How can I control what scalar form PyYAML uses for my data?

Question

I've got an object with a short string attribute, and a long multi-line string attribute. I want to write the short string as a YAML quoted scalar, and the multi-line string as a literal scalar:

my_obj.short = "Hello"
my_obj.long = "Line1\nLine2\nLine3"

I'd like the YAML to look like this:

short: "Hello"
long: |
  Line1
  Line2
  Line3

How can I instruct PyYAML to do this? If I call yaml.dump(my_obj), it produces a dict-like output:

{long: 'line1

    line2

    line3

    ', short: Hello}

(Not sure why long is double-spaced like that...)

Can I dictate to PyYAML how to treat my attributes? I'd like to affect both the order and style.

Answer 1

Based on Any yaml libraries in Python that support dumping of long strings as block literals or folded blocks?

import yaml
from collections import OrderedDict

class quoted(str):
    pass

def quoted_presenter(dumper, data):
    return dumper.represent_scalar('tag:yaml.org,2002:str', data, style='"')
yaml.add_representer(quoted, quoted_presenter)

class literal(str):
    pass

def literal_presenter(dumper, data):
    return dumper.represent_scalar('tag:yaml.org,2002:str', data, style='|')
yaml.add_representer(literal, literal_presenter)

def ordered_dict_presenter(dumper, data):
    return dumper.represent_dict(data.items())
yaml.add_representer(OrderedDict, ordered_dict_presenter)

d = OrderedDict(short=quoted("Hello"), long=literal("Line1\nLine2\nLine3\n"))

print(yaml.dump(d))

Output

short: "Hello"
long: |
  Line1
  Line2
  Line3