Interpolates aliases in YAML

example.yml

_anchors:
  checkout_repo: &checkout_repo
    name: Checkout repo
    uses: actions/checkout@v2
    with:
      fetch-depth: 1
      submodules: recursive

  install_pip_requirements: &install_pip_requirements
    name: Install pip requirements
    run: |
      pip install -r requirements.txt

name: Build

on:
push:
  branches: [ '*' ]
  tags: [ '*' ]

jobs:
  build:
    name: Build
    runs-on: ubuntu-latest

    steps:
      - *checkout_repo
      - *install_pip_requirements

interpolate-yaml-aliases.py

#!/usr/bin/env python

# Copyright © 2020 Elijah Shaw-Rutschman

# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the “Software”), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.


"""
Interpolates aliases in YAML.

Reads YAML from standard input, writes YAML to standard output. Comments and
formatting are retained.

This is useful if, for instance, you want to use aliases with a platform
that does not support them, such as Github Actions. You can define your anchors
in a top-level _anchors section, and that section will be omitted from the
output. The script can either be run manually prior to committing your changes,
or can be automated via a git hook.

Example input:

  _anchors:
    checkout_repo: &checkout_repo
      name: Checkout repo
      uses: actions/checkout@v2
      with:
        fetch-depth: 1
        submodules: recursive

    install_pip_requirements: &install_pip_requirements
      name: Install pip requirements
      run: |
        pip install -r requirements.txt

  name: Build

  on:
  push:
    branches: [ '*' ]
    tags: [ '*' ]

  jobs:
    build:
      name: Build
      runs-on: ubuntu-latest

      steps:
        - *checkout_repo
        - *install_pip_requirements

The output would be:

  name: Build

  on: null
  push:
    branches: ['*']
    tags: ['*']

  jobs:
    build:
      name: Build
      runs-on: ubuntu-latest

      steps:
      - name: Checkout repo
        uses: actions/checkout@v2
        with:
          fetch-depth: 1
          submodules: recursive

      - name: Install pip requirements
        run: |
          pip install -r requirements.txt
"""

import sys

from ruamel import yaml


class InterpolatingDumper(yaml.RoundTripDumper):
    def ignore_aliases(self, data):
        # Always interpolate aliases
        return True


def interpolate_aliases(in_stream, out_stream):
    data = yaml.load(in_stream, Loader=yaml.RoundTripLoader)
    if '_anchors' in data:
        # Remove top-level _anchors section
        del data['_anchors']
    out_stream.write(yaml.dump(data, Dumper=InterpolatingDumper))


if __name__ == '__main__':
    if '-h' in sys.argv:
        print(__doc__)
        sys.exit(0)

    interpolate_aliases(sys.stdin, sys.stdout)

@yurishkuro You could use a git pre-commit hook to interpolate the file automatically. I have haven't tested this, but something like:

#!/bin/sh

set -ueo pipefail

files=( $(git diff-index --cached HEAD --name-only) )

for file in ${files[@]}
do
	if [[ "$file" == "build.yml" ]]
	then
		echo "Change detected, interpolating"
		cat build.yml | python interpolate-yaml-aliases.py > .github/workflows/build.yml
		git add .github/workflows/build.yml
	fi
done