code: def source_from_cache(path):
      
      #if sys.implementation.cache_tag is None:
      #    raise NotImplementedError('sys.implementation.cache_tag is None')
      #path = os.fspath(path)
      head, pycache_filename = os.path.split(path)
      head, pycache = os.path.split(head)
      if pycache != _PYCACHE:
          raise ValueError('{} not bottom-level directory in '
                           '{!r}'.format(_PYCACHE, path))
      dot_count = pycache_filename.count('.')
      if dot_count not in {2, 3}:
          raise ValueError('expected only 2 or 3 dots in '
                           '{!r}'.format(pycache_filename))
      elif dot_count == 3:
          optimization = pycache_filename.rsplit('.', 2)[-2]
          if not optimization.startswith(_OPT):
              raise ValueError("optimization portion of filename does not start "
                               "with {!r}".format(_OPT))
          opt_level = optimization[len(_OPT):]
          if not opt_level.isalnum():
              raise ValueError("optimization level {!r} is not an alphanumeric "
                               "value".format(optimization))
      base_filename = pycache_filename.partition('.')[0]
      return os.path.join(head, base_filename + SOURCE_SUFFIXES[0])
docs: Given the path to a .pyc. file, return the path to its .py file.
      The .pyc file does not need to exist; this simply returns the path to
      the .py file calculated to correspond to the .pyc file.  If path does
      not conform to PEP 3147/488 format, ValueError will be raised. If
      sys.implementation.cache_tag is None then NotImplementedError is raised.