bpo-38530: Offer suggestions on AttributeError

Add News entry

Make 'name' and 'obj' public attributes of AttributeError

Fix various issues with the implementation

Add more tests

Simplify implementation and rename public function

More cosmetic changes

Add more tests

Add more tests

Author: pablogsal

Include/cpython/pyerrors.h

@@ -62,6 +62,12 @@ typedef struct {
     PyObject *value;
 } PyStopIterationObject;
 
+typedef struct {
+    PyException_HEAD
+    PyObject *obj;
+    PyObject *name;
+} PyAttributeErrorObject;
+
 /* Compatibility typedefs */
 typedef PyOSErrorObject PyEnvironmentErrorObject;
 #ifdef MS_WINDOWS

Include/internal/pycore_suggestions.h

@@ -0,0 +1,13 @@
+#include "Python.h"
+
+#ifndef Py_INTERNAL_SUGGESTIONS_H
+#define Py_INTERNAL_SUGGESTIONS_H
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+int _Py_offer_suggestions_for_attribute_error(PyAttributeErrorObject* exception_value);
+
+
+#endif /* !Py_INTERNAL_SUGGESTIONS_H */

Include/pyerrors.h

@@ -221,7 +221,6 @@ PyAPI_FUNC(PyObject *) PyErr_NewExceptionWithDoc(
     const char *name, const char *doc, PyObject *base, PyObject *dict);
 PyAPI_FUNC(void) PyErr_WriteUnraisable(PyObject *);
 
-
 /* In signalmodule.c */
 PyAPI_FUNC(int) PyErr_CheckSignals(void);
 PyAPI_FUNC(void) PyErr_SetInterrupt(void);

Lib/test/test_exceptions.py

@@ -1414,6 +1414,165 @@ class TestException(MemoryError):
             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.assertNotIn("blech", err.getvalue())
+
+
 class ImportErrorTests(unittest.TestCase):
 
     def test_attributes(self):

Makefile.pre.in

@@ -387,6 +387,7 @@ PYTHON_OBJS=	\
 		Python/dtoa.o \
 		Python/formatter_unicode.o \
 		Python/fileutils.o \
+		Python/suggestions.o \
 		Python/$(DYNLOADFILE) \
 		$(LIBOBJS) \
 		$(MACHDEP_OBJS) \
@@ -1161,6 +1162,7 @@ PYTHON_HEADERS= \
 		$(srcdir)/Include/internal/pycore_list.h \
 		$(srcdir)/Include/internal/pycore_long.h \
 		$(srcdir)/Include/internal/pycore_object.h \
+		$(srcdir)/Include/internal/pycore_suggestions.h \
 		$(srcdir)/Include/internal/pycore_pathconfig.h \
 		$(srcdir)/Include/internal/pycore_pyarena.h \
 		$(srcdir)/Include/internal/pycore_pyerrors.h \

Misc/NEWS.d/next/Core and Builtins/2019-10-27-20-20-07.bpo-38530.ZyoDNn.rst

@@ -0,0 +1,3 @@
+When printing :exc:`AttributeError`, :c:func:`PyErr_Display` will offer
+suggestions of simmilar attribute names in the object that the exception was
+raised from.

Objects/exceptions.c

@@ -1338,9 +1338,82 @@ SimpleExtendsException(PyExc_NameError, UnboundLocalError,
 /*
  *    AttributeError extends Exception
  */
-SimpleExtendsException(PyExc_Exception, AttributeError,
-                       "Attribute not found.");
 
