diff --git a/src/obj.h b/src/obj.h
index eb6bdcc2..18752ac8 100644
--- a/src/obj.h
+++ b/src/obj.h
@@ -14,14 +14,26 @@ class VM;
 typedef PyObject* (*NativeFuncC)(VM*, ArgsView);
 typedef int (*LuaStyleFuncC)(VM*);
 
+union UserData{
+    void* _p;
+    void (*_fp)(void);
+    char _char;
+    int _int;
+    float _float;
+    bool _bool;
+};
+
 struct NativeFunc {
     NativeFuncC f;
     int argc;       // DONOT include self
     bool method;
 
     // this is designed for lua style C bindings
-    // access it via `CAST(NativeFunc&, args[-2])._lua_f`
+    // access it via `_CAST(NativeFunc&, args[-2])._lua_f`
+    // (-2) or (-1) depends on the calling convention
     LuaStyleFuncC _lua_f;
+
+    UserData userdata;
     
     NativeFunc(NativeFuncC f, int argc, bool method) : f(f), argc(argc), method(method), _lua_f(nullptr) {}
     PyObject* operator()(VM* vm, ArgsView args) const;
@@ -389,4 +401,14 @@ struct Py_<DummyModule> final: PyObject {
 };
 
 
+template<typename T>
+T lambda_get_fp(ArgsView args){
+    void (*f)();
+    if(args[-1] != PY_NULL) f = OBJ_GET(NativeFunc, args[-1]).userdata._fp;
+    else f = OBJ_GET(NativeFunc, args[-2]).userdata._fp;
+    return reinterpret_cast<T>(f);
+}
+
+
+
 }   // namespace pkpy
\ No newline at end of file
diff --git a/src/vm.h b/src/vm.h
index 65bbbbad..e06e1ead 100644
--- a/src/vm.h
+++ b/src/vm.h
@@ -339,11 +339,12 @@ public:
     }
 
 #define BIND_UNARY_SPECIAL(name)                                                        \
-    void bind##name(Type type, PyObject* (*f)(VM* vm, PyObject*)){                      \
+    void bind##name(Type type, PyObject* (*f)(VM*, PyObject*)){                         \
         _all_types[type].m##name = f;                                                   \
-        bind_method<0>(_t(type), #name, [](VM* vm, ArgsView args){                      \
-            return vm->_inst_type_info(args[0])->m##name(vm, args[0]);                  \
+        PyObject* nf = bind_method<0>(_t(type), #name, [](VM* vm, ArgsView args){       \
+            return lambda_get_fp<PyObject*(*)(VM*, PyObject*)>(args)(vm, args[0]);      \
         });                                                                             \
+        OBJ_GET(NativeFunc, nf).userdata._fp = reinterpret_cast<void(*)()>(f);          \
     }
 
     BIND_UNARY_SPECIAL(__repr__)
@@ -360,13 +361,14 @@ public:
 
 
 #define BIND_LOGICAL_SPECIAL(name)                                                      \
-    void bind##name(Type type, bool (*f)(VM* vm, PyObject* lhs, PyObject* rhs)){        \
+    void bind##name(Type type, bool (*f)(VM*, PyObject*, PyObject*)){                   \
         PyObject* obj = _t(type);                                                       \
         _all_types[type].m##name = f;                                                   \
-        bind_method<1>(obj, #name, [](VM* vm, ArgsView args){                           \
-            bool ok = vm->_inst_type_info(args[0])->m##name(vm, args[0], args[1]);      \
+        PyObject* nf = bind_method<1>(obj, #name, [](VM* vm, ArgsView args){            \
+            bool ok = lambda_get_fp<bool(*)(VM*, PyObject*, PyObject*)>(args)(vm, args[0], args[1]); \
             return ok ? vm->True : vm->False;                                           \
         });                                                                             \
+        OBJ_GET(NativeFunc, nf).userdata._fp = reinterpret_cast<void(*)()>(f);          \
     }
 
     BIND_LOGICAL_SPECIAL(__eq__)
@@ -381,12 +383,13 @@ public:
 
 
 #define BIND_BINARY_SPECIAL(name)                                                       \
-    void bind##name(Type type, PyObject* (*f)(VM* vm, PyObject* lhs, PyObject* rhs)){   \
+    void bind##name(Type type, PyObject* (*f)(VM*, PyObject*, PyObject*)){              \
         PyObject* obj = _t(type);                                                       \
         _all_types[type].m##name = f;                                                   \
-        bind_method<1>(obj, #name, [](VM* vm, ArgsView args){                           \
-            return vm->_inst_type_info(args[0])->m##name(vm, args[0], args[1]);         \
+        PyObject* nf = bind_method<1>(obj, #name, [](VM* vm, ArgsView args){                           \
+            return lambda_get_fp<PyObject*(*)(VM*, PyObject*, PyObject*)>(args)(vm, args[0], args[1]); \
         });                                                                             \
+        OBJ_GET(NativeFunc, nf).userdata._fp = reinterpret_cast<void(*)()>(f);          \
     }
 
