Fancy YAML proof of concept in Python

fancy_yaml.py

#!/usr/bin/env python3

from typing import Optional, Dict

import os
import yaml
import jinja2

class FancyLoader(yaml.SafeLoader):
    def __init__(self, stream):
        self.definitions = dict()
        self.add_constructor("!env", self._yaml_tag_env)
        self.add_constructor("!define", self._yaml_tag_define)
        self.add_constructor("!ref", self._yaml_tag_ref)
        self.add_constructor("!template", self._yaml_tag_template)
        self.add_constructor("!literal", self._yaml_tag_literal)
        yaml.SafeLoader.__init__(self, stream)

    def _yaml_tag_env(self, loader, node) -> Optional[str]:
        value: str = loader.construct_scalar(node)
        return os.environ.get(value)

    def _yaml_tag_define(self, loader, node):
        mapping: Dict = loader.construct_mapping(node)
        for k, v in mapping.items():
            if k in self.definitions:
                print(f"Warning: overwriting variable {k} = {self.definitions[k]}")
            self.definitions[k] = v
        return mapping

    def _yaml_tag_ref(self, loader, node):
        value: str = loader.construct_scalar(node)
        return self.definitions[value]

    def _yaml_tag_template(self, loader, node):
        value: str = loader.construct_scalar(node)
        tmpl = jinja2.Template(value)
        return tmpl.render(**self.definitions)

    def _yaml_tag_literal(self, loader, node):
        value: str = loader.construct_scalar(node)
        tmpl = jinja2.Template(value)
        render = tmpl.render(**self.definitions)
        if render in ["true", "True", "TRUE", "yes", "Yes", "YES"]:
            return True
        elif render in ["false", "False", "FALSE", "no", "No", "NO"]:
            return False
        elif "." in render:
            return float(render)
        else:
            return int(render)

TEST = """
variables: !define
    what: "World"
    expr: "!"
    who: !env "USER"
    number: 35.45
    answer: 42
output: !template "Hello {{who}} to the {{what}}{{expr}}"
answer: !ref answer
literal: !literal "{{number}}"
"""

ERROR_1 = """
variables:
    number: 45
literal: !!int !template "{{number}}"
"""

if __name__ == "__main__":
    data = yaml.load(TEST, Loader=FancyLoader)
    print(yaml.dump(data))