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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 available_domains(self):
 """ Return list of available domains for use in email address. """

        if not hasattr(self, '_available_domains'):
            url = 'http://{0}/request/domains/format/json/'.format(
                self.api_domain)
            req = requests.get(url)
            domains = req.json()
            setattr(self, '_available_domains', domains)
        return self._available_domains