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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 get_backend_expiry(self, expiry=DEFAULT_EXPIRY):
 """ Return the expiry value usable by this backend based upon the provided timeout. """

        if expiry == DEFAULT_EXPIRY:
            expiry = self.default_expiry
        elif expiry == 0:
            # avoid time.time() related precision issues
            expiry = -1
        return None if expiry is None else time.time() + expiry