LambdaFunction does not implement IRuleTarget

In the Python CDK, attempting to pass aws_events_targets.LambdaFunction as the target of an events.Rule fails to typecheck:

$ mypy test.py
test.py:19: error: List item 0 has incompatible type "LambdaFunction"; expected "IRuleTarget"
test.py:19: note: 'LambdaFunction' is missing following 'IRuleTarget' protocol member:
test.py:19: note:     __jsii_proxy_class__
Found 1 error in 1 file (checked 1 source file)

It does run perfectly fine, however – this is purely an error in the typesystem.

Reproduction Steps

from aws_cdk import aws_events, aws_events_targets, aws_lambda, core

class Example(core.Construct):
    def __init__(self, scope: core.Construct):
        super().__init__(scope, "some-name")
        some_lambda = aws_lambda.Function(
            self,
            "some-handler",
            runtime=aws_lambda.Runtime.PYTHON_3_8,
            code=aws_lambda.AssetCode("some/path"),
            handler="some_package.handler",
            retry_attempts=0,
        )
        aws_events.Rule(
            self,
            "some-rule",
            schedule=aws_events.Schedule.cron(minute="*/5"),
            targets=[aws_events_targets.LambdaFunction(some_lambda)],
        )

pip install mypy && mypy test.py

Error Log

test.py:19: note: 'LambdaFunction' is missing following 'IRuleTarget' protocol member:
test.py:19: note:     __jsii_proxy_class__

Environment

• CLI Version: 1.17.13
• Framework Version: 1.31.0
• OS: OS X 10.14.6
• Language: Python 3.8.0 / mypy 0.770

Other

It is also possible this is a bug in mypy.

---

This is 🐛 Bug Report

[salimhamed]

I’m running into the same issue with aws_cloudwatch.Metric not correctly implementing IMetric. Here’s a snippet that demonstrates the issue:

import aws_cdk.aws_cloudwatch as cloudwatch
import aws_cdk.aws_kinesis as kds
from aws_cdk import core


class MyStack(core.Stack):
    def __init__(self, scope: core.Construct, _id: str, **kwargs) -> None:
        super().__init__(scope, _id, **kwargs)

        self.template_options.description = "Demo"

        stream = kds.Stream(self, "InputStream", shard_count=16)

        dashboard = cloudwatch.Dashboard(self, "Dashboard", dashboard_name=core.Aws.STACK_NAME)

        incoming_records = cloudwatch.Metric(
            namespace="AWS/Kinesis",
            metric_name="IncomingRecords",
            dimensions={"StreamName": stream.stream_name},
            period=core.Duration.minutes(1),
            statistic="sum",
        )

        dashboard.add_widgets(
            cloudwatch.GraphWidget(
                left=[incoming_records],  # 'Metric' is missing following 'IMetric' protocol member
                width=24,
                title="Kinesis data stream (incoming)",
                left_y_axis=cloudwatch.YAxisProps(min=0),
                right_y_axis=cloudwatch.YAxisProps(min=0),
            )
        )

Checking types with Mypy results in the following error:

'Metric' is missing following 'IMetric' protocol member:
__jsii_proxy_class__
List item 0 has incompatible type "Metric"; expected "IMetric"

[alexmv]

Sorry for not making clear that yeah, this is not about the functionality (which works great!), but about the type-correctness of the jsii-generated typing of the code.

I’ve found two other ones: neither aws_cloudwatch.MathExpression nor aws_cloudwatch.Metric implement IMetric as they are documented to, also because they don’t implement __jsii_proxy_class__.