code: def is_subdirectory(path_a, path_b):
      
      path_a = os.path.realpath(path_a)
      path_b = os.path.realpath(path_b)
      relative = os.path.relpath(path_a, path_b)
      return (not relative.startswith(os.pardir + os.sep))
docs: Returns True if `path_a` is a subdirectory of `path_b`.