README

Overview

A custom Jinja filter for Sceptre for unquoting resolvers appearing in var files.

Motivation

In Sceptre, resolvers are an essential feature for simplifying complex configurations by enabling dynamic data retrieval. Resolvers such as !stack_output_external and !stack_output are particularly useful for referencing output values from other stacks or external sources, among many others.

When using resolvers in var files, however, they must be protected from being interpreted as YAML tags by the YAML loader.

This plugin addresses this challenge by allowing you to safely include resolvers in your var files. It ensures that resolvers are not processed as YAML tags by the loader, and then turned back into YAML tags after Jinja interpolation in the generated config.

Installation

Installation instructions

To install directly from PyPI

pip install jinja-unquote-resolvers-filter

To install from the git repo

pip install git+https://github.com/Sceptre/jinja-unquote-resolvers-filter.git

Usage/Examples

In your var file:

Subnets:
  - '!stack_output_external mystack::subnet_a'  # Quotes needed to protect a YAML tag.
  - '!stack_output_external mystack::subnet_b'

VPC: '!stack_output_external mystack::vpc_id'

In your config:

j2_environment:
  extensions:
    - jinja_unquote_resolvers_filter.UnquoteResolversFilterExtension

sceptre_user_data:
  subnets:
    {{ var.Subnets | unquote_resolvers(output_indent=4, trim=True) }}
  vpc: {{ var.VPC }}  # This filter is not needed if the quoted resolvers are passed in as scalars.

Arguments

indent (optional, default=2): The number of spaces to use for indentation of nested structures in the output YAML.
output_indent (optional, default=0): The number of spaces to use for indentation of the entire output YAML.
trim (optional, default=False): Whether to trim leading and trailing spaces in the output YAML.

Limitations

At this time, resolver expressions must be wrapped in a single line of text. That is, instead of:

my_multiline_resolver: |
  !from_json
    - !request http://www.whatever.com

Instead write:

my_multiline_resolver: '!from_json [!request http://www.whatever.com]'