diff --git a/nac3artiq/src/lib.rs b/nac3artiq/src/lib.rs
index ffc0058b2..343c86e54 100644
--- a/nac3artiq/src/lib.rs
+++ b/nac3artiq/src/lib.rs
@@ -10,6 +10,7 @@ use inkwell::{
     targets::*,
     OptimizationLevel,
 };
+use nac3core::toplevel::builtins::get_exn_constructor;
 use nac3core::typecheck::typedef::{TypeEnum, Unifier};
 use nac3parser::{
     ast::{self, ExprKind, Stmt, StmtKind, StrRef},
@@ -82,8 +83,6 @@ struct Nac3 {
     time_fns: &'static (dyn TimeFns + Sync),
     primitive: PrimitiveStore,
     builtins: Vec<(StrRef, FunSignature, Arc<GenCall>)>,
-    builtins_ty: HashMap<StrRef, Type>,
-    builtins_def: HashMap<StrRef, DefinitionId>,
     pyid_to_def: Arc<RwLock<HashMap<u64, DefinitionId>>>,
     primitive_ids: PrimitivePythonId,
     working_directory: TempDir,
@@ -260,6 +259,34 @@ impl Nac3 {
     }
 }
 
+fn add_exceptions(
+    composer: &mut TopLevelComposer,
+    builtin_def: &mut HashMap<StrRef, DefinitionId>,
+    builtin_ty: &mut HashMap<StrRef, Type>,
+    error_names: &[&str]
+) -> Vec<Type> {
+    let mut types = Vec::new();
+    // note: this is only for builtin exceptions, i.e. the exception name is "0:{exn}"
+    for name in error_names {
+        let def_id = composer.definition_ast_list.len();
+        let (exception_fn, exception_class, exception_cons, exception_type) = get_exn_constructor(
+            name,
+            // class id
+            def_id,
+            // constructor id
+            def_id + 1,
+            &mut composer.unifier,
+            &composer.primitives_ty
+        );
+        composer.definition_ast_list.push((Arc::new(RwLock::new(exception_class)), None));
+        composer.definition_ast_list.push((Arc::new(RwLock::new(exception_fn)), None));
+        builtin_ty.insert((*name).into(), exception_cons);
+        builtin_def.insert((*name).into(), DefinitionId(def_id));
+        types.push(exception_type);
+    }
+    types
+}
+
 #[pymethods]
 impl Nac3 {
     #[new]
@@ -321,68 +348,42 @@ impl Nac3 {
                 }))),
             ),
         ];
-        let (_, builtins_def, builtins_ty) = TopLevelComposer::new(
-            builtins.clone(),
-            ComposerConfig { kernel_ann: Some("Kernel"), kernel_invariant_ann: "KernelInvariant" },
-        );
 
