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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 _helper_for_model(self, model_type):
 """ Get the helper for a given type of Docker model. For use by resource definitions. """

        if model_type is models.containers.Container:
            return self.containers
        if model_type is models.images.Image:
            return self.images
        if model_type is models.networks.Network:
            return self.networks
        if model_type is models.volumes.Volume:
            return self.volumes

        raise ValueError('Unknown model type {}'.format(model_type))