Warn about creating global variables by __setattr__ that shadow builtin

names.  Unfortunately, this is not bulletproof since the module
dictionary can be modified directly.

Misc/NEWS

@@ -43,6 +43,12 @@ Core and builtins
   instead of going through __getitem__.  If __getitem__ access is
   preferred, then __iter__ can be overriden.
 
+- Creating an attribute on a module (i.e. a global variable created by
+  __setattr__) that causes a builtin name to be shadowed now raises a
+  DeprecationWarning.  In future versions of Python the effect may be
+  undefined (in order to allow for optimization of global and builtin
+  name lookups).
+
 Extension modules
 -----------------
 

Objects/moduleobject.c

@@ -198,6 +198,71 @@ module_repr(PyModuleObject *m)
 	return PyString_FromFormat("<module '%s' from '%s'>", name, filename);
 }
 
+static PyObject *
+find_builtin_names(void)
+{
+	PyObject *builtins, *names, *key, *value;
+	int pos = 0;
+	builtins = PyEval_GetBuiltins();
+	if (builtins == NULL || !PyDict_Check(builtins)) {
+  		PyErr_SetString(PyExc_SystemError, "no builtins dict!");
+		return NULL;
+	}
+	names = PyDict_New();
+	if (names == NULL)
+		return NULL;
+	while (PyDict_Next(builtins, &pos, &key, &value)) {
+		if (PyString_Check(key) &&
+				PyString_Size(key) > 0 &&
+				PyString_AS_STRING(key)[0] != '_') {
+			if (PyDict_SetItem(names, key, Py_None) < 0) {
+				Py_DECREF(names);
+				return NULL;
+			}
+		}
+	}
+	return names;
+}
+
+/* returns 0 or 1 (and -1 on error) */
+static int
+shadows_builtin(PyObject *globals, PyObject *name)
+{
+	static PyObject *builtin_names = NULL;
+	if (builtin_names == NULL) {
+		builtin_names = find_builtin_names();
+		if (builtin_names == NULL)
+			return -1;
+	}
+	if (!PyString_Check(name))
+		return 0;
+	if (PyDict_GetItem(globals, name) == NULL &&
+	    PyDict_GetItem(builtin_names, name) != NULL) {
+		return 1;
+	}
+	else {
+		return 0;
+	}
+}
+
+static int
+module_setattr(PyObject *m, PyObject *name, PyObject *value)
+{
+	PyObject *globals = ((PyModuleObject *)m)->md_dict;
+	PyObject *builtins = PyEval_GetBuiltins();
+	if (globals != NULL && globals != builtins) {
+		int shadows = shadows_builtin(globals, name);
+		if (shadows == 1) {
+			if (PyErr_Warn(PyExc_DeprecationWarning,
+					"assignment shadows builtin") < 0)
+				return -1;
+		}
+		else if (shadows == -1)
+			return -1;
+	}
+	return PyObject_GenericSetAttr(m, name, value);
+}
+
 /* We only need a traverse function, no clear function: If the module
    is in a cycle, md_dict will be cleared as well, which will break
    the cycle. */
@@ -234,7 +299,7 @@ PyTypeObject PyModule_Type = {
 	0,					/* tp_call */
 	0,					/* tp_str */
 	PyObject_GenericGetAttr,		/* tp_getattro */
-	PyObject_GenericSetAttr,		/* tp_setattro */
+	module_setattr,				/* tp_setattro */
 	0,					/* tp_as_buffer */
 	Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
 		Py_TPFLAGS_BASETYPE,		/* tp_flags */