- Bug #1683368: The object.__init__() and object.__new__() methods are

now stricter in rejecting excess arguments.  The only time when
  either allows excess arguments is when it is not overridden and the
  other one is.  For backwards compatibility, when both are
  overridden, it is a deprecation warning (for now; maybe a Py3k
  warning later).

When merging this into 3.0, the warnings should become errors.

Note: without the change to string.py, lots of spurious warnings happen.
What's going on there?

Lib/string.py

@@ -108,7 +108,9 @@ class _TemplateMetaclass(type):
     """
 
     def __init__(cls, name, bases, dct):
-        super(_TemplateMetaclass, cls).__init__(name, bases, dct)
+        # A super call makes no sense since type() doesn't define __init__().
+        # (Or does it? And should type.__init__() accept three args?)
+        # super(_TemplateMetaclass, cls).__init__(name, bases, dct)
         if 'pattern' in dct:
             pattern = cls.pattern
         else:

Misc/NEWS

@@ -1,4 +1,4 @@
-+++++++++++
+__init+++++++++++
 Python News
 +++++++++++
 
@@ -17,6 +17,13 @@ Core and builtins
 
 - Remove unused file Python/fmod.c.
 
+- Bug #1683368: The object.__init__() and object.__new__() methods are
+  now stricter in rejecting excess arguments.  The only time when
+  either allows excess arguments is when it is not overridden and the
+  other one is.  For backwards compatibility, when both are
+  overridden, it is a deprecation warning (for now; maybe a Py3k
+  warning later).
+
 - Patch #1675423: PyComplex_AsCComplex() now tries to convert an object
   to complex using its __complex__() method before falling back to the
   __float__() method. Therefore, the functions in the cmath module now

Objects/typeobject.c

@@ -2391,27 +2391,103 @@ PyTypeObject PyType_Type = {
 
 /* The base type of all types (eventually)... except itself. */
 
+/* You may wonder why object.__new__() only complains about arguments
+   when object.__init__() is not overridden, and vice versa.
+
+   Consider the use cases:
+
+   1. When neither is overridden, we want to hear complaints about
+      excess (i.e., any) arguments, since their presence could
+      indicate there's a bug.
+
+   2. When defining an Immutable type, we are likely to override only
+      __new__(), since __init__() is called too late to initialize an
+      Immutable object.  Since __new__() defines the signature for the
+      type, it would be a pain to have to override __init__() just to
+      stop it from complaining about excess arguments.
+
+   3. When defining a Mutable type, we are likely to override only
+      __init__().  So here the converse reasoning applies: we don't
+      want to have to override __new__() just to stop it from
+      complaining.
+
+   4. When __init__() is overridden, and the subclass __init__() calls
+      object.__init__(), the latter should complain about excess
+      arguments; ditto for __new__().
+
+   Use cases 2 and 3 make it unattractive to unconditionally check for
+   excess arguments.  The best solution that addresses all four use
+   cases is as follows: __init__() complains about excess arguments
+   unless __new__() is overridden and __init__() is not overridden
+   (IOW, if __init__() is overridden or __new__() is not overridden);
+   symmetrically, __new__() complains about excess arguments unless
+   __init__() is overridden and __new__() is not overridden
+   (IOW, if __new__() is overridden or __init__() is not overridden).
+
+   However, for backwards compatibility, this breaks too much code.
+   Therefore, in 2.6, we'll *warn* about excess arguments when both
+   methods are overridden; for all other cases we'll use the above
+   rules.
+
+*/
+
+/* Forward */
+static PyObject *
+object_new(PyTypeObject *type, PyObject *args, PyObject *kwds);
+
+static int
+excess_args(PyObject *args, PyObject *kwds)
+{
+	return PyTuple_GET_SIZE(args) ||
+		(kwds && PyDict_Check(kwds) && PyDict_Size(kwds));
+}
+
 static int
 object_init(PyObject *self, PyObject *args, PyObject *kwds)
 {
-	return 0;
+	int err = 0;
+	if (excess_args(args, kwds)) {
+		PyTypeObject *type = self->ob_type;
+		if (type->tp_init != object_init &&
+		    type->tp_new != object_new)
+		{
+			err = PyErr_WarnEx(PyExc_DeprecationWarning,
+				   "object.__init__() takes no parameters",
+				   1);
+		}
+		else if (type->tp_init != object_init ||
+			 type->tp_new == object_new)
+		{
+			PyErr_SetString(PyExc_TypeError,
+				"object.__init__() takes no parameters");
+			err = -1;
+		}
+	}
+	return err;
 }
 
-/* If we don't have a tp_new for a new-style class, new will use this one.
-   Therefore this should take no arguments/keywords.  However, this new may
-   also be inherited by objects that define a tp_init but no tp_new.  These
-   objects WILL pass argumets to tp_new, because it gets the same args as
-   tp_init.  So only allow arguments if we aren't using the default init, in
-   which case we expect init to handle argument parsing. */
 static PyObject *
 object_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 {
-	if (type->tp_init == object_init && (PyTuple_GET_SIZE(args) ||
-	     (kwds && PyDict_Check(kwds) && PyDict_Size(kwds)))) {
-		PyErr_SetString(PyExc_TypeError,
-				"default __new__ takes no parameters");
-		return NULL;
+	int err = 0;
+	if (excess_args(args, kwds)) {
+		if (type->tp_new != object_new &&
+		    type->tp_init != object_init)
+		{
+			err = PyErr_WarnEx(PyExc_DeprecationWarning,
+				   "object.__new__() takes no parameters",
+				   1);
 		}
+		else if (type->tp_new != object_new ||
+			 type->tp_init == object_init)
+		{
+			PyErr_SetString(PyExc_TypeError,
+				"object.__new__() takes no parameters");
+			err = -1;
+		}
+	}
+	if (err < 0)
+		return NULL;
 	return type->tp_alloc(type, 0);
 }