How to use the traittypes.utils.Sentinel function in traittypes

class _DelayedImportError(object):
    def __init__(self, package_name):
        self.package_name = package_name

    def __getattribute__(self, name):
        package_name = super(_DelayedImportError, self).__getattribute__('package_name')
        raise RuntimeError('Missing dependency: %s' % package_name)

try:
    import numpy as np
except ImportError:
    np = _DelayedImportError('numpy')


Empty = Sentinel('Empty', 'traittypes',
"""
Used in traittypes to specify that the default value should
be an empty dataset
""")


class SciType(TraitType):

    """A base trait type for numpy arrays, pandas dataframes, pandas series, xarray datasets and xarray dataarrays."""

    def __init__(self, **kwargs):
        super(SciType, self).__init__(**kwargs)
        self.validators = []

    def valid(self, *validators):
        """