Issue #14385: Support other types than dict for __builtins__

It is now possible to use a custom type for the __builtins__ namespace, instead
of a dict. It can be used for sandboxing for example.  Raise also a NameError
instead of ImportError if __build_class__ name if not found in __builtins__.

Lib/test/test_builtin.py

@@ -554,6 +554,39 @@ class BuiltinTest(unittest.TestCase):
             del l['__builtins__']
         self.assertEqual((g, l), ({'a': 1}, {'b': 2}))
 
+    def test_exec_globals(self):
+        code = compile("print('Hello World!')", "", "exec")
+        # no builtin function
+        self.assertRaisesRegex(NameError, "name 'print' is not defined",
+                               exec, code, {'__builtins__': {}})
+        # __builtins__ must be a mapping type
+        self.assertRaises(TypeError,
+                          exec, code, {'__builtins__': 123})
+
+        # no __build_class__ function
+        code = compile("class A: pass", "", "exec")
+        self.assertRaisesRegex(NameError, "__build_class__ not found",
+                               exec, code, {'__builtins__': {}})
+
+        class frozendict_error(Exception):
+            pass
+
+        class frozendict(dict):
+            def __setitem__(self, key, value):
+                raise frozendict_error("frozendict is readonly")
+
+        # read-only builtins
+        frozen_builtins = frozendict(__builtins__)
+        code = compile("__builtins__['superglobal']=2; print(superglobal)", "test", "exec")
+        self.assertRaises(frozendict_error,
+                          exec, code, {'__builtins__': frozen_builtins})
+
+        # read-only globals
+        namespace = frozendict({})
+        code = compile("x=1", "test", "exec")
+        self.assertRaises(frozendict_error,
+                          exec, code, namespace)
+
     def test_exec_redirected(self):
         savestdout = sys.stdout
         sys.stdout = None # Whatever that cannot flush()

Misc/NEWS

@@ -10,6 +10,11 @@ What's New in Python 3.3.0 Alpha 3?
 Core and Builtins
 -----------------
 
+- Issue #14385: It is now possible to use a custom type for the __builtins__
+  namespace, instead of a dict. It can be used for sandboxing for example.
+  Raise also a NameError instead of ImportError if __build_class__ name if not
+  found in __builtins__.
+
 - Issue #12599: Be more strict in accepting None compared to a false-like
   object for importlib.util.module_for_loader and
   importlib.machinery.PathFinder.

Objects/frameobject.c

@@ -614,10 +614,8 @@ PyFrame_New(PyThreadState *tstate, PyCodeObject *code, PyObject *globals,
         if (builtins) {
             if (PyModule_Check(builtins)) {
                 builtins = PyModule_GetDict(builtins);
-                assert(!builtins || PyDict_Check(builtins));
+                assert(builtins != NULL);
             }
-            else if (!PyDict_Check(builtins))
-                builtins = NULL;
         }
         if (builtins == NULL) {
             /* No builtins!              Make up a minimal one
@@ -636,7 +634,7 @@ PyFrame_New(PyThreadState *tstate, PyCodeObject *code, PyObject *globals,
         /* If we share the globals, we share the builtins.
            Save a lookup and a call. */
         builtins = back->f_builtins;
-        assert(builtins != NULL && PyDict_Check(builtins));
+        assert(builtins != NULL);
         Py_INCREF(builtins);
     }
     if (code->co_zombieframe != NULL) {

Python/ceval.c

@@ -1932,12 +1932,27 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
         TARGET(LOAD_BUILD_CLASS)
         {
             _Py_IDENTIFIER(__build_class__);
+
+            if (PyDict_CheckExact(f->f_builtins)) {
                 x = _PyDict_GetItemId(f->f_builtins, &PyId___build_class__);
                 if (x == NULL) {
-                PyErr_SetString(PyExc_ImportError,
+                    PyErr_SetString(PyExc_NameError,
+                                    "__build_class__ not found");
+                    break;
+                }
+            }
+            else {
+                PyObject *build_class_str = _PyUnicode_FromId(&PyId___build_class__);
+                if (build_class_str == NULL)
+                    break;
+                x = PyObject_GetItem(f->f_builtins, build_class_str);
+                if (x == NULL) {
+                    if (PyErr_ExceptionMatches(PyExc_KeyError))
+                        PyErr_SetString(PyExc_NameError,
                                         "__build_class__ not found");
                     break;
                 }
+            }
             Py_INCREF(x);
             PUSH(x);
             break;
@@ -2078,6 +2093,7 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
             if (x == NULL) {
                 x = PyDict_GetItem(f->f_globals, w);
                 if (x == NULL) {
+                    if (PyDict_CheckExact(f->f_builtins)) {
                         x = PyDict_GetItem(f->f_builtins, w);
                         if (x == NULL) {
                             format_exc_check_arg(
@@ -2086,6 +2102,17 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
                             break;
                         }
                     }
+                    else {
+                        x = PyObject_GetItem(f->f_builtins, w);
+                        if (x == NULL) {
+                            if (PyErr_ExceptionMatches(PyExc_KeyError))
+                                format_exc_check_arg(
+                                            PyExc_NameError,
+                                            NAME_ERROR_MSG, w);
+                            break;
+                        }
+                    }
+                }
                 Py_INCREF(x);
             }
             PUSH(x);
@@ -2093,6 +2120,8 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 
         TARGET(LOAD_GLOBAL)
             w = GETITEM(names, oparg);
+            if (PyDict_CheckExact(f->f_globals)
+                && PyDict_CheckExact(f->f_builtins)) {
                 if (PyUnicode_CheckExact(w)) {
                     /* Inline the PyDict_GetItem() calls.
                        WARNING: this is an extreme speed hack.
@@ -2143,6 +2172,23 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
                 Py_INCREF(x);
                 PUSH(x);
                 DISPATCH();
+            }
+
+            /* Slow-path if globals or builtins is not a dict */
+            x = PyObject_GetItem(f->f_globals, w);
+            if (x == NULL) {
+                x = PyObject_GetItem(f->f_builtins, w);
+                if (x == NULL) {
+                    if (PyErr_ExceptionMatches(PyExc_KeyError))
+                        format_exc_check_arg(
+                                    PyExc_NameError,
+                                    GLOBAL_NAME_ERROR_MSG, w);
+                    break;
+                }
+            }
+            Py_INCREF(x);
+            PUSH(x);
+            DISPATCH();
 
         TARGET(DELETE_FAST)
             x = GETLOCAL(oparg);