code: def test_istask():
      assert istask((inc, 1))
      assert not istask(1)
      assert not istask((1, 2))
      f = namedtuple("f", ["x", "y"])
      assert not istask(f(sum, 2))
  
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 6 for calling a global function with example usage: test_istask() without return types