Conditional expression evaluates unreachable substitution

[wschult23]

Terraform Version 0.11.5

I have a conditional data resource and I want to obtain a value from this resource. If the ressource doesn't exist, I want to set the value to an empty string.

data "aws_instance" "not_exist" {
 
  filter {
    name   = "tag:Name"
    values = ["non_existing"]
  }
}

locals {
  name = "${data.aws_instance.not_exist.count==0?"":data.aws_instance.not_exist.tags["Name"]}"
}

Expected Behavior

Due to the fact that the expression evaluates to true, I would expect, that the second term

data.aws_instance.not_exist.tags["Name"]

would not be evaluated and the value of locals.name is "".

Actual Behavior

Terraform produces a error message, that it can't find the ressource after applying the plan:

* local.name: local.name: Resource 'data.aws_instance.some_ressource_that_does_not_exist' not found for variable 'data.aws_instance.not_exist.tags'

Steps to Reproduce

terraform apply

[jbardin]

Hi @wschult23,

Thanks for filing the issue. This is correct that conditional statements don't short-circuit.
We are working on including this in a suite of changes to the configuration language that will come in a future release.

[jbardin]

We're leaving the earlier issue #15605 open to track this feature.

[wschult23]

Hi James, is there a workaround? I want to access data within a „data“ resource only if the resource itself exists. Best regards, Wolfgang.

…

Am 27.04.2018 um 20:59 schrieb James Bardin ***@***.***>: Hi @wschult23 <https://github.com/wschult23>, Thanks for filing the issue. This is correct that conditional statements don't short-circuit. We are working on including this in a suite of changes to the configuration language that will come in a future release. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#17960 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AFMdbYchpRdm3sm4KrjiU4x7e6ymQQ5oks5ts2qZgaJpZM4Tq2nu>.

[jbardin]

Hi @wschult23,

There's no workaround if the resource does not exist in the configuration at all (I'm actually not sure yet if that will work in the next version, but it's not something that should be counted on).

If the resource doesn't exist because the count is 0, you can reference it as an empty list using the * operator. There's an example in the 0.11 upgrade guide here: https://www.terraform.io/upgrade-guides/0-11.html#referencing-attributes-from-resources-with-count-0

[ghost]

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