How to create a custom yaml mapping dumper for ruamel.yaml?

Question

I\'m trying to make a custom YAML dumper/loader for some configuration objects. For simplicity, assuming we want to dump a object of class Hero to a hero.

Answer 1

The defintion for the RoundTripConstructor.construct_mapping is::

def construct_mapping(self, node, maptyp=None, deep=False)

and it needs to know what kind of mapping it is expected to construct. There is some expectation in the RoundTripDumper on what can be attached to such an object, so you best of emulating what the routines in the RoundTripDumper pass: CommentedMap (a normal dict is not going to work).

So you will need to do something like:

from ruamel.yaml import YAML
from ruamel.yaml.comments import CommentedMap

yaml = YAML()

class Hero:
    yaml_tag = '!Hero'
    def __init__(self, name, age):
        self.name = name
        self.age = age

    @classmethod
    def to_yaml(cls, representer, data):
        return representer.represent_mapping(cls.yaml_tag,
                                             {'name': data.name, 'age': data.age})

    @classmethod
    def from_yaml(cls, constructor, node):
        data = CommentedMap()
        constructor.construct_mapping(node, data, deep=True)
        return cls(**data)

    def __str__(self):
        return "Hero(name -> {}, age -> {})".format(self.name, self.age)


yaml.register_class(Hero)

ante_hero = Hero('Saber', 15)
with open('config.yml', 'w') as fout:
    yaml.dump(ante_hero, fout)

with open('config.yml') as fin:
    post_hero = yaml.load(fin)

print(post_hero)

which gives:

Hero(name -> Saber, age -> 15)

The above works because your class is relatively simple, if it could have recursive parts, you would need to follow a two-step creation process, with initial yield of the object created, so that it can be used during the recursion.

That the maptyp defaults to None is historical, it must be set. E.g. one of the first things that construct_mapping does is trying to attach comments (if any were available on the node). I'll remove the default value in 0.15.55, which gives a more sensible error if you leave it out, like you did.