When working with Terraform, you may encounter situations where you want to ignore changes to certain attributes of a resource. The ignore_changes lifecycle meta-argument allows you to specify which attributes of a resource should be ignored when Terraform detects changes. However, configuring ignore_changes correctly can sometimes be tricky, especially if you encounter errors like "A static list expression is required."

Lets see how to properly use ignore_changes and address common issues:

Common Issue: "A static list expression is required"

This error occurs because Terraform requires ignore_changes to be a statically defined list. Dynamic expressions, such as interpolations or index lookups within the list, are not allowed. Here's an example of code that would trigger this error:
 

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  tags = {
    Name = "example-instance"
  }

  lifecycle {
    ignore_changes = [
      tags["Name"]  # This will cause an error
    ]
  }
}

To fix this, ensure that the list contains only static strings, not expressions. Here’s how you can rewrite it:
 
resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  tags = {
    Name = "example-instance"
    Environment = "production"
  }

  lifecycle {
    ignore_changes = [
      "tags.Name",  # Correct usage with a static string
      "tags.Environment"  # You can include multiple static strings
    ]
  }
}


Best Practices

Explicitly List Attributes: Always use explicit attribute paths in ignore_changes. This ensures Terraform knows exactly which attributes to ignore.

Test Changes: After adding ignore_changes, run terraform plan to ensure that the changes are correctly ignored. This helps verify that your configuration is working as expected.