code: def test_iskey():
      class C:
          pass
  
      assert iskey("x")
      assert iskey(1)
      assert not iskey(C())  # Custom hashables can't be dask keys
      assert not iskey((C(),))
      assert iskey((1, 2))
      assert iskey(())
      assert iskey(("x",))
      assert not iskey([1, 2])
      assert not iskey({1, 2})
      assert not iskey({1: 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 3 for calling a global function with example usage: test_iskey() without return types