code: def test_flatten_dict_keep_empty(self):
      xs = {"foo": 1, "bar": {"a": 2, "b": {}}}
      flat_xs = traverse_util.flatten_dict(xs, keep_empty_nodes=True)
      self.assertEqual(
          flat_xs,
          {
              ("foo",): 1,
              ("bar", "a"): 2,
              ("bar", "b"): traverse_util.empty_node,
          },
      )
      xs_restore = traverse_util.unflatten_dict(flat_xs)
      self.assertEqual(xs, xs_restore)
  
imports: import collections
  import jax
  import jax.numpy
  import numpy
  import optax
  from absl.testing import absltest
  import flax
  from flax import traverse_util
  from flax.core import freeze
label_desc: Use image node_id 17 for calling the TraversalTest obj's underlying member method code with example usage: obj.test_flatten_dict_keep_empty() without return types