Issue #28385: An error message when non-empty format spec is passed to

object.__format__ now contains the name of actual type.

Lib/test/test_builtin.py

@@ -11,6 +11,7 @@ import os
 import pickle
 import platform
 import random
+import re
 import sys
 import traceback
 import types
@@ -1447,21 +1448,14 @@ class BuiltinTest(unittest.TestCase):
 
         # --------------------------------------------------------------------
         # Issue #7994: object.__format__ with a non-empty format string is
-        #  deprecated
-        def test_deprecated_format_string(obj, fmt_str, should_raise):
-            if should_raise:
-                self.assertRaises(TypeError, format, obj, fmt_str)
-            else:
-                format(obj, fmt_str)
-
-        fmt_strs = ['', 's']
-
+        # disallowed
         class A:
             def __format__(self, fmt_str):
                 return format('', fmt_str)
 
-        for fmt_str in fmt_strs:
-            test_deprecated_format_string(A(), fmt_str, False)
+        self.assertEqual(format(A()), '')
+        self.assertEqual(format(A(), ''), '')
+        self.assertEqual(format(A(), 's'), '')
 
         class B:
             pass
@@ -1470,8 +1464,12 @@ class BuiltinTest(unittest.TestCase):
             pass
 
         for cls in [object, B, C]:
-            for fmt_str in fmt_strs:
-                test_deprecated_format_string(cls(), fmt_str, len(fmt_str) != 0)
+            obj = cls()
+            self.assertEqual(format(obj), str(obj))
+            self.assertEqual(format(obj, ''), str(obj))
+            with self.assertRaisesRegex(TypeError,
+                                        r'\b%s\b' % re.escape(cls.__name__)):
+                format(obj, 's')
         # --------------------------------------------------------------------
 
         # make sure we can take a subclass of str as a format spec

Lib/test/test_bytes.py

@@ -1259,6 +1259,15 @@ class AssortedBytesTest(unittest.TestCase):
             self.assertEqual(f(b"'"), '''b"'"''') # '''
             self.assertEqual(f(b"'\""), r"""b'\'"'""") # '
 
+    @check_bytes_warnings
+    def test_format(self):
+        for b in b'abc', bytearray(b'abc'):
+            self.assertEqual(format(b), str(b))
+            self.assertEqual(format(b, ''), str(b))
+            with self.assertRaisesRegex(TypeError,
+                                        r'\b%s\b' % re.escape(type(b).__name__)):
+                format(b, 's')
+
     def test_compare_bytes_to_bytearray(self):
         self.assertEqual(b"abc" == bytes(b"abc"), True)
         self.assertEqual(b"ab" != bytes(b"abc"), True)

Objects/typeobject.c

@@ -4318,13 +4318,6 @@ PyDoc_STRVAR(object_subclasshook_doc,
 "NotImplemented, the normal algorithm is used.  Otherwise, it\n"
 "overrides the normal algorithm (and the outcome is cached).\n");
 
-/*
-   from PEP 3101, this code implements:
-
-   class object:
-       def __format__(self, format_spec):
-           return format(str(self), format_spec)
-*/
 static PyObject *
 object_format(PyObject *self, PyObject *args)
 {
@@ -4335,22 +4328,19 @@ object_format(PyObject *self, PyObject *args)
     if (!PyArg_ParseTuple(args, "U:__format__", &format_spec))
         return NULL;
 
-    self_as_str = PyObject_Str(self);
-    if (self_as_str != NULL) {
     /* Issue 7994: If we're converting to a string, we
        should reject format specifications */
     if (PyUnicode_GET_LENGTH(format_spec) > 0) {
-            PyErr_SetString(PyExc_TypeError,
-               "non-empty format string passed to object.__format__");
-            goto done;
+        PyErr_Format(PyExc_TypeError,
+                     "unsupported format string passed to %.200s.__format__",
+                     self->ob_type->tp_name);
+        return NULL;
     }
-
+    self_as_str = PyObject_Str(self);
+    if (self_as_str != NULL) {
         result = PyObject_Format(self_as_str, format_spec);
+        Py_DECREF(self_as_str);
     }
-
-done:
-    Py_XDECREF(self_as_str);
-
     return result;
 }