[Python] How to flatten a mapping constructed from a tagged scalar using ruamel.yaml

My aim is to create a YAML loader that can construct mappings from tagged scalars. Here is a stripped-down version of the loader which constructs an object containing names from a scalar tagged !fullname.

import ruamel.yaml

class MyLoader(ruamel.yaml.YAML):
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
self.constructor.add_constructor("!fullname", self._fullname_constructor)

@staticmethod
def _fullname_constructor(constructor, node):
value = constructor.construct_scalar(node)
first, *middle, last = value.split()
return {
"first_name": first,
"middle_names": middle,
"last_name": last

}

myyaml = MyLoader()


The loader can successfully substitute objects for tagged scalars i.e.

>>> myyaml.load("""
- !fullname Albus Percival Wulfric Brian Dumbledore
- !fullname Severus Snape""")
[
{'first_name': 'Albus', 'middle_names': ['Percival', 'Wulfric', 'Brian'], 'last_name': 'Dumbledore'},
{'first_name': 'Severus', 'middle_names': [], 'last_name': 'Snape'}
]


However, the construction fails when I try to merge the constructed mapping into an enclosing object

>>> yaml.load("""
id: 0
<<: !fullname Albus Percival Wulfric Brian Dumbledore""")
ruamel.yaml.constructor.ConstructorError: while constructing a mapping (...)
expected a mapping or list of mappings for merging, but found scalar


My understanding is that the type of the node is still a ScalarNode, so the constructor is unable to process it even though it ultimately resolves to a mapping. How to modify my code, such that !fullname {scalar} can be merged into the object?

Continue reading...