def fast_relpath(path, start):
  """A prefix-based relpath, with no normalization or support for returning `..`."""
  relpath = fast_relpath_optional(path, start)
  if relpath is None:
    raise ValueError('{} is not a directory containing {}'.format(start, path))
  return relpath
def fast_relpath ( path , start ) : relpath = fast_relpath_optional ( path , start ) if relpath is None : raise ValueError ( '{} is not a directory containing {}' . format ( start , path ) ) return relpath
A prefix-based relpath, with no normalization or support for returning `..`.