diff --git a/third_party/python/Include/abstract.h b/third_party/python/Include/abstract.h
index 0f69acb95..b60b01d26 100644
--- a/third_party/python/Include/abstract.h
+++ b/third_party/python/Include/abstract.h
@@ -14,6 +14,9 @@ PyObject *PyObject_Call(PyObject *callable_object, PyObject *args,
 
 #ifndef Py_LIMITED_API
 PyObject *_PyStack_AsTuple(PyObject **stack, Py_ssize_t nargs);
+PyObject *_PyStack_AsTupleSlice(PyObject **stack, Py_ssize_t nargs,
+                                Py_ssize_t start, Py_ssize_t end);
+
 PyObject *_PyStack_AsDict(PyObject **values, PyObject *kwnames);
 
 PyObject ** _PyStack_UnpackDict(
diff --git a/third_party/python/Objects/abstract.c b/third_party/python/Objects/abstract.c
index 35beea0d5..151c695d2 100644
--- a/third_party/python/Objects/abstract.c
+++ b/third_party/python/Objects/abstract.c
@@ -2300,6 +2300,30 @@ _PyStack_AsTuple(PyObject **stack, Py_ssize_t nargs)
     return args;
 }
 
+PyObject*
+_PyStack_AsTupleSlice(PyObject **stack, Py_ssize_t nargs,
+                      Py_ssize_t start, Py_ssize_t end)
+{
+    PyObject *args;
+    Py_ssize_t i;
+
+    assert(0 <= start);
+    assert(end <= nargs);
+    assert(start <= end);
+
+    args = PyTuple_New(end - start);
+    if (args == NULL) {
+        return NULL;
+    }
+
+    for (i=start; i < end; i++) {
+        PyObject *item = stack[i];
+        Py_INCREF(item);
+        PyTuple_SET_ITEM(args, i - start, item);
+    }
+    return args;
+}
+
 PyObject *
 _PyObject_FastCallDict(PyObject *callable, PyObject **args, Py_ssize_t nargs,
                        PyObject *kwargs)
diff --git a/third_party/python/Python/bltinmodule.c b/third_party/python/Python/bltinmodule.c
index a8df11b34..1500d4677 100644
--- a/third_party/python/Python/bltinmodule.c
+++ b/third_party/python/Python/bltinmodule.c
@@ -239,51 +239,45 @@ _Py_IDENTIFIER(stderr);
 
 /* AC: cannot convert yet, waiting for *args support */
 static PyObject *
-builtin___build_class__(PyObject *self, PyObject *args, PyObject *kwds)
+builtin___build_class__(PyObject *self, PyObject **args, Py_ssize_t nargs,
+                        PyObject *kwnames)
 {
     PyObject *func, *name, *bases, *mkw, *meta, *winner, *prep, *ns;
     PyObject *cls = NULL, *cell = NULL;
-    Py_ssize_t nargs;
     int isclass = 0;   /* initialize to prevent gcc warning */
 
-    assert(args != NULL);
-    if (!PyTuple_Check(args)) {
-        PyErr_SetString(PyExc_TypeError,
-                        "__build_class__: args is not a tuple");
-        return NULL;
-    }
-    nargs = PyTuple_GET_SIZE(args);
     if (nargs < 2) {
         PyErr_SetString(PyExc_TypeError,
                         "__build_class__: not enough arguments");
         return NULL;
     }
-    func = PyTuple_GET_ITEM(args, 0); /* Better be callable */
+    func = args[0];   /* Better be callable */
     if (!PyFunction_Check(func)) {
         PyErr_SetString(PyExc_TypeError,
                         "__build_class__: func must be a function");
         return NULL;
     }
-    name = PyTuple_GET_ITEM(args, 1);
+    name = args[1];
     if (!PyUnicode_Check(name)) {
         PyErr_SetString(PyExc_TypeError,
                         "__build_class__: name is not a string");
         return NULL;
     }
-    bases = PyTuple_GetSlice(args, 2, nargs);
+    bases = _PyStack_AsTupleSlice(args, nargs, 2, nargs);
     if (bases == NULL)
         return NULL;
 
-    if (kwds == NULL) {
+    if (kwnames == NULL) {
         meta = NULL;
         mkw = NULL;
     }
     else {
-        mkw = PyDict_Copy(kwds); /* Don't modify kwds passed in! */
+        mkw = _PyStack_AsDict(args + nargs, kwnames);
         if (mkw == NULL) {
             Py_DECREF(bases);
             return NULL;
         }
+
         meta = _PyDict_GetItemId(mkw, &PyId_metaclass);
         if (meta != NULL) {
             Py_INCREF(meta);
@@ -354,13 +348,6 @@ builtin___build_class__(PyObject *self, PyObject *args, PyObject *kwds)
         Py_DECREF(bases);
         return NULL;
     }
-    if (!PyMapping_Check(ns)) {
-        PyErr_Format(PyExc_TypeError,
-                     "%.200s.__prepare__() must return a mapping, not %.200s",
-                     isclass ? ((PyTypeObject *)meta)->tp_name : "<metaclass>",
-                     Py_TYPE(ns)->tp_name);
-        goto error;
-    }
     cell = PyEval_EvalCodeEx(PyFunction_GET_CODE(func), PyFunction_GET_GLOBALS(func), ns,
                              NULL, 0, NULL, 0, NULL, 0, NULL,
                              PyFunction_GET_CLOSURE(func));
@@ -2825,7 +2812,7 @@ PyTypeObject PyZip_Type = {
 
 static PyMethodDef builtin_methods[] = {
     {"__build_class__", (PyCFunction)builtin___build_class__,
-     METH_VARARGS | METH_KEYWORDS, build_class_doc},
+     METH_FASTCALL, build_class_doc},
     {"__import__",      (PyCFunction)builtin___import__, METH_VARARGS | METH_KEYWORDS, import_doc},
     BUILTIN_ABS_METHODDEF
     BUILTIN_ALL_METHODDEF