ruamel.yaml: modify dump() to add extra '-' at the very start

Question

I'm using ruamel.yaml library to dump a dictionary into a yaml file. However it does not behave the way I'd like it to.

How could I get:

---
- a: 
    - foo: bar
    - bar: foo

instead of:

a:
    - foo: bar
    - bar: foo

with yaml.dump()

I'm extracting the data from:

data = {
    'a': [
         {'foo':'bar'},
         {'bar':'foo'}
       ]
     }

Any other library with the same functionality is open for proposal.

Answer 1

What you are looking for is setting explicit_start = True on the YAML instance. In addition you need to make the top-level a list if you want - a instead of just a on the second line:

import sys
import ruamel.yaml

yaml_str = """\
---
-a:
    -foo: bar
    -bar: foo
"""

data = [{
    'a': [
         {'foo':'bar'},
         {'bar':'foo'}
       ]
     }]

yaml = ruamel.yaml.YAML()
yaml.explicit_start = True
yaml.dump(data, sys.stdout)

gives:

---
- a:
  - foo: bar
  - bar: foo