yaml.py

from io import StringIO
from ruamel.yaml import YAML  # pip install ruamel.yaml

def my_string_representer(dumper, data):
    # '' generates `my_multiline: |-` and works perfectly
    # '\n' generates `my_multiline: |` but load(save(J)) != J due to trailing
    FIX = ''
    is_multiline = '\n' in data
    return dumper.represent_scalar(
        'tag:yaml.org,2002:str',
        data.strip() + FIX if is_multiline else data,
        style='|' if is_multiline else None
    )

# Initialize YAML object
yaml = YAML()
yaml.indent(mapping=2, sequence=4, offset=2)
yaml.default_flow_style = False

# Register the custom representer
yaml.Representer.add_representer(str, my_string_representer)

def load(filename):
    with open(filename) as f:
        return yaml.load(f)

def save(filename, data):
    with open(filename, 'w') as f:
        yaml.dump(data, f)


def stringify(data):
    ss = StringIO()
    yaml.dump(data, ss)
    return ss.getvalue()

if __name__ == '__main__':
    from textwrap import dedent
    J = {
        'foo': {
            'i': 42,
            'bar': {
                'baz': dedent('''
                    multiline stuff
                    here
                    and here
                ''').strip()
            },
            'qux': "single line stuff"
        },
        'fizz': 'buzz'
    }

    save('/tmp/test.yaml', J)
    J2 = load('/tmp/test.yaml')

    # print(stringify(J))
    # print(stringify(J2))
    # print(f"-{J['foo']['bar']['baz']}-")
    # print(f"-{J2['foo']['bar']['baz']}-")

    J = {'shape': 'a a', 'colours': '#0=97 #2=37 #5=16'}
    print(stringify(J))