Rudimentary support for changing __class__

src/runtime/objmodel.cpp

@@ -1651,6 +1651,51 @@ bool dataDescriptorSetSpecialCases(Box* obj, Box* val, Box* descr, SetattrRewrit
 void setattrInternal(Box* obj, const std::string& attr, Box* val, SetattrRewriteArgs* rewrite_args) {
     assert(gc::isValidGCObject(val));
 
+    if (obj->cls == type_cls) {
+        BoxedClass* cobj = static_cast<BoxedClass*>(obj);
+        if (!isUserDefined(cobj)) {
+            raiseExcHelper(TypeError, "can't set attributes of built-in/extension type '%s'", getNameOfClass(cobj));
+        }
+    }
+
+    if (attr == "__class__") {
+        if (!isSubclass(val->cls, type_cls))
+            raiseExcHelper(TypeError, "__class__ must be set to new-style class, not '%s' object", val->cls->tp_name);
+
+        auto new_cls = static_cast<BoxedClass*>(val);
+
+        // Conservative Pyston checks: make sure that both classes are derived only from Pyston types,
+        // and that they don't define any extra C-level fields
+        RELEASE_ASSERT(val->cls == type_cls, "");
+        RELEASE_ASSERT(obj->cls->cls == type_cls, "");
+        for (auto _base : static_cast<BoxedTuple*>(obj->cls->tp_mro)->elts) {
+            BoxedClass* base = static_cast<BoxedClass*>(_base);
+            RELEASE_ASSERT(base->is_pyston_class, "");
+        }
+        for (auto _base : static_cast<BoxedTuple*>(new_cls->tp_mro)->elts) {
+            BoxedClass* base = static_cast<BoxedClass*>(_base);
+            RELEASE_ASSERT(base->is_pyston_class, "");
+        }
+
+        RELEASE_ASSERT(obj->cls->tp_basicsize == object_cls->tp_basicsize + sizeof(HCAttrs) + sizeof(Box**), "");
+        RELEASE_ASSERT(new_cls->tp_basicsize == object_cls->tp_basicsize + sizeof(HCAttrs) + sizeof(Box**), "");
+        RELEASE_ASSERT(obj->cls->attrs_offset != 0, "");
+        RELEASE_ASSERT(new_cls->attrs_offset != 0, "");
+        RELEASE_ASSERT(obj->cls->tp_weaklistoffset != 0, "");
+        RELEASE_ASSERT(new_cls->tp_weaklistoffset != 0, "");
+
+        // Normal Python checks.
+        // TODO there are more checks to add here, and they should throw errors not asserts
+        RELEASE_ASSERT(obj->cls->tp_basicsize == new_cls->tp_basicsize, "");
+        RELEASE_ASSERT(obj->cls->tp_dictoffset == new_cls->tp_dictoffset, "");
+        RELEASE_ASSERT(obj->cls->tp_weaklistoffset == new_cls->tp_weaklistoffset, "");
+        RELEASE_ASSERT(obj->cls->attrs_offset == new_cls->attrs_offset, "");
+
+        obj->cls = new_cls;
+        return;
+    }
+
+
     // Lookup a descriptor
     Box* descr = NULL;
     RewriterVar* r_descr = NULL;
@@ -1744,18 +1789,9 @@ void setattrInternal(Box* obj, const std::string& attr, Box* val, SetattrRewrite
 }
 
 extern "C" void setattr(Box* obj, const char* attr, Box* attr_val) {
-    assert(strcmp(attr, "__class__") != 0);
-
     static StatCounter slowpath_setattr("slowpath_setattr");
     slowpath_setattr.log();
 
-    if (obj->cls == type_cls) {
-        BoxedClass* cobj = static_cast<BoxedClass*>(obj);
-        if (!isUserDefined(cobj)) {
-            raiseExcHelper(TypeError, "can't set attributes of built-in/extension type '%s'", getNameOfClass(cobj));
-        }
-    }
-
     std::unique_ptr<Rewriter> rewriter(
         Rewriter::createRewriter(__builtin_extract_return_addr(__builtin_return_address(0)), 3, "setattr"));
 

test/tests/class_changing.py

-# expected: fail
-# - class changing not supported yet
-
 # Tests to make sure that setting __class__ changes the class, and that it's ok to disallow
 # having anything other than a type as the class
 class C(object):
@@ -17,5 +14,7 @@ c.foo()
 c.__class__ = D
 c.foo()
 
-# This should err:
-c.__class__ = 1
+try:
+    c.__class__ = 1
+except TypeError as e:
+    print e