Proof of concept for yaml inherit tag. Unfortunately, defining the tag is ugly. This was based on the code in [this comment](https://gist.github.com/joshbode/569627ced3076931b02f#gistcomment-2309157).

example.yml

# Unforunately, the tag can't be a standalone item by itelf.
# After all, the following document is invalid:
#     foo
#     a: bar
# Therefore the tag needs to fit in with the structure of the document.
# I have used a null key and then discard it in the loader
# so that it is not in the final output.

null: !inherit parent.yml

b: baz
c: bar

inherit_yaml.py

import yaml
import os.path


def merge(a, b):
    """ Merge two documents.  TODO: implement deep merge"""
    a.update(b)
    return a


class InheritLoaderMeta(type):

    def __new__(metacls, __name__, __bases__, __dict__):
        """Add include constructer to class."""

        # register the inherit constructor on the class
        cls = super().__new__(metacls, __name__, __bases__, __dict__)
        cls.add_constructor('!inherit', cls.construct_inherit)

        return cls


class InheritLoader(yaml.Loader, metaclass=InheritLoaderMeta):
    """YAML Loader with `!inherit` constructor."""

    def __init__(self, stream):
        """Initialise Loader."""

        try:
            self._root = os.path.split(stream.name)[0]
        except AttributeError:
            self._root = os.path.curdir

        self._parent_data = None

        super().__init__(stream)

    def construct_inherit(self, node):
        """load parent file referenced at node."""

        filename = os.path.abspath(os.path.join(
            self._root, self.construct_scalar(node)
        ))
        extension = os.path.splitext(filename)[1].lstrip('.')

        with open(filename, 'r') as f:
            if extension in ('yaml', 'yml'):
                self._parent_data = yaml.load(f, InheritLoader)
            else:
                self._parent_data = ''.join(f.readlines())
        return ''

    def construct_document(self, node):
        """ Merge parent with document."""
        doc = super().construct_document(node)
        parent = self._parent_data
        if parent is not None:
            # Reset parent data for a multi-document file
            self._parent_data = None
            if None in doc:
                # remove empty None key
                doc.pop(None)
            return merge(parent, doc)
        return doc


if __name__ == '__main__':
    with open('example.yml', 'r') as f:
        data = yaml.load(f, Loader=InheritLoader)
    print(data)

parent.yml

a: foo
b: bar

Hey waylan,

for a project I also ended up writing a yaml parser like yours that supports inheritance. For the declaration of the base file I chose this:

!extend : base.yml

So the value after the constructor is simply None, and then the parser knows it has to evaluate what's been assigned to this key.
Although one tends to forget the ":" in between, which leads to confusing error messages, it's quite clean and readable. And the wording "extend" is taken directly from jinja2, which I hoped would be intuitive for users that know it.

Nice to see I'm not the only one missing a de-facto tool for DRY and extendable config files. 👍