         let builtins_mod = PyModule::import(py, "builtins").unwrap();
         let id_fn = builtins_mod.getattr("id").unwrap();
         let numpy_mod = PyModule::import(py, "numpy").unwrap();
         let typing_mod = PyModule::import(py, "typing").unwrap();
         let types_mod = PyModule::import(py, "types").unwrap();
+
+        let get_id = |x| id_fn.call1((x,)).unwrap().extract().unwrap();
+        let get_attr_id = |obj: &PyModule, attr| id_fn.call1((obj.getattr(attr).unwrap(),))
+            .unwrap().extract().unwrap();
         let primitive_ids = PrimitivePythonId {
-            virtual_id: id_fn
-                .call1((builtins_mod
-                    .getattr("globals")
-                    .unwrap()
-                    .call0()
-                    .unwrap()
-                    .get_item("virtual")
-                    .unwrap(),))
-                .unwrap()
-                .extract()
-                .unwrap(),
+            virtual_id: get_id(
+                            builtins_mod
+                            .getattr("globals")
+                            .unwrap()
+                            .call0()
+                            .unwrap()
+                            .get_item("virtual")
+                            .unwrap(
+                        )),
             generic_alias: (
-                id_fn
-                    .call1((typing_mod.getattr("_GenericAlias").unwrap(),))
-                    .unwrap()
-                    .extract()
-                    .unwrap(),
-                id_fn
-                    .call1((types_mod.getattr("GenericAlias").unwrap(),))
-                    .unwrap()
-                    .extract()
-                    .unwrap(),
+                get_attr_id(typing_mod, "_GenericAlias"),
+                get_attr_id(types_mod, "GenericAlias"),
             ),
-            none: id_fn.call1((builtins_mod.getattr("None").unwrap(),)).unwrap().extract().unwrap(),
-            typevar: id_fn
-                .call1((typing_mod.getattr("TypeVar").unwrap(),))
-                .unwrap()
-                .extract()
-                .unwrap(),
-            int: id_fn.call1((builtins_mod.getattr("int").unwrap(),)).unwrap().extract().unwrap(),
-            int32: id_fn.call1((numpy_mod.getattr("int32").unwrap(),)).unwrap().extract().unwrap(),
-            int64: id_fn.call1((numpy_mod.getattr("int64").unwrap(),)).unwrap().extract().unwrap(),
-            uint32: id_fn.call1((numpy_mod.getattr("uint32").unwrap(),)).unwrap().extract().unwrap(),
-            uint64: id_fn.call1((numpy_mod.getattr("uint64").unwrap(),)).unwrap().extract().unwrap(),
-            bool: id_fn.call1((builtins_mod.getattr("bool").unwrap(),)).unwrap().extract().unwrap(),
-            float: id_fn
-                .call1((builtins_mod.getattr("float").unwrap(),))
-                .unwrap()
-                .extract()
-                .unwrap(),
-            list: id_fn.call1((builtins_mod.getattr("list").unwrap(),)).unwrap().extract().unwrap(),
-            tuple: id_fn
-                .call1((builtins_mod.getattr("tuple").unwrap(),))
-                .unwrap()
-                .extract()
-                .unwrap(),
-            exception: id_fn
-                .call1((builtins_mod.getattr("Exception").unwrap(),))
-                .unwrap()
-                .extract()
-                .unwrap(),
+            none: get_attr_id(builtins_mod, "None"),
+            typevar: get_attr_id(typing_mod, "TypeVar"),
+            int: get_attr_id(builtins_mod, "int"),
+            int32: get_attr_id(numpy_mod, "int32"),
+            int64: get_attr_id(numpy_mod, "int64"),
+            uint32: get_attr_id(numpy_mod, "uint32"),
+            uint64: get_attr_id(numpy_mod, "uint64"),
+            bool: get_attr_id(builtins_mod, "bool"),
+            float: get_attr_id(builtins_mod, "float"),
+            list: get_attr_id(builtins_mod, "list"),
+            tuple: get_attr_id(builtins_mod, "tuple"),
+            exception: get_attr_id(builtins_mod, "Exception"),
         };
 
         let working_directory = tempfile::Builder::new().prefix("nac3-").tempdir().unwrap();
@@ -393,8 +394,6 @@ impl Nac3 {
             time_fns,
             primitive,
             builtins,
-            builtins_ty,
-            builtins_def,
             primitive_ids,
             top_levels: Default::default(),
             pyid_to_def: Default::default(),
@@ -440,7 +439,7 @@ impl Nac3 {
         embedding_map: &PyAny,
         py: Python,
     ) -> PyResult<()> {
-        let (mut composer, _, _) = TopLevelComposer::new(
+        let (mut composer, mut builtins_def, mut builtins_ty) = TopLevelComposer::new(
             self.builtins.clone(),
             ComposerConfig { kernel_ann: Some("Kernel"), kernel_invariant_ann: "KernelInvariant" },
         );
@@ -462,9 +461,16 @@ impl Nac3 {
             store_obj: store_obj.clone(),
             store_str,
         };
-        let mut module_to_resolver_cache: HashMap<u64, _> = HashMap::new();
 
