> Having had apps with YAML for config, and having had nightmares trying to copy/paste config directives from various sources, I just find whitespace to be unwieldy.

Convert your YaML into JSON and save it in your YaML file. There is probably an online converter, but writing one in your language of choice should be less than ten lines of code.

Do the same YaML→JSON for the “source” configuration you want to copy from, and copy-paste the parts you want. Leave them as JSON.

Complaining about Python's significant whitespace, I get it. I don't mind it personally, but it's obligatory and you can't overcome it (unless you do `coding: with_braces` tricks, of course). But why one would complain about YaML's whitespace? It is not obligatory.

    some_key:
      attr1: val1
      attr2: 12312

is equivalent to

    {some_key: {attr1: val1, attr2: 12312}}

is equivalent to

    {"some_key": {"attr1": "val1", "attr2": 12312}}

is equivalent to

    {"some_key": {
    "attr1":
    "val1", "attr2": 12312
    }
    }

and they're all valid YaML (and on the plus side you can leave dangling commas at the end of sequences, but it won't be valid JSON anymore).