'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'

def compute_etag(self):

    version_hash = self._get_cached_version(self.absolute_path)
    if (not version_hash):
        return None
    return ('"%s"' % (version_hash,))
