bpo-38530: Offer suggestions on AttributeError

Doc/library/exceptions.rst

@@ -149,6 +149,11 @@ The following exceptions are the exceptions that are usually raised.
    assignment fails.  (When an object does not support attribute references or
    attribute assignments at all, :exc:`TypeError` is raised.)
 
+   The :attr:`name` and :attr:`obj` attributes can be set using keyword-only
+   arguments to the constructor. When set they represent the name of the attribute
+   that was attempted to be accessed and the object that was accessed for said
+   attribute, respectively.
+
 
 .. exception:: EOFError
 

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 +1,12 @@
+#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(PyObject* exception, PyObject* value);
+
+#endif /* !Py_INTERNAL_SUGGESTIONS_H */

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 +1338,76 @@ 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 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, BaseException_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"
 
@@ -884,29 +885,60 @@ _PyObject_SetAttrId(PyObject *v, _Py_Identifier *name, PyObject *w)
     return result;
 }
 
+static inline int
+set_attribute_error_context(PyObject* v, PyObject* name)
+{
+    assert(PyErr_Occurred());
+    _Py_IDENTIFIER(name);
+    _Py_IDENTIFIER(obj);
+    // 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_SetAttrId(value, &PyId_name, name) ||
+             _PyObject_SetAttrId(value, &PyId_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,
                      "attribute name must be string, not '%.200s'",
                      Py_TYPE(name)->tp_name);
         return NULL;
     }
-    if (tp->tp_getattro != NULL)
-        return (*tp->tp_getattro)(v, name);
-    if (tp->tp_getattr != NULL) {
+    if (tp->tp_getattro != NULL) {
+        result = (*tp->tp_getattro)(v, name);
+    }
+    else if (tp->tp_getattr != NULL) {
         const char *name_str = PyUnicode_AsUTF8(name);
-        if (name_str == NULL)
+        if (name_str == NULL) {
             return NULL;
-        return (*tp->tp_getattr)(v, (char *)name_str);
+        }
+        result = (*tp->tp_getattr)(v, (char *)name_str);
     }
-    PyErr_Format(PyExc_AttributeError,
-                 "'%.50s' object has no attribute '%U'",
-                 tp->tp_name, name);
-    return NULL;
+    else {
+        PyErr_Format(PyExc_AttributeError,
+                    "'%.50s' object has no attribute '%U'",
+                    tp->tp_name, name);
+    }
+
+    if (result == NULL) {
+        set_attribute_error_context(v, name);
+    }
+    return result;
 }
 
 int
@@ -1165,6 +1197,8 @@ _PyObject_GetMethod(PyObject *obj, PyObject *name, PyObject **method)
     PyErr_Format(PyExc_AttributeError,
                  "'%.50s' object has no attribute '%U'",
                  tp->tp_name, name);
+
+    set_attribute_error_context(obj, name);
     return 0;
 }