from importlib import import_module
from typing import Union


def import_attr(
    attr_name: str,
    module_name: Union[str, None],
    package: Union[str, None],
) -> object:
    """Import an attribute from a module located in a package.

    This utility function is used in custom __getattr__ methods within __init__.py
    files to dynamically import attributes.

    Args:
        attr_name: The name of the attribute to import.
        module_name: The name of the module to import from. If None, the attribute
            is imported from the package itself.
        package: The name of the package where the module is located.
    """
    if module_name == "__module__" or module_name is None:
        try:
            result = import_module(f".{attr_name}", package=package)
        except ModuleNotFoundError:
            msg = f"module '{package!r}' has no attribute {attr_name!r}"
            raise AttributeError(msg) from None
    else:
        try:
            module = import_module(f".{module_name}", package=package)
        except ModuleNotFoundError:
            msg = f"module '{package!r}.{module_name!r}' not found"
            raise ImportError(msg) from None
        result = getattr(module, attr_name)
    return result