After working on #100817 I've noticed that there are multiple things that can be improved in terms of copy module tests:

1. First of all, I had submitted some broken code in gh-100817: Speed up copy.deepcopy calls on slice objects #100818 but, our test cases were not able to detect it. Solution: add a new test case in test_slice.py with copy and deepcopy calls. I think it should be in test_slice and not in test_copy, because there's nothing special about it: copy does not change its behaviour or special case it.
2. This test ensures that after modifing copyreg we can now copy an object, but does not assert the result:

   cpython/Lib/test/test_copy.py

   Lines 42 to 55 in e47b139

   	def test_copy_registry(self):
   	class C(object):
   	def __new__(cls, foo):
   	obj = object.__new__(cls)
   	obj.foo = foo
   	return obj
   	def pickle_C(obj):
   	return (C, (obj.foo,))
   	x = C(42)
   	self.assertRaises(TypeError, copy.copy, x)
   	copyreg.pickle(C, pickle_C, C)
   	y = copy.copy(x)
   	def test_copy_reduce_ex(self):

   Right now it can be whatever. The same happens in

   cpython/Lib/test/test_copy.py

   Lines 302 to 315 in e47b139

   	def test_deepcopy_registry(self):
   	class C(object):
   	def __new__(cls, foo):
   	obj = object.__new__(cls)
   	obj.foo = foo
   	return obj
   	def pickle_C(obj):
   	return (C, (obj.foo,))
   	x = C(42)
   	self.assertRaises(TypeError, copy.deepcopy, x)
   	copyreg.pickle(C, pickle_C, C)
   	y = copy.deepcopy(x)
   	def test_deepcopy_reduce_ex(self):

   Solution: add required assertions
3. test_deepcopy_atomic misses several important types: bytes, types.EllipsisType, NotImplementedType.

   cpython/Lib/test/test_copy.py

   Lines 350 to 359 in e47b139

   	def test_deepcopy_atomic(self):
   	class NewStyle:
   	pass
   	def f():
   	pass
   	tests = [None, 42, 2**100, 3.14, True, False, 1j,
   	"hello", "hello\u1234", f.__code__,
   	NewStyle, range(10), max, property()]
   	for x in tests:
   	self.assertIs(copy.deepcopy(x), x)

PR is incoming.

Linked PRs

• gh-100871: Improve copy module tests #100872
• [3.10] gh-100871: Improve copy module tests (GH-100872) #100975
• [3.11] gh-100871: Improve copy module tests (GH-100872) #100976