[FEATURE] Support repeated key=value in form data

Is your feature request related to a problem

Yes.

Given some URL encoded data like this…

choices=parrot&choices=spider

…only the last key=value wins.

This does not work like I expected:

choices: list = Form(...)

You can only validate against the last value.

The solution you would like

Perhaps FastAPI should collect repeated keys in the 2-tuple list that request.form() gives and assign those values as a list to the same key before validation happens.

[HitLuca]

I think we still have an issue with Form and List. When passing a list of elements as a Form field, it doesn’t get parsed correctly:

@app.put("/test1")
def test1(values: List[str] = Form(...)) -> None:
    return values


@app.put("/test2")
def test2(values: List[str] = Query(...)) -> None:
    return values

test1 example

curl -X 'PUT' \
  'http://localhost:5050/test1' \
  -H 'accept: application/json' \
  -H 'Content-Type: application/x-www-form-urlencoded' \
  -d 'values=string,string'

[
  "string,string"
]

test2 example

curl -X 'PUT' \
  'http://localhost:5050/test2?values=string&values=string' \
  -H 'accept: application/json'

[
  "string",
  "string"
]

we can see how Query correctly parses the list, whereas Form doesn’t. Changing List[str] to list doesn’t fix the problem. Accepting list of numbers with List[int] for example returns an error as shown below, indicating that typing.List isn’t handled as expected

@app.put("/test3")
def test3(values: List[int] = Form(...)) -> None:
    print(type(values), values)
    return values

test3 example

curl -X 'PUT' \
  'http://localhost:5050/test3' \
  -H 'accept: application/json' \
  -H 'Content-Type: application/x-www-form-urlencoded' \
  -d 'values=1,2'

{
  "detail": [
    {
      "loc": [
        "body",
        "values",
        0
      ],
      "msg": "value is not a valid integer",
      "type": "type_error.integer"
    }
  ]
}

The curl commands have been copy pasted from the swagger web interface but I don’t see any change when running them from terminal. Using fastapi==0.65.2

[tiangolo]

Thanks @StephenCarboni for the report 🔍 🕵️‍♂️

Thanks @nsidnev for the fix! 🎉 🚀

And thanks everyone here for the discussion.

The fix is available in the latest release.