def get_dir_from_zipfile(zip_file, fallback=None):
    """Return the name of the root folder in a zip file.

    :param zip_file: a :class:`zipfile.ZipFile` instance.
    :param fallback: if `None`, the name of the zip file is used. This is
        returned if the zip file contains more than one top-level directory,
        or none at all.
    """
    fallback = fallback or zip_file.filename
    directories = [name for name in zip_file.namelist() if name.endswith("/")
                   and len(PurePath(name).parts) == 1]
    return fallback if len(directories) > 1 else directories[0]
def demag(isa, ax=0):
    """plot demag of NC/I7/ISA7 model
    Args:
      isa: Isa7/NC object
    """
    emag = [e for e in isa.elements if e.is_magnet()]
    demag = np.array([e.demagnetization(isa.MAGN_TEMPERATURE) for e in emag])
    _contour(ax, f'Demagnetization at {isa.MAGN_TEMPERATURE} °C',
             emag, demag, '-H / kA/m', isa)
    logger.info("Max demagnetization %f", np.max(demag))