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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 default_from_address(self):
 """ Cache the coinbase address so that we don't make two requests for every single transaction. """

        if self._coinbase_cache_til is not None:
            if time.time - self._coinbase_cache_til > 30:
                self._coinbase_cache_til = None
                self._coinbase_cache = None

        if self._coinbase_cache is None:
            self._coinbase_cache = self.get_coinbase()

        return self._coinbase_cache