bpo-38530: Offer suggestions on AttributeError (#16856)

When printing AttributeError, PyErr_Display will offer suggestions of similar 
attribute names in the object that the exception was raised from:

>>> collections.namedtoplo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'collections' has no attribute 'namedtoplo'. Did you mean: namedtuple?

1 files changed, 159 insertions, 0 deletions

diff --git a/Lib/test/test_exceptions.py b/Lib/test/test_exceptions.py
index 9dc3a81..e1a5ec7 100644
--- a/Lib/test/test_exceptions.py
+++ b/Lib/test/test_exceptions.py
@@ -1414,6 +1414,165 @@ class ExceptionTests(unittest.TestCase):
             gc_collect()
 
 
+class AttributeErrorTests(unittest.TestCase):
+    def test_attributes(self):
+        # Setting 'attr' should not be a problem.
+        exc = AttributeError('Ouch!')
+        self.assertIsNone(exc.name)
+        self.assertIsNone(exc.obj)
+
+        sentinel = object()
+        exc = AttributeError('Ouch', name='carry', obj=sentinel)
+        self.assertEqual(exc.name, 'carry')
+        self.assertIs(exc.obj, sentinel)
+
+    def test_getattr_has_name_and_obj(self):
+        class A:
+            blech = None
+
+        obj = A()
+        try:
+            obj.bluch
+        except AttributeError as exc:
+            self.assertEqual("bluch", exc.name)
+            self.assertEqual(obj, exc.obj)
+
+    def test_getattr_has_name_and_obj_for_method(self):
+        class A:
+            def blech(self):
+                return
+
+        obj = A()
+        try:
+            obj.bluch()
+        except AttributeError as exc:
+            self.assertEqual("bluch", exc.name)
+            self.assertEqual(obj, exc.obj)
+
+    def test_getattr_suggestions(self):
+        class Substitution:
+            noise = more_noise = a = bc = None
+            blech = None
+
+        class Elimination:
+            noise = more_noise = a = bc = None
+            blch = None
+
+        class Addition:
+            noise = more_noise = a = bc = None
+            bluchin = None
+
+        class SubstitutionOverElimination:
+            blach = None
+            bluc = None
+
+        class SubstitutionOverAddition:
+            blach = None
+            bluchi = None
+
+        class EliminationOverAddition:
+            blucha = None
+            bluc = None
+
+        for cls, suggestion in [(Substitution, "blech?"),
+                                (Elimination, "blch?"),
+                                (Addition, "bluchin?"),
+                                (EliminationOverAddition, "bluc?"),
+                                (SubstitutionOverElimination, "blach?"),
+                                (SubstitutionOverAddition, "blach?")]:
+            try:
+                cls().bluch
+            except AttributeError as exc:
+                with support.captured_stderr() as err:
+                    sys.__excepthook__(*sys.exc_info())
+
+            self.assertIn(suggestion, err.getvalue())
+
+    def test_getattr_suggestions_do_not_trigger_for_long_attributes(self):
+        class A:
+            blech = None
+
+        try:
+            A().somethingverywrong
+        except AttributeError as exc:
+            with support.captured_stderr() as err:
+                sys.__excepthook__(*sys.exc_info())
+
+        self.assertNotIn("blech", err.getvalue())
+
+    def test_getattr_suggestions_do_not_trigger_for_big_dicts(self):
+        class A:
+            blech = None
+        # A class with a very big __dict__ will not be consider
+        # for suggestions.
+        for index in range(101):
+            setattr(A, f"index_{index}", None)
+
+        try:
+            A().bluch
+        except AttributeError as exc:
+            with support.captured_stderr() as err:
+                sys.__excepthook__(*sys.exc_info())
+
+        self.assertNotIn("blech", err.getvalue())
+
+    def test_getattr_suggestions_no_args(self):
+        class A:
+            blech = None
+            def __getattr__(self, attr):
+                raise AttributeError()
+
+        try:
+            A().bluch
+        except AttributeError as exc:
+            with support.captured_stderr() as err:
+                sys.__excepthook__(*sys.exc_info())
+
+        self.assertIn("blech", err.getvalue())
+
+        class A:
+            blech = None
+            def __getattr__(self, attr):
+                raise AttributeError
+
+        try:
+            A().bluch
+        except AttributeError as exc:
+            with support.captured_stderr() as err:
+                sys.__excepthook__(*sys.exc_info())
+
+        self.assertIn("blech", err.getvalue())
+
+    def test_getattr_suggestions_invalid_args(self):
+        class NonStringifyClass:
+            __str__ = None
+            __repr__ = None
+
+        class A:
+            blech = None
+            def __getattr__(self, attr):
+                raise AttributeError(NonStringifyClass())
+
+        class B:
+            blech = None
+            def __getattr__(self, attr):
+                raise AttributeError("Error", 23)
+
+        class C:
+            blech = None
+            def __getattr__(self, attr):
+                raise AttributeError(23)
+
+        for cls in [A, B, C]:
+            try:
+                cls().bluch
+            except AttributeError as exc:
+                with support.captured_stderr() as err:
+                    sys.__excepthook__(*sys.exc_info())
+
+            self.assertIn("blech", err.getvalue())
+
+
 class ImportErrorTests(unittest.TestCase):
 
     def test_attributes(self):