Helm: Template unable to read external files

There are some external files (e.g - "panels/pods/memory.txt") which is needed to read dynamically based on parameters inside configMap.yaml file

=> cat templates/configMap.yaml
fileContent_via_template: {{ include "readfile" (dict "metrics" "panels/pods/memory.txt") }}

=> cat panels/pods/memory.txt
This is dummy content which is displayed from memory.txt file

To read external file, defined a template -

=> templates/_helpers.tpl
{{- define "readfile" -}}
  {{- $content := .Files.Get .metrics -}}
  {{ $content }}
{{- end -}

It throws below error -

Error: template: grafana/templates/configMap.yaml:2:22: executing "grafana/templates/configMap.yaml" at <include "readfile" (dict "metrics" "panels/pods/memory.txt")>: error calling include: template: grafana/templates/_helpers.tpl:42:24: executing "readfile" at <.Files.Get>: nil pointer evaluating interface {}.Get

If same external file is read from configMap.yaml directly, then it works -

=> cat templates/configMap.yaml
fileContent: {{ .Files.Get "panels/pods/memory.txt" }}

It returns the below output:

---
# Source: grafana/templates/configMap.yaml
fileContent: This is dummy content which is displayed from memory.txt file

Inside the readfile template, you refer to .Files.Get. This is normal Go template syntax: from the object ., get the field Files, and within that object, get the field Get. However, . is no longer the top-level Helm object but rather the parameter to the template.

You could fix this without changing the inner template code by explicitly passing .Files as a parameter

{{ include "readfile" (dict "Files" .Files "metrics" "panels/pods/memory") }}
{{/*                        ^^^^^^^^^^^^^^                               */}}

If you also needed other characterictics like .Values, or if you needed to pass the whole top-level object on to some other template, you may need to pass the top-level object as a parameter. In my templates I like to call this top but I don't think I've seen a specific convention for this in the wild.

In this case, you'd need to both change the template to refer to .Files out of the top object

{{/* Reads a file as in .Files.Get and ....  Takes a dictionary with two
     fields.  `metrics` is the name of the file to read, and `top` is the
     top-level Helm object. */}}
{{ define "readfile" }}
  {{- $content := .top.Files.Get .metrics -}}
  {{/*-           ^^^^                  -*/}}
  {{ $content }}
{{- end }}

At the call site, again, you need to add the parameter.

{{ include "readfile" (dict "top" . "metrics" "panels/pods/memory") }}
{{/*                        ^^^^^^^                               */}}