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?

2 files changed, 112 insertions, 12 deletions

diff --git a/Objects/exceptions.c b/Objects/exceptions.c
index dfa069e..4bb4153 100644
--- a/Objects/exceptions.c
+++ b/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
diff --git a/Objects/object.c b/Objects/object.c
index 4b67840..854cc85 100644
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -884,29 +884,60 @@ _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);
-
     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) {
+
+    PyObject* result = 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 +1196,8 @@ _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;
 }