def get_format(self):
        format = self.request.GET.get('format', self.default_format)
        if format not in self.allowed_formats:
            format = self.default_format
        return format
device_name=None means getting bdm of root instance device
dataset/ETHPy150Open ofri/Open-Knesset/hashnav/base.py/View.get_format--crossover with--dataset/ETHPy150Open openstack/ec2-api/ec2api/tests/functional/base.py/EC2TestCase.get_instance_bdm