here..
curl -X POST "http://127.0.0.1:8001/items" -H "accept: application/json" -H "Content-Type: application/x-www-form-urlencoded" -d ""
you are sending no params... the form will take it as empty and returns None

curl -X POST "http://127.0.0.1:8001/items" -H "accept: application/json" -H "Content-Type: application/x-www-form-urlencoded" -d "item_description="
you are sending param... but no values assigned to it, even '' also and the form will take the param value as None.

yes, it depends on the default value.

I can't use item_description: str = Form(""), because it wouldn't differentiate a missing parameter from a parameter with empty string.

Have you tried this one?

curl -X POST "http://127.0.0.1:8001/items" -H "accept: application/json" -H "Content-Type: application/x-www-form-urlencoded" -d "item_description='' "
There is a difference between without passing any params value or passing blank param values.
....
curl -X POST "http://127.0.0.1:8001/items" -H "accept: application/json" -H "Content-Type: application/x-www-form-urlencoded" -d "item_description="
you are sending param... but no values assigned to it, even '' also and the form will take the param value as None.

Ok, it seems to be working ! I was missing the empty string in my data, so it was defaulting to None.

There is indead a difference between:
-d "item_description=''" => I get the empty string in my variable

and
-d "item_description=" => defaulting to None

Now I just need to find how to send an empty string with axios. it seems to replace it with empty data, so the backend default to None..

Thank you so much for your help.

Have you tried this one?

curl -X POST "http://127.0.0.1:8001/items" -H "accept: application/json" -H "Content-Type: application/x-www-form-urlencoded" -d "item_description='' "
There is a difference between without passing any params value or passing blank param values.

Here..
curl -X POST "http://127.0.0.1:8001/items" -H "accept: application/json" -H "Content-Type: application/x-www-form-urlencoded" -d ""
you are sending no params... the form will take it as empty and returns None

curl -X POST "http://127.0.0.1:8001/items" -H "accept: application/json" -H "Content-Type: application/x-www-form-urlencoded" -d "item_description="
you are sending param... but no values assigned to it, even '' also and the form will take the param value as None.

I'm reopening because I find it is a very strange behavior.

Is there no way to differentiate no param curl -d "" and blank param values curl -d "item_description=" ?

-d "item_description=''" is really awkward to handle, I have to do something like this:

if item_description == "''":
    do_something()

-d "item_description=''" is really awkward to handle

It is awkward indeed 🤣

Wouldn't it be better if you handle all the validation logic inside the Pydantic model?

Yes, awkward solution for an awkward curl in the description..😉

As @ycd suggested, you can handle it in pydantic class as per your usecase.

Ok, I don't have this problem when I am using pydantic objects

something like this:

from pydantic import BaseModel, validator

class Item(BaseModel):
    name: Optional[str] = None
    description: Optional[str] = None

@app.post("/items")
async def create_item(
        item: Item,
):
    return

The data send in the request is a json, and I can ommit some field or send empty value.
curl ..... -d "{\"name\":\"\"}"

name would be emptied, and description left untouched !

Thank you all for your help!

Thanks for the help here everyone! 👏 🙇

Thanks for reporting back and closing the issue @VictorBac 👍

Sorry for the long delay! 🙈 I wanted to personally address each issue/PR and they piled up through time, but now I'm checking each one in order.

I agree with the thread starter @VictorBac

That in FastAPI (fastapi==0.111.0, pydantic==2.7.1) in the Form() parameter there is no way to distinguish between 2 states:

1. no parameter at all
   curl -d ''.

2. parameter passed but without value
   curl -d 'key='.

In both cases I get the value None or empty string if I change the default.

The main problem is that these 2 states are mixed together and I cannot distinguish them.

For example, my old backend application written on flask-restx has such a possibility to differentiate these 2 states.

htmx sends unquoted form fields and so when a textbox is empty, we get missing field instead of empty string. greenfield is not so bad, but it seems awkward to have to give a default value for a required field if empty string is a valid value. I'd like to reserve 422 for when we actually have a bug and the field is actually missing.