Required Body argument with None is not supported

[roynico]

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Example Code

from fastapi import FastAPI, Body
from typing import Optional

app = FastAPI()

@app.post("/")
def post_data(data: Union[str, None] = Body(...)):
      pass

Description

According to the documentation, it is possible to handle a None value for required argument provided by the user. First of all, the example in the documentation doesn't really make sense, since a Python None (i.e. a JSON null) can not be provided in the query a GET request, but has to be sent through the body of a POST/PUT request.
Nevertheless, this is my use case: I have a POST endpoint, with a required (without default) argument, but I want the user to be able to send a null value.
There has been a issue (#1661) for 2 years exactly about this point. It has been claimed, that this is solved in 0.85.1, but I claim that this is not. I report here below more or less the same MWE:

• store the above code in a file example.py
• start the fastapi server with uvicorn example:app
• send a POST request with:

$ curl -X POST "http://localhost:8000/" -d 'null' -H "Content-Type: application/json"
{"detail":[{"loc":["body"],"msg":"field required","type":"value_error.missing"}]}

Expected would be:

$ curl -X POST "http://localhost:8000/" -d 'null' -H "Content-Type: application/json"
null

I tried with other (more recent) versions of fastapi without more success.

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.85.1

Python Version

Python 3.9.2

Additional Context

No response

[haoyun]

I think this might be a bug. Related function is request_body_to_args():

https://github.com/tiangolo/fastapi/blob/efc12c5cdbede6922a033a5234c70cb22c4d204a/fastapi/dependencies/utils.py#L624-L627

https://github.com/tiangolo/fastapi/blob/efc12c5cdbede6922a033a5234c70cb22c4d204a/fastapi/dependencies/utils.py#L645

https://github.com/tiangolo/fastapi/blob/efc12c5cdbede6922a033a5234c70cb22c4d204a/fastapi/dependencies/utils.py#L657-L658

https://github.com/tiangolo/fastapi/blob/efc12c5cdbede6922a033a5234c70cb22c4d204a/fastapi/dependencies/utils.py#L665-L667

With your MWE, there are two cases the body will be loaded as Python's None:

1. sending an empty body
2. sending raw JSON null.

In both cases, request_body_to_args got received_body = None, hence the value in code quoted above is None. However, according to your declaration, field.required is True. So, an error is added by FastAPI before Pydantic validates the model, which happens later in

https://github.com/tiangolo/fastapi/blob/efc12c5cdbede6922a033a5234c70cb22c4d204a/fastapi/dependencies/utils.py#L696

So it is FastAPI that raise this problem but not Pydantic.

Now, the question is how to distinguish empty body and null as body. The only way I can come up with is to pass the bytes of received_body, instead of its loaded version to the function request_body_to_args() quoted above. Or we should pass some extra data to distinguish nothing and null.

A workaround that slightly changes your API, is to use an Object as the requested body instead of the bare value. More precisely, use:

from typing import Union

from fastapi import FastAPI
from pydantic import BaseModel, Field


class Payload(BaseModel):
    data: Union[str, None] = Field(...)


app = FastAPI()


@app.post("/")
def post_data(q: Payload):
    pass

Then compare (with httpie):

http POST :8000 --raw='{"data": null}'
http POST :8000 --raw='{"data": "hello"}'
http POST :8000 --raw='{}'
http POST :8000 --raw=''

---

There is some imperfection on the Pydantic side, the JSON schema that Pydantic generates for the above Payload is

    Payload.schema(): {
        'title': 'Payload',
        'type': 'object',
        'properties': {
            'data': {
                'title': 'Data',
                'type': 'string',
            },
        },
        'required': ['data'],
    } (dict) len=4

so the json

{
    "data": null
}

does not pass the validation of that JSON schema.

[antonio-pedro99]

@roynico how can I reproduce this?