code: def test_flatten(self):
      features = Features(
          {
              "foo": {
                  "bar1": Value("int32"),
                  "bar2": {"foobar": Value("string")},
              }
          }
      )
      _features = features.copy()
      flattened_features = features.flatten()
      assert flattened_features == {
          "foo.bar1": Value("int32"),
          "foo.bar2.foobar": Value("string"),
      }
      assert (
          features == _features
      ), "calling flatten shouldn't alter the current features"
  
imports: import datetime
  from unittest import TestCase
  from unittest.mock import patch
  import numpy
  import pandas
  import pyarrow
  import pytest
  from datasets import Array2D
  from datasets.arrow_dataset import Dataset
  from datasets.features import Audio, ClassLabel, Features, Image, Sequence, Value
  from datasets.features.features import _arrow_to_datasets_dtype, _cast_to_python_objects, cast_to_python_objects, encode_nested_example, generate_from_dict, string_to_arrow
  from datasets.features.translation import Translation, TranslationVariableLanguages
  from datasets.info import DatasetInfo
  from datasets.utils.py_utils import asdict
  from ..utils import require_jax, require_tf, require_torch
label_desc: Use image node_id 8 for calling the FeaturesTest obj's underlying member method code with example usage: obj.test_flatten() without return types