Is it possible to define a class with arbitrary extra keys, or alternatively a Mapping with some required keys?

[edgarrmondragon]

I'm trying to build a template for Python logging configuration.

The problem:

Each of the values in the handlers mapping has a required class key, a few optional pre-defined keys, and zero or more arbitrary keys which are passed through as keyword arguments to the handler’s constructor defined in class.

I'm stuck defining the Pkl class for that

class Handler {
    `class`: String
    level: String?
    formatter: String?
    filters: List<String>?
    // TODO: Support extra handler keyword arguments
}

So I'm currently forced to use a Mapping, which loses a lot of the information compared to the class:

handlers = new Mapping {
    ["stdout"] = new Mapping {
        ["class"] = "logging.StreamHandler"
        ["level"] = "INFO"
        ["formatter"] = "simple"
        ["stream"] = "ext://sys.stdout"
    }
    ["stderr"] = new Mapping {
        ["class"] = "logging.StreamHandler"
        ["level"] = "ERROR"
        ["formatter"] = "simple"
        ["stream"] = "ext://sys.stderr"
    }
    ["file"] = new Mapping {
        ["class"] = "logging.FileHandler"
        ["formatter"] = "simple"
        ["filename"] = "app.log"
        ["mode"] = "w"
    }
}

So, it is possible to define a class with arbitrary extra keys, or alternatively a Mapping with some required keys?

Thanks!