from typing import Callable


def check_callback(callback):
    """
    Check if callback is a callable or a list of callables.
    """
    if callback is not None:
        if isinstance(callback, Callable):
            return [callback]

        elif (isinstance(callback, list) and
              all([isinstance(c, Callable) for c in callback])):
            return callback

        else:
            raise ValueError("callback should be either a callable or "
                             "a list of callables.")
    else:
        return []