how to resolve values in custom constructors

[vielleicht]

Dynamic resolution in values seems quite handy to me. I tried to add custom constructors, but failed quickly, as the values do not get resolved anymore - at least it doesn't work in my example:

#!/usr/bin/env python3
from typing import Any
import dynamic_yaml

class Command():
    def __init__(self, value):
        self.value = value

    def __call__(self, *args: Any, **kwds: Any) -> Any:
        return self.value

def command_constructor(loader, node):
    value = loader.construct_scalar(node)
    print(value)
    return Command(value)

dynamic_yaml.DynamicYamlLoader.add_constructor("!command", command_constructor)

yaml = '''
a: name
b: !command test/{a}
'''

cfg = dynamic_yaml.load(yaml)
print(cfg.b())

please give advice, how this could work.