code: def test_getitem_nested_immutable(self):
      f = data_structures.to_immutable_dict({"a": {"b": "c"}})
      with self.assertRaisesRegex(
          TypeError, "does not support item assignment"
      ):
          f["a"]["b"] = "d"
  
imports: import collections
  import copy
  import pickle
  import threading
  from absl.testing import absltest
  from absl.testing import parameterized
  import cloudpickle
  import dill
  from haiku._src import data_structures
  import jax
  import tree
label_desc: Use image node_id 7 for calling the FlatMappingTest obj's underlying member method code with example usage: obj.test_getitem_nested_immutable() without return types