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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 sha1_hexdigest(self):
 # type: () -> str """ A SHA-1 digest of the whole object for easy differentiation. .. versionadded:: 18.1.0 """

        if self._sha1_hexdigest is None:
            self._sha1_hexdigest = hashlib.sha1(self._pem_bytes).hexdigest()

        return self._sha1_hexdigest