object inspection: produce deterministic descriptions for nested collection datastructures (#11312)

``util.inspect.object_description`` already attempts to sort collections, but this can fail.
This commit handles the failure case by using string-based object descriptions
as a fallback deterministic sort ordering, and protects against recursive collections.

Co-authored-by: Chris Lamb <lamby@debian.org>
Co-authored-by: Faidon Liambotis <paravoid@debian.org>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

tests/test_util_inspect.py

@@ -503,10 +503,32 @@ def test_set_sorting():
     assert description == "{'a', 'b', 'c', 'd', 'e', 'f', 'g'}"
 
 
+def test_set_sorting_enum():
+    class MyEnum(enum.Enum):
+        a = 1
+        b = 2
+        c = 3
+
+    set_ = set(MyEnum)
+    description = inspect.object_description(set_)
+    assert description == "{MyEnum.a, MyEnum.b, MyEnum.c}"
+
+
 def test_set_sorting_fallback():
     set_ = {None, 1}
     description = inspect.object_description(set_)
-    assert description in ("{1, None}", "{None, 1}")
+    assert description == "{1, None}"
+
+
+def test_deterministic_nested_collection_descriptions():
+    # sortable
+    assert inspect.object_description([{1, 2, 3, 10}]) == "[{1, 2, 3, 10}]"
+    assert inspect.object_description(({1, 2, 3, 10},)) == "({1, 2, 3, 10},)"
+    # non-sortable (elements of varying datatype)
+    assert inspect.object_description([{None, 1}]) == "[{1, None}]"
+    assert inspect.object_description(({None, 1},)) == "({1, None},)"
+    assert inspect.object_description([{None, 1, 'A'}]) == "[{'A', 1, None}]"
+    assert inspect.object_description(({None, 1, 'A'},)) == "({'A', 1, None},)"
 
 
 def test_frozenset_sorting():
@@ -518,7 +540,39 @@ def test_frozenset_sorting():
 def test_frozenset_sorting_fallback():
     frozenset_ = frozenset((None, 1))
     description = inspect.object_description(frozenset_)
-    assert description in ("frozenset({1, None})", "frozenset({None, 1})")
+    assert description == "frozenset({1, None})"
+
+
+def test_nested_tuple_sorting():
+    tuple_ = ({"c", "b", "a"},)  # nb. trailing comma
+    description = inspect.object_description(tuple_)
+    assert description == "({'a', 'b', 'c'},)"
+
+    tuple_ = ({"c", "b", "a"}, {"f", "e", "d"})
+    description = inspect.object_description(tuple_)
+    assert description == "({'a', 'b', 'c'}, {'d', 'e', 'f'})"
+
+
+def test_recursive_collection_description():
+    dict_a_, dict_b_ = {"a": 1}, {"b": 2}
+    dict_a_["link"], dict_b_["link"] = dict_b_, dict_a_
+    description_a, description_b = (
+        inspect.object_description(dict_a_),
+        inspect.object_description(dict_b_),
+    )
+    assert description_a == "{'a': 1, 'link': {'b': 2, 'link': dict(...)}}"
+    assert description_b == "{'b': 2, 'link': {'a': 1, 'link': dict(...)}}"
+
+    list_c_, list_d_ = [1, 2, 3, 4], [5, 6, 7, 8]
+    list_c_.append(list_d_)
+    list_d_.append(list_c_)
+    description_c, description_d = (
+        inspect.object_description(list_c_),
+        inspect.object_description(list_d_),
+    )
+
+    assert description_c == "[1, 2, 3, 4, [5, 6, 7, 8, list(...)]]"
+    assert description_d == "[5, 6, 7, 8, [1, 2, 3, 4, list(...)]]"
 
 
 def test_dict_customtype():