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

pablogsal · web-flow · commit 37494b441ace · 2021-04-14T02:36:07.000+01:00
When printing AttributeError, PyErr_Display will offer suggestions of similar 
attribute names in the object that the exception was raised from:

&gt;&gt;&gt; collections.namedtoplo
Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in &lt;module&gt;
AttributeError: module 'collections' has no attribute 'namedtoplo'. Did you mean: namedtuple?
diff --git a/Doc/library/exceptions.rst b/Doc/library/exceptions.rst
@@ -149,6 +149,13 @@ 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.
+
+   .. versionchanged:: 3.10
+      Added the :attr:`name` and :attr:`obj` attributes.
 
 .. exception:: EOFError
 
diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst
@@ -125,8 +125,11 @@ Check :pep:`617` for more details.
 in :issue:`12782` and :issue:`40334`.)
 
 
-Better error messages in the parser
------------------------------------
+Better error messages
+---------------------
+
+SyntaxErrors
+~~~~~~~~~~~~
 
 When parsing code that contains unclosed parentheses or brackets the interpreter
 now includes the location of the unclosed bracket of parentheses instead of displaying
@@ -167,6 +170,23 @@ These improvements are inspired by previous work in the PyPy interpreter.
 (Contributed by Pablo Galindo in :issue:`42864` and Batuhan Taskaya in
 :issue:`40176`.)
 
+
+AttributeErrors
+~~~~~~~~~~~~~~~
+
+When printing :exc:`AttributeError`, :c:func:`PyErr_Display` will offer
+suggestions of simmilar attribute names in the object that the exception was
+raised from:
+
+.. code-block:: python
+
+    >>> collections.namedtoplo
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in <module>
+    AttributeError: module 'collections' has no attribute 'namedtoplo'. Did you mean: namedtuple?
+
+(Contributed by Pablo Galindo in :issue:`38530`.)
+
 PEP 626: Precise line numbers for debugging and other tools
 -----------------------------------------------------------
 
diff --git a/Include/cpython/pyerrors.h b/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
diff --git a/Include/internal/pycore_pyerrors.h b/Include/internal/pycore_pyerrors.h
@@ -86,6 +86,8 @@ PyAPI_FUNC(int) _PyErr_CheckSignalsTstate(PyThreadState *tstate);
 
 PyAPI_FUNC(void) _Py_DumpExtensionModules(int fd, PyInterpreterState *interp);
 
+extern PyObject* _Py_Offer_Suggestions(PyObject* exception);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/Lib/test/test_exceptions.py b/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.assertIn("blech", err.getvalue())
+
+
 class ImportErrorTests(unittest.TestCase):
 
     def test_attributes(self):
diff --git a/Makefile.pre.in b/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) \
diff --git a/Misc/NEWS.d/next/Core and Builtins/2019-10-27-20-20-07.bpo-38530.ZyoDNn.rst b/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. Patch by Pablo Galindo
diff --git 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
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
diff --git a/Python/pythonrun.c b/Python/pythonrun.c
diff --git a/Python/suggestions.c b/Python/suggestions.c

Original file line number	Diff line number	Diff line change
`@@ -86,6 +86,8 @@ PyAPI_FUNC(int) _PyErr_CheckSignalsTstate(PyThreadState *tstate);`
`86`	`86`
`87`	`87`	`PyAPI_FUNC(void) _Py_DumpExtensionModules(int fd, PyInterpreterState *interp);`
`88`	`88`
	`89`	`+extern PyObject* _Py_Offer_Suggestions(PyObject* exception);`
	`90`	`+`
`89`	`91`	`#ifdef __cplusplus`
`90`	`92`	`}`
`91`	`93`	`#endif`
Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	+When printing :exc:`AttributeError`, :c:func:`PyErr_Display` will offer
	`2`	`+suggestions of simmilar attribute names in the object that the exception was`
	`3`	`+raised from. Patch by Pablo Galindo`