code: def test_flatten(nested, result):
      if result is None:
          val = flatten(nested, result)
          assert len(val) == 20
      else:
          _result = copy(
              result
          )  # because pytest passes parameters as is
          nexisting = len(_result)
          val = flatten(nested, _result)
          assert len(val) == len(_result) == 20 + nexisting
  
      assert issubclass(type(val), tuple)
  
imports: import random
  from copy import copy
  import pytest
  import networkx
  from networkx.utils import PythonRandomInterface, arbitrary_element, create_py_random_state, create_random_state, dict_to_numpy_array, discrete_sequence, flatten, groups, make_list_of_ints, pairwise, powerlaw_sequence
  from networkx.utils.misc import _dict_to_numpy_array1, _dict_to_numpy_array2
label_desc: Use image node_id 1 for calling a global function with example usage: test_flatten(nested, result) without return types