code: def compute_etag(self) -> Optional[str]:
          
          assert self.absolute_path is not None
          version_hash = self._get_cached_version(self.absolute_path)
          if not version_hash:
              return None
          return '"%s"' % (version_hash,)
docs: Sets the ``Etag`` header based on static url version.
  
          This allows efficient ``If-None-Match`` checks against cached
          versions, and sends the correct ``Etag`` for a partial response
          (i.e. the same ``Etag`` as the full file).
  
          .. versionadded:: 3.1