helm: 'lookup' function always returns empty map

Question

The relevant docs: https://helm.sh/docs/chart_template_guide/functions_and_pipelines/#using-the-lookup-function

My helm version:

$ helm version
version.BuildInfo{Version:"v3.4.1", 
GitCommit:"c4e74854886b2efe3321e185578e6db9be0a6e29", 
GitTreeState:"dirty", GoVersion:"go1.15.4"}

Minimal example to reproduce:

1. Create a new helm chart and install it.

   $ helm create my-chart
   $ helm install my-chart ./my-chart
   

2. Create a simple ConfigMap.

   # my-chart/templates/configmap.yaml
   
   apiVersion: v1
   kind: ConfigMap
   metadata:
     name: my-configmap
   data:
     someKey: someValue
   

   Upgrade the existing chart so that the ConfigMap is applied.

   $ helm upgrade my-chart ./my-chart
   

3. Confirm that the ConfigMap exists.

   $ kubectl -n default get configmap my-configmap
   

   Which returns as expected:

   NAME           DATA   AGE
   my-configmap   1      12m
   

4. Try to use the lookup function to reference the existing ConfigMap.

   # my-chart/templates/configmap.yaml
   
   apiVersion: v1
   kind: ConfigMap
   metadata:
     name: my-configmap
   data:
     someKey: someValue
     someOtherKey: {{ (lookup "v1" "ConfigMap" "default" "my-configmap").data.someValue }}
   

   Then do a dry-run of the upgrade.

   $ helm upgrade my-chart ./my-chart --dry-run
   

   You will be met with a nil pointer error:

   Error: UPGRADE FAILED: template: my-chart/templates/configmap.yaml:9:54: executing "my-
   chart/templates/configmap.yaml" at <"my-configmap">: nil pointer evaluating interface 
   {}.someValue
   

What am I doing wrong?

Answer 1

This is an expected behavior if you are using --dry-run flag.

From documentation

Keep in mind that Helm is not supposed to contact the Kubernetes API Server during a helm template or a helm install|update|delete|rollback --dry-run, so the lookup function will return an empty list (i.e. dict) in such a case.