'Create a new descriptor.
Parameters:
attribute_name (str): name of the attribute to be
searched for in the Package instance
default_handler (callable, optional): default function to be
called if the attribute was not found in the Package
instance'

def __init__(self, attribute_name, default_handler=None):

    self.attribute_name = attribute_name
    if (default_handler is None):
        default_handler = (lambda descriptor, spec, cls: None)
    self.default = default_handler
