Disallow unordered sequences in pytest.approx (#9709)

Fix #9692

testing/python/approx.py

@@ -858,13 +858,21 @@ class TestApprox:
         assert approx(expected, rel=5e-7, abs=0) == actual
         assert approx(expected, rel=5e-8, abs=0) != actual
 
-    def test_generic_sized_iterable_object(self):
-        class MySizedIterable:
-            def __iter__(self):
-                return iter([1, 2, 3, 4])
+    def test_generic_ordered_sequence(self):
+        class MySequence:
+            def __getitem__(self, i):
+                return [1, 2, 3, 4][i]
 
             def __len__(self):
                 return 4
 
-        expected = MySizedIterable()
-        assert [1, 2, 3, 4] == approx(expected)
+        expected = MySequence()
+        assert [1, 2, 3, 4] == approx(expected, abs=1e-4)
+
+        expected_repr = "approx([1 ± 1.0e-06, 2 ± 2.0e-06, 3 ± 3.0e-06, 4 ± 4.0e-06])"
+        assert repr(approx(expected)) == expected_repr
+
+    def test_allow_ordered_sequences_only(self) -> None:
+        """pytest.approx() should raise an error on unordered sequences (#9692)."""
+        with pytest.raises(TypeError, match="only supports ordered sequences"):
+            assert {1, 2, 3} == approx({1, 2, 3})