     BIND_BINARY_SPECIAL(__add__)
@@ -406,30 +409,33 @@ public:
 
 #undef BIND_BINARY_SPECIAL
 
-    void bind__getitem__(Type type, PyObject* (*f)(VM* vm, PyObject* lhs, PyObject* rhs)){
+    void bind__getitem__(Type type, PyObject* (*f)(VM*, PyObject*, PyObject*)){
         PyObject* obj = _t(type);
         _all_types[type].m__getitem__ = f;
-        bind_method<1>(obj, "__getitem__", [](VM* vm, ArgsView args){
-            return vm->_inst_type_info(args[0])->m__getitem__(vm, args[0], args[1]);
+        PyObject* nf = bind_method<1>(obj, "__getitem__", [](VM* vm, ArgsView args){
+            return lambda_get_fp<PyObject*(*)(VM*, PyObject*, PyObject*)>(args)(vm, args[0], args[1]);
         });
+        OBJ_GET(NativeFunc, nf).userdata._fp = reinterpret_cast<void(*)()>(f);
     }
 
-    void bind__setitem__(Type type, void (*f)(VM* vm, PyObject* lhs, PyObject* rhs, PyObject* value)){
+    void bind__setitem__(Type type, void (*f)(VM*, PyObject*, PyObject*, PyObject*)){
         PyObject* obj = _t(type);
         _all_types[type].m__setitem__ = f;
-        bind_method<2>(obj, "__setitem__", [](VM* vm, ArgsView args){
-            vm->_inst_type_info(args[0])->m__setitem__(vm, args[0], args[1], args[2]);
+        PyObject* nf = bind_method<2>(obj, "__setitem__", [](VM* vm, ArgsView args){
+            lambda_get_fp<void(*)(VM* vm, PyObject*, PyObject*, PyObject*)>(args)(vm, args[0], args[1], args[2]);
             return vm->None;
         });
+        OBJ_GET(NativeFunc, nf).userdata._fp = reinterpret_cast<void(*)()>(f);
     }
 
-    void bind__delitem__(Type type, void (*f)(VM* vm, PyObject* lhs, PyObject* rhs)){
+    void bind__delitem__(Type type, void (*f)(VM*, PyObject*, PyObject*)){
         PyObject* obj = _t(type);
         _all_types[type].m__delitem__ = f;
-        bind_method<1>(obj, "__delitem__", [](VM* vm, ArgsView args){
-            vm->_inst_type_info(args[0])->m__delitem__(vm, args[0], args[1]);
+        PyObject* nf = bind_method<1>(obj, "__delitem__", [](VM* vm, ArgsView args){
+            lambda_get_fp<void(*)(VM*, PyObject*, PyObject*)>(args)(vm, args[0], args[1]);
             return vm->None;
         });
+        OBJ_GET(NativeFunc, nf).userdata._fp = reinterpret_cast<void(*)()>(f);
     }
 
     bool py_equals(PyObject* lhs, PyObject* rhs){
@@ -447,32 +453,32 @@ public:
     }
 
     template<int ARGC>
-    void bind_func(Str type, Str name, NativeFuncC fn) {
-        bind_func<ARGC>(_find_type_object(type), name, fn);
+    PyObject* bind_func(Str type, Str name, NativeFuncC fn) {
+        return bind_func<ARGC>(_find_type_object(type), name, fn);
     }
 
     template<int ARGC>
-    void bind_method(Str type, Str name, NativeFuncC fn) {
-        bind_method<ARGC>(_find_type_object(type), name, fn);
+    PyObject* bind_method(Str type, Str name, NativeFuncC fn) {
+        return bind_method<ARGC>(_find_type_object(type), name, fn);
     }
 
     template<int ARGC, typename __T>
-    void bind_constructor(__T&& type, NativeFuncC fn) {
+    PyObject* bind_constructor(__T&& type, NativeFuncC fn) {
         static_assert(ARGC==-1 || ARGC>=1);
-        bind_func<ARGC>(std::forward<__T>(type), "__new__", fn);
+        return bind_func<ARGC>(std::forward<__T>(type), "__new__", fn);
     }
 
     template<typename T, typename __T>
-    void bind_default_constructor(__T&& type) {
-        bind_constructor<-1>(std::forward<__T>(type), [](VM* vm, ArgsView args){
+    PyObject* bind_default_constructor(__T&& type) {
+        return bind_constructor<-1>(std::forward<__T>(type), [](VM* vm, ArgsView args){
             Type t = OBJ_GET(Type, args[0]);
             return vm->heap.gcnew<T>(t, T());
         });
     }
 
