def get_partial(self, base=None, depth=-1):
if base:
base = Path(base)
if not base.is_directory:
raise RuntimeError("Base path must reference a directory.")
elif base.is_relative:
base = self.base.join(base)
elif not base.descends_from(self.base):
raise RuntimeError("Path does not descend from base.")
listing = self.get(base)
else:
listing = self
base = self.base
partial = super(FileListing, listing).get_partial(depth)
partial.base = base
return partial
