Python: Ec2Service.placeSpreadAccross example is broken

[rix0rrr]

The example https://github.com/aws-samples/aws-cdk-examples/tree/master/python/ecs/ecs-service-with-task-placement is broken in Python:

service.place_spread_across(ecs.BuiltInAttributes.AVAILABILITY_ZONE)

leads to

npx: installed 271 in 3.688s
Traceback (most recent call last):
  File "app.py", line 50, in <module>
    service.place_spread_across([ecs.BuiltInAttributes.AVAILABILITY_ZONE])
  File "/tmp/.venv/lib/python3.6/site-packages/aws_cdk/aws_ecs/__init__.py", line 2683, in place_spread_across
    return jsii.invoke(self, "placeSpreadAcross", [fields])
  File "/tmp/.venv/lib/python3.6/site-packages/jsii/_kernel/__init__.py", line 104, in wrapped
    return _recursize_dereference(kernel, fn(kernel, *args, **kwargs))
  File "/tmp/.venv/lib/python3.6/site-packages/jsii/_kernel/__init__.py", line 258, in invoke
    args=_make_reference_for_native(self, args),
  File "/tmp/.venv/lib/python3.6/site-packages/jsii/_kernel/__init__.py", line 115, in _make_reference_for_native
    return [_make_reference_for_native(kernel, i) for i in d]
  File "/tmp/.venv/lib/python3.6/site-packages/jsii/_kernel/__init__.py", line 115, in <listcomp>
    return [_make_reference_for_native(kernel, i) for i in d]
  File "/tmp/.venv/lib/python3.6/site-packages/jsii/_kernel/__init__.py", line 121, in _make_reference_for_native
    d.__jsii__type__ = "Object"
AttributeError: 'tuple' object has no attribute '__jsii__type__'

Method is variadic, is that the problem? Or it might be about the constant definition.

https://alpha-docs-aws.amazon.com/CDK/api/preview/docs/@aws-cdk_aws-ecs.Ec2Service.html#placespreadacrossfields-void

[danlaramay]

I'm getting the same error when using add_user_data from ec2_autoscaling.AutoScalingGroup

  File "app.py", line 13, in <module>
    App1Stack(app, "aws-cdk-example3-app1", vpc_stack.vpc)
  File "/Users/test/.pyenv/versions/3.7.2/lib/python3.7/site-packages/jsii/_runtime.py", line 66, in __call__
    inst = super().__call__(*args, **kwargs)
  File "/private/tmp/example3/stacks/app1.py", line 38, in __init__
    "echo '<?php phpinfo(); ?>' > /var/www/html/phpinfo.php\n")
  File "/private/tmp/example3/venv/lib/python3.7/site-packages/aws_cdk/aws_autoscaling/__init__.py", line 2310, in add_user_data
    return jsii.invoke(self, "addUserData", [script_lines])
  File "/Users/test/.pyenv/versions/3.7.2/lib/python3.7/site-packages/jsii/_kernel/__init__.py", line 104, in wrapped
    return _recursize_dereference(kernel, fn(kernel, *args, **kwargs))
  File "/Users/test/.pyenv/versions/3.7.2/lib/python3.7/site-packages/jsii/_kernel/__init__.py", line 258, in invoke
    args=_make_reference_for_native(self, args),
  File "/Users/test/.pyenv/versions/3.7.2/lib/python3.7/site-packages/jsii/_kernel/__init__.py", line 115, in _make_reference_for_native
    return [_make_reference_for_native(kernel, i) for i in d]
  File "/Users/test/.pyenv/versions/3.7.2/lib/python3.7/site-packages/jsii/_kernel/__init__.py", line 115, in <listcomp>
    return [_make_reference_for_native(kernel, i) for i in d]
  File "/Users/test/.pyenv/versions/3.7.2/lib/python3.7/site-packages/jsii/_kernel/__init__.py", line 121, in _make_reference_for_native
    d.__jsii__type__ = "Object"
AttributeError: 'tuple' object has no attribute '__jsii__type__'```

[garnaat]

Taking a look now.

[carterv]

I ran into this same error when using aws_cdk.aws_sqs.Queue.grant(). It appears that the variadic args end up being passed into jsii._kernel._make_reference_for_native() as a tuple, which falls to the default handling of object. The solution would be to change isinstance(d, list) to isinstance(d, (list, tuple)) in the function here.

As an aside, fixing that reveals another issue with the grant() function which is probably more appropriately discussed in the aws_cdk repo (the variadic args should be expanded when passed to the invoke method).

[garnaat]

I think there are a couple of problems. First, the code generator for variadic args is wrapping the tuple passed to the method in a list, like this:

    @jsii.member(jsii_name="placeSpreadAcross")
    def place_spread_across(self, *fields: str) -> None:
        return jsii.invoke(self, "placeSpreadAcross", [fields])

The list gets unpacked and then the tuple contained within it isn't handled properly and causes the above tuple error.

The second problem is that the Python kernel is not handling tuples, only lists. So code like:

def _recursize_dereference(kernel, d):
    if isinstance(d, dict):
        return {k: _recursize_dereference(kernel, v) for k, v in d.items()}
    elif isinstance(d, list):
        return [_recursize_dereference(kernel, i) for i in d]
    elif isinstance(d, ObjRef):
        return _reference_map.resolve_reference(kernel, d)
    elif isinstance(d, EnumRef):
        return _recursize_dereference(kernel, d.ref)(d.member)
    else:
        return d

needs to handle tuples as well as lists.

[carterv]

I was unaware that the Python CDK was auto-generated, that's pretty nifty. I guess it also means that the issue I opened on the CDK repo isn't super relevant over there.

Digging into the code generator, it seems like this line in emitJsiiMethodCall() might be the problem:

code.line(`${methodPrefix}jsii.${this.jsiiMethod}(${jsiiMethodParams.join(", ")}, [${paramNames.join(", ")}])`);

I don't have any experience with TypeScript, but this seems like you could solve the unpacking issue like so:

// If the args are variadic, expand the last one
let modifiedParamNames: string[] = paramNames.slice();
if (this.parameters.length >= 1 && this.parameters[this.parameters.length - 1].variadic) {
    modifiedParamNames.push("*" + modifiedParamNames.pop());
}

code.line(`${methodPrefix}jsii.${this.jsiiMethod}(${jsiiMethodParams.join(", ")}, [${modifiedParamNames.join(", ")}])`);