[Feature] Allow empty Form fields

I see that FastAPI does support Form data input. However it does not seem to support empty form fields and throws error:

{"detail":[{"loc":["body","searchstr"],"msg":"field required","type":"value_error.missing"}]}

Basically, fastapi expects all fields that are going to be handled should not be empty, which is not realistic IMHO. Or can someone show me a workaround?

[nsidnev]

@borako FastAPI uses the pydantic’s convention to mark fields as required. As described here, if you use ... in the definition of a query parameter, then FastAPI will require this field in the request input. The same is true for the Form, Body and other parameters from FastAPI. You can set a default value for your field if it can be omitted in the form data. Additionally, you can mark the field as typing.Optional if you don’t want a static analyzer like mypy to complain about types. Here is an example:

from typing import Optional

from fastapi import FastAPI, Form

app = FastAPI()


@app.post("/my-form-endpoint")
def my_endpoint(
    my_field_with_default: str = Form("some value"),
    my_optional_field: Optional[int] = Form(None),
    my_required_field: str = Form(...),
) -> dict:
    return {
        "my_field_with_default": my_field_with_default,
        "my_optional_field": my_optional_field,
        "my_required_field": my_required_field,
    }

There are 3 form fields defined for this endpoint, but FastAPI will require that only my_required_field be present in the user input. Here are some examples of requests to this application using the httpie client:

$ http -f POST :8000/my-form-endpoint my_required_field=something
HTTP/1.1 200 OK
content-length: 95
content-type: application/json
date: Tue, 14 Jan 2020 01:43:21 GMT
server: uvicorn

{
    "my_field_with_default": "some value",
    "my_optional_field": null,
    "my_required_field": "something"
}

$ http -f POST :8000/my-form-endpoint my_required_field=something my_optional_field=42
HTTP/1.1 200 OK
content-length: 93
content-type: application/json
date: Tue, 14 Jan 2020 01:43:37 GMT
server: uvicorn

{
    "my_field_with_default": "some value",
    "my_optional_field": 42,
    "my_required_field": "something"
}

$ http -f POST :8000/my-form-endpoint my_required_field=something my_optional_field=42 my_field_with_default=non-default-value
HTTP/1.1 200 OK
content-length: 100
content-type: application/json
date: Tue, 14 Jan 2020 01:44:06 GMT
server: uvicorn

{
    "my_field_with_default": "non-default-value",
    "my_optional_field": 42,
    "my_required_field": "something"
}

[borako]

This is great, very helpful. I understand fastapi borrows a lot of pieces from others (starlette, pydantic, etc.). Being not familiar with those technologies, fastapi user guide sections for Form data look too simplistic. It’ll be wonderful if this makes into those sections in the future. Thanks! Bora