+static int
+AttributeError_init(PyAttributeErrorObject *self, PyObject *args, PyObject *kwds)
+{
+    static char *kwlist[] = {"name", "obj", NULL};
+    PyObject *name = NULL;
+    PyObject *obj = NULL;
+
+    if (BaseException_init((PyBaseExceptionObject *)self, args, NULL) == -1) {
+        return -1;
+    }
+
+    PyObject *empty_tuple = PyTuple_New(0);
+    if (!empty_tuple) {
+        return -1;
+    }
+    if (!PyArg_ParseTupleAndKeywords(empty_tuple, kwds, "|$OO:AttributeError", kwlist,
+                                     &name, &obj)) {
+        Py_DECREF(empty_tuple);
+        return -1;
+    }
+    Py_DECREF(empty_tuple);
+
+    Py_XINCREF(name);
+    Py_XSETREF(self->name, name);
+
+    Py_XINCREF(obj);
+    Py_XSETREF(self->obj, obj);
+
+    return 0;
+}
+
+static int
+AttributeError_clear(PyAttributeErrorObject *self)
+{
+    Py_CLEAR(self->obj);
+    Py_CLEAR(self->name);
+    return BaseException_clear((PyBaseExceptionObject *)self);
+}
+
+static void
+AttributeError_dealloc(PyAttributeErrorObject *self)
+{
+    _PyObject_GC_UNTRACK(self);
+    AttributeError_clear(self);
+    Py_TYPE(self)->tp_free((PyObject *)self);
+}
+
+static int
+AttributeError_traverse(PyAttributeErrorObject *self, visitproc visit, void *arg)
+{
+    Py_VISIT(self->obj);
+    Py_VISIT(self->name);
+    return BaseException_traverse((PyBaseExceptionObject *)self, visit, arg);
+}
+
+static PyObject *
+AttributeError_str(PyAttributeErrorObject *self)
+{
+    return BaseException_str((PyBaseExceptionObject *)self);
+}
+
+static PyMemberDef AttributeError_members[] = {
+    {"name", T_OBJECT, offsetof(PyAttributeErrorObject, name), 0, PyDoc_STR("attribute name")},
+    {"obj", T_OBJECT, offsetof(PyAttributeErrorObject, obj), 0, PyDoc_STR("object")},
+    {NULL}  /* Sentinel */
+};
+
+static PyMethodDef AttributeError_methods[] = {
+    {NULL}  /* Sentinel */
+};
+
+ComplexExtendsException(PyExc_Exception, AttributeError,
+                        AttributeError, 0,
+                        AttributeError_methods, AttributeError_members,
+                        0, AttributeError_str, "Attribute not found.");
 
 /*
  *    SyntaxError extends Exception

Objects/object.c

@@ -12,6 +12,7 @@
 #include "pycore_pystate.h"       // _PyThreadState_GET()
 #include "pycore_symtable.h"      // PySTEntry_Type
 #include "pycore_unionobject.h"   // _Py_UnionType
+#include "pycore_suggestions.h"
 #include "frameobject.h"
 #include "interpreteridobject.h"
 
@@ -884,10 +885,31 @@ _PyObject_SetAttrId(PyObject *v, _Py_Identifier *name, PyObject *w)
     return result;
 }
 
+static inline int
+add_context_to_attribute_error_exception(PyObject* v, PyObject* name)
+{
+    assert(PyErr_Occurred());
+    // Intercept AttributeError exceptions and augment them to offer
+    // suggestions later.
+    if (PyErr_ExceptionMatches(PyExc_AttributeError)){
+        PyObject *type, *value, *traceback;
+        PyErr_Fetch(&type, &value, &traceback);
+        PyErr_NormalizeException(&type, &value, &traceback);
+        if (PyErr_GivenExceptionMatches(value, PyExc_AttributeError) &&
+            (PyObject_SetAttrString(value, "name", name) ||
+             PyObject_SetAttrString(value, "obj", v))) {
+            return 1;
+        }
+        PyErr_Restore(type, value, traceback);
+    }
+    return 0;
+}
+
 PyObject *
 PyObject_GetAttr(PyObject *v, PyObject *name)
 {
     PyTypeObject *tp = Py_TYPE(v);
+    PyObject* result = NULL;
 
     if (!PyUnicode_Check(name)) {
         PyErr_Format(PyExc_TypeError,
@@ -896,17 +918,23 @@ PyObject_GetAttr(PyObject *v, PyObject *name)
         return NULL;
     }
     if (tp->tp_getattro != NULL)
-        return (*tp->tp_getattro)(v, name);
-    if (tp->tp_getattr != NULL) {
+        result = (*tp->tp_getattro)(v, name);
+    else if (tp->tp_getattr != NULL) {
         const char *name_str = PyUnicode_AsUTF8(name);
         if (name_str == NULL)
             return NULL;
-        return (*tp->tp_getattr)(v, (char *)name_str);
+        result = (*tp->tp_getattr)(v, (char *)name_str);
+    } else {
+        PyErr_Format(PyExc_AttributeError,
+                    "'%.50s' object has no attribute '%U'",
+                    tp->tp_name, name);
     }
-    PyErr_Format(PyExc_AttributeError,
-                 "'%.50s' object has no attribute '%U'",
-                 tp->tp_name, name);
-    return NULL;
+
+    if (!result && add_context_to_attribute_error_exception(v, name)) {
+        return NULL;
+    }
+
+    return result;
 }
 
 int
@@ -1165,6 +1193,11 @@ _PyObject_GetMethod(PyObject *obj, PyObject *name, PyObject **method)
     PyErr_Format(PyExc_AttributeError,
                  "'%.50s' object has no attribute '%U'",
                  tp->tp_name, name);
+
+    if (add_context_to_attribute_error_exception(obj, name)) {
+        return 0;
+    }
+
     return 0;
 }