flatten dict keys into dot notation

flatten_dict.py

def _flatten_dict(dct: dict[str, Any]) -> list[str]:
    # for each key:
    # - find values
    # - if dict: recurse
    # - elif list: recurse for each item
    # - else: check if bool.
    # - if bool and True, compose a string that you pass around
    #   to join the param names.
    # This was coded for a specific use-case and YAML design; it should
    # be fairly straightforward to adapt to produce just the keys.
    def recur(entry, dot_str: str = "") -> list[str] | str | None:
        if isinstance(entry, bool):
            if entry:
                return dot_str
            return None
        dot_strs: list[str] = []
        if isinstance(entry, dict):
            for key in entry:
                passed_str = dot_str + f".{key}" if dot_str else key
                res = recur(entry[key], passed_str)
                if res is not None:
                    if isinstance(res, list):
                        dot_strs += res
                    else:
                        dot_strs.append(res)
            return dot_strs
        if isinstance(entry, list):
            for i, e in enumerate(entry):
                res = recur(e, dot_str + f".{i}")
                if res is not None:
                    if isinstance(res, list):
                        dot_strs += res
                    else:
                        dot_strs.append(res)
            return dot_strs
        msg = "Ignore config should only have entries with `True` or `False` to ignore or not ignore validation."
        raise ValueError(msg)
    return recur(dct)