If you have python installed on your system, you can do pip install ruamel.yaml.cmd¹ and then:
yaml merge-expand input.yaml output.yaml
(replace output.yaml with - to write to stdout). This implements the merge expanding with preservation of key order and comments.
The above is actually a few lines of code that utilizes ruamel.yaml¹
so if you have Python (2.7 or 3.4+) and install that using pip install ruamel.yaml and save the following as expand.py:
import sys
from ruamel.yaml import YAML
yaml = YAML(typ='safe')
yaml.default_flow_style=False
with open(sys.argv[1]) as fp:
data = yaml.load(fp)
with open(sys.argv[2], 'w') as fp:
yaml.dump(data, fp)
you can already do:
python expand.py input.yaml output.yaml
That will get you YAML that is semantically equivalent to what you requested (in output.yaml the keys of the mappings are sorted, in this programs output they are not).
The above assumes you don't have any tags in your YAML, nor care about preserving any comments. Most of those, and the key ordering, can be preserved by using a patched version of the standard YAML() instance. Patching is necessary because the standard YAML() instance preserves the merges on round-trip as well, which is exactly what you don't want:
import sys
from ruamel.yaml import YAML, SafeConstructor
yaml = YAML()
yaml.Constructor.flatten_mapping = SafeConstructor.flatten_mapping
yaml.default_flow_style=False
yaml.allow_duplicate_keys = True
# comment out next line if you want "normal" anchors/aliases in your output
yaml.representer.ignore_aliases = lambda x: True
with open(sys.argv[1]) as fp:
data = yaml.load(fp)
with open(sys.argv[2], 'w') as fp:
yaml.dump(data, fp)
with this input:
default: &DEFAULT
URL: website.com
mode: production
site_name: Website
some_setting: h2i8yiuhef
some_other_setting: 3600 # an hour?
development:
<<: *DEFAULT
URL: website.local # local web
mode: dev
test:
<<: *DEFAULT
URL: test.website.qa
mode: test
that will give this output (note that comments on the merged in keys get duplicated):
default:
URL: website.com
mode: production
site_name: Website
some_setting: h2i8yiuhef
some_other_setting: 3600 # an hour?
development:
URL: website.local # local web
mode: dev
site_name: Website
some_setting: h2i8yiuhef
some_other_setting: 3600 # an hour?
test:
URL: test.website.qa
mode: test
site_name: Website
some_setting: h2i8yiuhef
some_other_setting: 3600 # an hour?
The above is what the yaml merge-expand command, mentioned at the start of this answer, does.
¹ Disclaimer: I am the author of that package.
