def get_axis_variables(ds):
    """
    Returns a list of variables that define an axis of the dataset

    :param netCDF4.Dataset ds: An open netCDF4 Dataset
    """
    axis_variables = []
    for ncvar in ds.get_variables_by_attributes(axis=lambda x: x is not None):
        axis_variables.append(ncvar.name)
    return axis_variables