pyyamlについて：タグの加え方

module.py

import yaml
from yaml import ScalarNode, SequenceNode


def _flatten(_, node):
    def _inner(n):
        if isinstance(n, ScalarNode):
            yield n
        elif isinstance(n, SequenceNode):
            for e in n.value:
                yield from _inner(e)
        else:
            assert False

    return [v.value for v in _inner(node)]


yaml.add_constructor('!Flatten', _flatten)


#
with open('sample.yml') as fin:
    data = yaml.load(fin.read(), Loader=yaml.FullLoader)
    
print(data)

sample.yml

a: &nums [4, 5, 6]
b: !!Flatten [1, 2, 3, *nums]