import yaml
class MyLoader(yaml.SafeLoader):
    # returns
    def construct_mapping(self, *args, **kwargs):
        super().add_constructor(None, construct_undefined)
        # when loading we want to skip keys that require construction,
        mapping = super().construct_mapping(*args, **kwargs)

        return mapping
import typing
class Tagged(typing.NamedTuple):
    tag: str
    value: object

def construct_undefined(self, node):
    if isinstance(node, yaml.nodes.ScalarNode):
        value = self.construct_scalar(node)
    elif isinstance(node, yaml.nodes.SequenceNode):
        value = self.construct_sequence(node)
    elif isinstance(node, yaml.nodes.MappingNode):
        value = self.construct_mapping(node)
    else:
        assert False, f"unexpected node: {node!r}"
    return Tagged(node.tag, value)