Python None maps to JS/TS undefined instead of null

[beck3905]

Describe the bug

It seems that JSII may be interpreting a Python None value or NoneType object as undefined rather than null. I am working with Step Functions state machines in the CDK in Python and when I use result_path=JsonPath.DISCARD this gets set to "ResultPath": None in the resulting state machine definition as a Python dictionary. But then either calling stack.resolve or just during the resolution process during cdk synth the ResultPath property get's removed entirely. I can see here where resolve removed properties with undefined value. https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/core/lib/private/resolve.ts#L222. But when converting from code to JSON, null is a valid value, which Python's None value usually maps to.

Expected Behavior

When creating a Step Function state with result_path=JsonPath.DISCARD I expect the resulting state machine definition to include "ResultPath": null.

Current Behavior

Currently, if I create a state with result_path=JsonPath.DISCARD the resulting state machine definition does not include the ResultPath field at all.

Reproduction Steps

from aws_cdk import Stack
from aws_cdk.aws_stepfunctions import JsonPath, Map, Pass, StateGraph
from constructs import Construct


class CdkMapStack(Stack):
    def __init__(self, scope: Construct, construct_id: str, **kwargs) -> None:
        super().__init__(scope, construct_id, **kwargs)

        map_state = Map(
            self,
            "Map",
            max_concurrency=1,
            items_path=JsonPath.string_at("$.inputForMap"),
            parameters={"foo": "foo", "bar": JsonPath.string_at("$.bar")},
            result_path=JsonPath.DISCARD,
        )
        map_state.iterator(Pass(self, "Pass State"))

        print(map_state.to_state_json(), file=open("./cdk.out/map-state.json", "w"))

        graph = StateGraph(map_state.start_state, "Test Graph")

        graph_json = graph.to_graph_json()

        print(
            graph_json,
            file=open("./cdk.out/state-graph.json", "w"),
        )

        print(
            self.resolve(graph_json),
            file=open("./cdk.out/state-graph-resolved.json", "w"),
        )

Run cdk synth with the stack above. Inspect the 3 files created in the cdk.out directory. The map-state.json and state-graph.json files will show the "ResultPath": null, but the state-graph-resolved.json file will not include the ResultPath field on the Map state at all.

Possible Solution

I'm not entirely sure of a solution as in some contexts it makes sense for Python None values to map to undefined and other times it makes sense for it to map to null. I don't believe Python has a way to distinguish the 2. Perhaps the CDK or JSII need an enum or constant to represent null in those contexts where null is valid and then None will always be treated as undefined.

Additional Information/Context

I have noticed this on multiple state types now, but both related to a Map state. I've found this behavior on both the Map state itself and when I add result_path=JsonPath.DISCARD to a task state within the Map state iterator. Neither had the ResultPath field in the synthesized/resolved state machine definition.

SDK version used

2.68.0 (build 25fda51)

Environment details (OS name and version, etc.)

Mac OS 13.2.1, Python 3.9.6

[RomainMuller]

Jsii APIs cannot distinguish between null and undefined by design, as most languages don't have a distinction between the two. The fix for this issue lies in the CDK, where a special JsonNull value needs to be used.

[github-actions]

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