bpo-38530: Offer suggestions on AttributeError

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
 

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
 -----------------------------------------------------------
 

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_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

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):

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) \

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

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