Ruamel thinks this list is a multi-document file

This is my valid YAML file:

# my_yaml.yaml
- aaa: zzzzz
  bbbb: 'lksdjflksjdflksdj'
  xxxx:
    qqqq:
      - sldflsdkjflks
  fffff: []

- aaa: zzzzz
  bbbb: 'lksdjflksjdflksdj'
  xxxx:
    qqqq:
      - sldflsdkjflks
  fffff: []

- aaa: zzzzz
  bbbb: 'lksdjflksjdflksdj'
  xxxx:
    qqqq:
      - sldflsdkjflks
  fffff: []

But this is what ruamel.yaml generates:

from ruamel.yaml import YAML

yaml = YAML()
yaml.preserve_quotes = True
yaml.default_flow_style = None
yaml.explicit_start = False
formatted_config = yaml.load(Path("my_yaml.yaml").open().read())
yaml.dump_all(formatted_config, Path("/tmp/derps.yaml"))

/tmp/derps.yaml looks like:

aaa: zzzzz
bbbb: 'lksdjflksjdflksdj'
xxxx:
  qqqq:
  - sldflsdkjflks
fffff: []
---
aaa: zzzzz
bbbb: 'lksdjflksjdflksdj'
xxxx:
  qqqq:
  - sldflsdkjflks
fffff: []
---
aaa: zzzzz
bbbb: 'lksdjflksjdflksdj'
xxxx:
  qqqq:
  - sldflsdkjflks
fffff: []

I don't understand why it's inserting --- for each list item. All valid YAML parsers just see this as a list.

How can I make ruamel process this correctly?

Those --- separators indicate multiple documents embedded in a single file.

Instead of yaml.dump_all, which accepts a list of documents, you should be using yaml.dump which accepts a single document:

from ruamel.yaml import YAML
from pathlib import Path

yaml = YAML()
yaml.preserve_quotes = True
yaml.default_flow_style = None
yaml.explicit_start = False

formatted_config = yaml.load(Path("my_yaml.yaml").open().read())
yaml.dump(formatted_config, Path("/tmp/derps.yaml"))