'Assigns one of the stack\'s registered Amazon EBS volumes to a
specified instance. The volume must first be registered with
the stack by calling RegisterVolume. For more information, see
`Resource Management`_.
**Required Permissions**: To use this action, an IAM user must
have a Manage permissions level for the stack, or an attached
policy that explicitly grants permissions. For more
information on user permissions, see `Managing User
Permissions`_.
:type volume_id: string
:param volume_id: The volume ID.
:type instance_id: string
:param instance_id: The instance ID.'

def assign_volume(self, volume_id, instance_id=None):

    params = {'VolumeId': volume_id}
    if (instance_id is not None):
        params['InstanceId'] = instance_id
    return self.make_request(action='AssignVolume', body=json.dumps(params))
