travis.yml DRY with anchors

Anchors and aliases are an amazing feature of YAML that allow you to re-use blocks of YAML elsewhere in the same file. Even tho they’re a part of the YAML spec (and have been since the early 2000s) not everyone supports them (I’m looking at you AWS CloudFormation) but fortunately travis-ci has embraced a modern implementation of YAML.

You use anchors and aliases like so:

_template: &_template
  key: value
  other_key: other_value
real_config:
- <<: *_template
  other_key: overridden_value
  new_key: new_value
- <<: *_template
  different_key: different_value
- *_template

Which is equivalent to:

real_config:
- key: value
  other_key: overridden_value
  new_key: new_value
- key: value
  other_key: other_value
  different_key: different_value
- key: value
  other_key: other_value

As you can see we’ve got a hash anchored at _template with key and other_key and in the first hash in the real_config array we override other_key and introduce new_key. In the second hash we introduce different_key.

In travis.yml this means you can keep it DRY and template things like deployments and API keys (and what ever else you can think of):

install: pip install -r requirements-tests.txt
script: make tests
before_deploy: mv build build-${TRAVIS_OS_NAME}
matrix:
  include:
    - language: python
      os: linux
      python: '3.6'
    - language: python
      os: linux
      python: '3.5'
    - os: osx
      language: generic
_deploy_provider: &_deploy_provider
  provider: releases
  api_key:
    secure: abcde=
  file: build-${TRAVIS_OS_NAME}
  skip_cleanup: true
  on: &_deploy_provider_on
    tags: true
deploy:
- <<: *_deploy_provider
  on:
    <<: *_deploy_provider_on
    python: '3.5'
- <<: *_deploy_provider
  on:
    <<: *_deploy_provider_on
    condition: "$TRAVIS_OS_NAME == osx"

This example is based off of a working travis.yml, feel free to adapt it to your needs.

At the time of writing this AWS Cloudformation returns “Template validation error: Template error: YAML aliases are not allowed in CloudFormation templates” if you try to use YAML anchors/aliases.