Pyyaml: Default dump behavior for inherited classes

Question

I have a class that inherits from one of the built-ins:

from yaml import YAMLObject, dump
class D(dict, YAMLObject):
   yaml_tag = u'!!map'
   ...

Is there a way to tell pyyaml that the dump of any instance of D should be treated like the parent (in this case dict)? I.e., I would like the following output:

d = {'a':1}
print( dump(d) )
>>> a: 1

Instead of

print( dump(D(d)) )
>>> !%21map {}

As you can see I already tried assigning the class a default tag, but that did not quite work out. Defining a custom dumper is unfortunately not an option.

Answer 1

You can provide a custom to_yaml method in your D class:

import yaml


class D(dict, yaml.YAMLObject):
    yaml_tag = "!map"

    @classmethod
    def to_yaml(cls, dumper, data):
        return dumper.represent_dict(dict(data))


d = D({"foo": "bar"})
print(yaml.dump(d))

The output of the above is:

foo: bar

Note that this only works with yaml.dump and not with yaml.safe_dump. The value of yaml_tag seems to be irrelevant, but it must be set; without setting it, the output of the above would be:

!!python/object/new:__main__.D
dictitems:
  foo: bar