'List the contents of the handler.
Args:
return_tuples (bool, optional): Defines if the Nattributes
are returns as a list of keys or as a list of `(key, value)`.
Returns:
nattributes (list): A list of keys `[key, key, ...]` or a
list of tuples `[(key, value), ...]` depending on the
setting of `return_tuples`.'

def all(self, return_tuples=False):

    if return_tuples:
        return [(key, value) for (key, value) in self._store.items() if (not key.startswith('_'))]
    return [key for key in self._store if (not key.startswith('_'))]
