code: def test_flatten():
      assert list(flatten(())) == []
      assert list(flatten("foo")) == ["foo"]
  
imports: from __future__ import annotations
  import pickle
  from collections import namedtuple
  import pytest
  from dask.core import flatten, get, get_dependencies, get_deps, getcycle, has_tasks, ishashable, iskey, istask, literal, preorder_traversal, quote, subs, validate_key
  from dask.utils_test import GetFunctionTestMixin, add, inc
label_desc: Use image node_id 18 for calling a global function with example usage: test_flatten() without return types