Bug: Form() field does not handle list parameters

I'm trying to parse a POST based Form that has a multi-valued string property.
However this doesn't appear to work?. It does work for the string parameter, but not for the list.

Minimal example:

from typing import Annotated
from aws_lambda_powertools.event_handler import APIGatewayHttpResolver
from aws_lambda_powertools.event_handler.openapi.params import Form
from aws_lambda_powertools.utilities.typing import LambdaContext

app = APIGatewayHttpResolver(
    enable_validation=True,
)

@app.post("/example")
def example(
    param: Annotated[
        str,
        Form(),
    ],
    param_list: Annotated[
        list[str],
        Form(),
    ] = []
):
     pass

def lambda_handler(event: dict, context: LambdaContext) -> dict:
    return app.resolve(event, context)

Tested by generating a dummy event to the lambda with body content like this:

import urlparse.parse

def test_example():
    # Also tested with tuple form [("param_list", ["value1", "value2"])]
    body  = urlparse.parse.urlencode({
        "param": "value",
        "param_list": ["value1", "value2"]
    }, doseq=True)
    event = {
        "headers": [
            "accept": "application/json",
            "content-type": "application/x-www-form-urlencoded"
        ],
        ...
        "body": body 
    }
   ... rest of test elided

But I always get response code of 422 like this:

{
  "statusCode": 422,
  "detail": [
    {
      "loc": [
        "body","param_list"
      ],
      "type": "list_type"
    }
  ]
}

Thanks!

Originally posted by @bdellegrazie in #7337

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Hi @bdellegrazie – Thanks for bringing this up, this seems like an oversight in the implementation.

Hi @bdellegrazie, I'll take a look at this soon! If you have a solution, I'd appreciate it if you could send me a PR with the fix.

Thanks

Hi @leandrodamascena, sorry, I don't have a solution at present.

I've tried to find the issue but couldn't reproduce it. Do you have a minimal example?

In

powertools-lambda-python/tests/functional/event_handler/_pydantic/test_openapi_validation_middleware.py

Line 1494 in 5496213

gw_event["body"] = "name=test&tags=tag1&tags=tag2"

is a test for this. I make a simple test local for an http event, but this works also.

Hi @tonnico , thanks for the pointer to the test. I'll try again in case I made a mistake, I did mention that I might have been mistaken in the original thread.

Hi @tonnico, @leandrodamascena I think I've figured out what the bug is at least:

If a single value is parsed to an array parameter, it doesn't parse.
If multiple values are supplied, it works.

It looks like the parser is unable to accept a single value for an array.

I can confirm this issue.

parse_qs will always create lists, and here it will create a single item, when the list is only one item.

I have worked around this by using the following type in the form definition:

@app.post("/example")
def example(
    param: Annotated[
        str,
        Form(),
    ],
    param_list: Annotated[
        None | str | list[str],
        Form(),
    ] = None
):

and checking the type in the resulting code. This works but adds unnecessary type checking in my code.

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