Tool to automatically expand YAML merges?

UPDATE: 2019-03-13 12:41:05

• This answer was modified pursuant to a comment by Anthon which correctly identified limitations with PyYAML. (See Pitfalls infra).

Context

• YAML file
• Python for parsing the YAML

Problem

• User jtYamlEnthusiast wishes to output a non-DRY version of a YAML file with aliases, anchors, and merge keys.

Solution(s)

• Alternative 1: use the ruamel library promoted by Anthon infra.
• Alternative 2: use Python pprint.pformat and simply do a load/dump round-trip transformation.

Rationale

• the ruamel library is great if you have the discretion to install another python library besides pyyaml, and you want a high degree of control over "round-trip" YAML transformations (such as the preservation of YAML comments, for example).
• if you do not need rigorous control over round-tripped YAML, or you are limited for some other reason to pyyaml, you can simply load and dump YAML directly, in order to obtain the "non-DRY" output.

Pitfalls

• as of this writing PyYAML has limitations relative to the ruamel library, regarding the handling of YAML v1.1 and YAML v1.2

• See also

  • ruamel docs
  • pyyaml repo

Example

    ##
    import pprint
    import yaml
    ##
    myrawyaml = '''
    default: &DEFAULT
      URL: website.com
      mode: production
      site_name: Website
      some_setting: h2i8yiuhef
      some_other_setting: 3600

    development:
      <<: *DEFAULT
      URL: website.local
      mode: dev

    test:
      <<: *DEFAULT
      URL: test.website.qa
      mode: test
    '''
    ##
    pynative  =   yaml.safe_load(myrawyaml)
    vout      =   pprint.pformat(pynative)
    print(vout)                             ##=> this is non-DRY and just happens to be well-formed YAML syntax
    print(yaml.safe_load(vout))             ##=> this proves we have well-formed YAML if it loads without exception

If you for some reason have a use case where you need to write the expanded YAML back to a file as YAML, you can:

• Use @Anthon's answer. As noted above, though, this approach might not be feasible if you can't install packages.

• Use @dreftymac's answer. It appears that this answer has worked for some people, but it didn't work for me; by my understanding, pprint.pformat returns the argument as a string of its Python representation, and yaml.safe_load expects the Python representation itself. Of course, you could eval the string returned by pprint.pformat, but using eval on even trusted input feels icky. (Again, the answer has a couple of upvotes so maybe I'm missing something here.)

Alternatively, you can do what I did:

import json
import yaml

def expand_yml(yml):
    return yaml.dump(json.loads(json.dumps(yml)))

expand_yml(my_yml_with_aliases)

Since JSON can (with some exceptions, such as aliases) be regarded as a strict subset of YAML, this approach should generally work. However, if performance is a concern, or if you're dealing with hairier YAML, this approach might not work for you.