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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def compute_etag(self) -> Optional[str]: """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 """

        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,)