code: def test_flatten():
      assert list(flatten(())) == []
      assert list(flatten("foo")) == ["foo"]
  
imports: import pickle
  from collections import namedtuple
  import pytest
  from flash.core.serve.dag.task import flatten, get, get_dependencies, get_deps, istask, literal, preorder_traversal, quote, subs
  from flash.core.serve.dag.utils_test import add, inc
  from flash.core.utilities.imports import _TOPIC_SERVE_AVAILABLE
label_desc: Use image node_id 12 for calling a global function with example usage: test_flatten() without return types