code: def test_flatten_dict_is_leaf(self):
      xs = {"foo": {"c": 4}, "bar": {"a": 2, "b": {}}}
      flat_xs = traverse_util.flatten_dict(
          xs, is_leaf=lambda k, x: len(k) == 1 and len(x) == 2
      )
      self.assertEqual(
          flat_xs,
          {
              ("foo", "c"): 4,
              ("bar",): {"a": 2, "b": {}},
          },
      )
      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 18 for calling the TraversalTest obj's underlying member method code with example usage: obj.test_flatten_dict_is_leaf() without return types