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New !unset YAML Function to Delete Inherited Keys

· 2 min read
Erik Osterman
Erik Osterman
Founder @ Cloud Posse

Atmos now supports the !unset YAML function, which removes a key entirely from a stack configuration during inheritance and merging. It's the clean way to drop a value that a parent stack or import defined, without resorting to workarounds.

The Problem​

Atmos configuration is built from layered imports and inheritance. That's a strength — until you need to remove something a base layer set. Setting a key to null doesn't remove it; it keeps the key with a null value, which downstream merges (and Terraform) still see:

import:
  - base   # defines vpc.vars.enable_vpn_gateway: true

components:
  terraform:
    vpc:
      vars:
        # Keeps the key, now as null — not the same as "not set".
        enable_vpn_gateway: null

There was no first-class way to say "pretend the parent never set this."

The Solution​

The !unset function marks a key for deletion. After processing, the key is gone — IsSet returns false and it never reaches the merged config:

import:
  - base

components:
  terraform:
    vpc:
      vars:
        # Remove the inherited value entirely.
        enable_vpn_gateway: !unset

        # Override a sibling as usual.
        enable_nat_gateway: false

The dev stack ends up with enable_nat_gateway: false, no enable_vpn_gateway key, and everything else inherited from base.

Use Cases​

Clean overrides by exception​

Keep a rich base catalog and remove only what a specific environment shouldn't have:

components:
  terraform:
    app:
      vars:
        config:
          database:
            # Drop the inherited backup config for dev.
            backup_enabled: !unset
            host: "dev.db.example.com"

Removing whole sections​

!unset works at any depth, including entire mapping sections:

settings:
  # Remove an inherited integration block wholesale.
  spacelift: !unset

Trimming inherited lists​

Use it to drop a list item that inheritance brought in, alongside the values you keep.

How It Works​

!unset is handled during YAML preprocessing and stack merging. The key is truly removed from the underlying configuration store rather than being set to null, so it does not appear in AllSettings() and won't be re-introduced by later merges. Sibling and ancestor keys are preserved.

Get Started​

The !unset function is available now. Check out the documentation for more examples and details, and browse the full set of Atmos YAML functions.