     template<int ARGC>
-    void bind_builtin_func(Str name, NativeFuncC fn) {
-        bind_func<ARGC>(builtins, name, fn);
+    PyObject* bind_builtin_func(Str name, NativeFuncC fn) {
+        return bind_func<ARGC>(builtins, name, fn);
     }
 
     int normalized_index(int index, int size){
@@ -581,9 +587,9 @@ public:
     PyObject* format(Str, PyObject*);
     void setattr(PyObject* obj, StrName name, PyObject* value);
     template<int ARGC>
-    void bind_method(PyObject*, Str, NativeFuncC);
+    PyObject* bind_method(PyObject*, Str, NativeFuncC);
     template<int ARGC>
-    void bind_func(PyObject*, Str, NativeFuncC);
+    PyObject* bind_func(PyObject*, Str, NativeFuncC);
     void _error(Exception);
     PyObject* _run_top_frame();
     void post_init();
@@ -594,6 +600,9 @@ inline PyObject* NativeFunc::operator()(VM* vm, ArgsView args) const{
     if(argc != -1 && args_size != argc) {
         vm->TypeError(fmt("expected ", argc, " arguments, but got ", args_size));
     }
+#if DEBUG_EXTRA_CHECK
+    if(f == nullptr) FATAL_ERROR();
+#endif
     return f(vm, args);
 }
 
@@ -1351,20 +1360,24 @@ inline void VM::setattr(PyObject* obj, StrName name, PyObject* value){
 }
 
 template<int ARGC>
-void VM::bind_method(PyObject* obj, Str name, NativeFuncC fn) {
+PyObject* VM::bind_method(PyObject* obj, Str name, NativeFuncC fn) {
     check_non_tagged_type(obj, tp_type);
     if(obj->attr().contains(name)){
         throw std::runtime_error(fmt("bind_method() failed: ", name.escape(), " already exists"));
     }
-    obj->attr().set(name, VAR(NativeFunc(fn, ARGC, true)));
+    PyObject* nf = VAR(NativeFunc(fn, ARGC, true));
+    obj->attr().set(name, nf);
+    return nf;
 }
 
 template<int ARGC>
-void VM::bind_func(PyObject* obj, Str name, NativeFuncC fn) {
+PyObject* VM::bind_func(PyObject* obj, Str name, NativeFuncC fn) {
     if(obj->attr().contains(name)){
         throw std::runtime_error(fmt("bind_func() failed: ", name.escape(), " already exists"));
     }
-    obj->attr().set(name, VAR(NativeFunc(fn, ARGC, false)));
+    PyObject* nf = VAR(NativeFunc(fn, ARGC, false));
+    obj->attr().set(name, nf);
+    return nf;
 }
 
 inline void VM::_error(Exception e){
@@ -1411,22 +1424,24 @@ PyObject* PyArrayGetItem(VM* vm, PyObject* obj, PyObject* index){
     return self[i];
 }
 
-inline void VM::bind__hash__(Type type, i64 (*f)(VM* vm, PyObject*)){
+inline void VM::bind__hash__(Type type, i64 (*f)(VM*, PyObject*)){
     PyObject* obj = _t(type);
     _all_types[type].m__hash__ = f;
-    bind_method<0>(obj, "__hash__", [](VM* vm, ArgsView args){
-        i64 ret = vm->_inst_type_info(args[0])->m__hash__(vm, args[0]);
+    PyObject* nf = bind_method<0>(obj, "__hash__", [](VM* vm, ArgsView args){
+        i64 ret = lambda_get_fp<i64(*)(VM*, PyObject*)>(args)(vm, args[0]);
         return VAR(ret);
     });
+    OBJ_GET(NativeFunc, nf).userdata._fp = reinterpret_cast<void(*)(void)>(f);
 }
 
-inline void VM::bind__len__(Type type, i64 (*f)(VM* vm, PyObject*)){
+inline void VM::bind__len__(Type type, i64 (*f)(VM*, PyObject*)){
     PyObject* obj = _t(type);
     _all_types[type].m__len__ = f;
-    bind_method<0>(obj, "__len__", [](VM* vm, ArgsView args){
-        i64 ret = vm->_inst_type_info(args[0])->m__len__(vm, args[0]);
+    PyObject* nf = bind_method<0>(obj, "__len__", [](VM* vm, ArgsView args){
+        i64 ret = lambda_get_fp<i64(*)(VM*, PyObject*)>(args)(vm, args[0]);
         return VAR(ret);
     });
+    OBJ_GET(NativeFunc, nf).userdata._fp = reinterpret_cast<void(*)(void)>(f);
 }