code: def test_keys(self):
      d = FlatMap({"key1": "value", "key2": "value2"})
      self.assertEqual(
          str(d.keys()), "KeysOnlyKeysView(['key1', 'key2'])"
      )
      self.assertEqual(
          repr(d.keys()), "KeysOnlyKeysView(['key1', 'key2'])"
      )
  
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 20 for calling the FlatMappingTest obj's underlying member method code with example usage: obj.test_keys() without return types