outputs an obscure subset of XML where the content of a given object must be either a string, or more objects, but not both. Also, the XML output attributes "__content__" and "__children__" are forbidden. Each child in the __children__ list must be a dict, with exactly one key: the data type of the child.

dict2xml.py

def dict2xml(x: dict, indent=0) -> str:
    assert isinstance(x, dict)
    assert len(x) == 1
    datatype = list(x.keys())[0]
    assert re.fullmatch(r"[a-zA-Z0-9_]+", datatype)
    data = x[datatype]
    body = ""
    if "__content__" in data:
        assert isinstance(data["__content__"], str)
        body += f"{' '*(indent+2)}{data['__content__']}"
        del data["__content__"]
    if "__children__" in data:
        for child in data["__children__"]:
            body += "\n"
            body += dict2xml(child, indent=indent + 2)
        del data["__children__"]
    open_bracket = f"<{datatype}"
    for attr_name, attr_value in data.items():
        assert re.fullmatch(r"[a-zA-Z0-9_]+", attr_name)
        open_bracket += f" {attr_name}="
        if isinstance(attr_value, str):
            open_bracket += f'"{attr_value}"'
        else:
            open_bracket += str(attr_value)
    open_bracket += ">"
    close_bracket = f"</{datatype}>"
    if body:
        if "\n" not in body.strip():
            return f"{' '*indent}{open_bracket}{body.strip()}{close_bracket}"
        else:
            return f"{' '*indent}{open_bracket}{body}\n{' '*indent}{close_bracket}"
    else:
        return f"{' '*indent}{open_bracket} />"

example-in.yml

FooType:
  attr1: "foo"
  attr2: "bar"
  __children__:
    - FooType:
        __content__: "string"
    - FooType:
        __children__:
          - FooType:
              attr4: 123
          - FooType:
              attr5: 456

example-out.xml

<FooType attr1="foo" attr2="bar">
  <FooType>string</FooType>
  <FooType>
    <FooType attr4=123> />
    <FooType attr5=456> />
  </FooType>
</FooType>