         let pyid_to_type = Arc::new(RwLock::new(HashMap::<u64, Type>::new()));
+        let exception_names = [
+            "ValueError",
+            "RuntimeError"
+        ];
+        add_exceptions(&mut composer, &mut builtins_def, &mut builtins_ty, &exception_names);
+
+        let mut module_to_resolver_cache: HashMap<u64, _> = HashMap::new();
+
         let global_value_ids = Arc::new(RwLock::new(HashSet::<u64>::new()));
         let mut rpc_ids = vec![];
         for (stmt, path, module) in self.top_levels.iter() {
@@ -494,8 +500,8 @@ impl Nac3 {
                         name_to_pyid.insert(key.into(), val);
                     }
                     let resolver = Arc::new(Resolver(Arc::new(InnerResolver {
-                        id_to_type: self.builtins_ty.clone().into(),
-                        id_to_def: self.builtins_def.clone().into(),
+                        id_to_type: builtins_ty.clone().into(),
+                        id_to_def: builtins_def.clone().into(),
                         pyid_to_def: self.pyid_to_def.clone(),
                         pyid_to_type: pyid_to_type.clone(),
                         primitive_ids: self.primitive_ids.clone(),
@@ -579,8 +585,8 @@ impl Nac3 {
         let mut synthesized =
             parse_program(&synthesized, "__nac3_synthesized_modinit__".to_string().into()).unwrap();
         let resolver = Arc::new(Resolver(Arc::new(InnerResolver {
-            id_to_type: self.builtins_ty.clone().into(),
-            id_to_def: self.builtins_def.clone().into(),
+            id_to_type: builtins_ty.clone().into(),
+            id_to_def: builtins_def.clone().into(),
             pyid_to_def: self.pyid_to_def.clone(),
             pyid_to_type: pyid_to_type.clone(),
             primitive_ids: self.primitive_ids.clone(),
diff --git a/nac3core/src/toplevel/builtins.rs b/nac3core/src/toplevel/builtins.rs
index d045545f9..64a52ea02 100644
--- a/nac3core/src/toplevel/builtins.rs
+++ b/nac3core/src/toplevel/builtins.rs
@@ -9,7 +9,92 @@ use inkwell::{types::BasicType, FloatPredicate, IntPredicate};
 
 type BuiltinInfo = (Vec<(Arc<RwLock<TopLevelDef>>, Option<Stmt>)>, &'static [&'static str]);
 
+pub fn get_exn_constructor(
+    name: &str,
+    class_id: usize,
+    cons_id: usize,
+    unifier: &mut Unifier,
+    primitives: &PrimitiveStore
+)-> (TopLevelDef, TopLevelDef, Type, Type) {
+    let int32 = primitives.int32;
+    let int64 = primitives.int64;
+    let string = primitives.str;
+    let exception_fields = vec![
+        ("__name__".into(), int32, true),
+        ("__file__".into(), string, true),
+        ("__line__".into(), int32, true),
+        ("__col__".into(), int32, true),
+        ("__func__".into(), string, true),
+        ("__message__".into(), string, true),
+        ("__param0__".into(), int64, true),
+        ("__param1__".into(), int64, true),
+        ("__param2__".into(), int64, true),
+    ];
+    let exn_cons_args = vec![
+        FuncArg {
+            name: "msg".into(),
+            ty: string,
+            default_value: Some(SymbolValue::Str("".into())),
+        },
+        FuncArg { name: "param0".into(), ty: int64, default_value: Some(SymbolValue::I64(0)) },
+        FuncArg { name: "param1".into(), ty: int64, default_value: Some(SymbolValue::I64(0)) },
+        FuncArg { name: "param2".into(), ty: int64, default_value: Some(SymbolValue::I64(0)) },
+    ];
+    let exn_type = unifier.add_ty(TypeEnum::TObj {
+        obj_id: DefinitionId(class_id),
+        fields: exception_fields.iter().map(|(a, b, c)| (*a, (*b, *c))).collect(),
+        params: Default::default(),
+    });
+    let signature = unifier.add_ty(TypeEnum::TFunc(FunSignature {
+        args: exn_cons_args,
+        ret: exn_type,
+        vars: Default::default(),
+    }));
+    let fun_def = TopLevelDef::Function {
+        name: format!("{}.__init__", name),
+        simple_name: "__init__".into(),
+        signature,
+        var_id: Default::default(),
+        instance_to_symbol: Default::default(),
+        instance_to_stmt: Default::default(),
+        resolver: None,
+        codegen_callback: Some(Arc::new(GenCall::new(Box::new(exn_constructor)))),
+        loc: None,
+    };
+    let class_def = TopLevelDef::Class {
+        name: name.into(),
+        object_id: DefinitionId(class_id),
+        type_vars: Default::default(),
+        fields: exception_fields,
+        methods: vec![("__init__".into(), signature, DefinitionId(cons_id))],
+        ancestors: vec![
+            TypeAnnotation::CustomClass { id: DefinitionId(class_id), params: Default::default() },
+            TypeAnnotation::CustomClass { id: DefinitionId(7), params: Default::default() },
+        ],
+        constructor: Some(signature),
+        resolver: None,
+        loc: None,
+    };
+    (fun_def, class_def, signature, exn_type)
+}
+
 pub fn get_builtins(primitives: &mut (PrimitiveStore, Unifier)) -> BuiltinInfo {
+    // please refer to the top_level_def_list below for the definition IDs
+    let (div_by_zero_fun, div_by_zero_class, _, _) = get_exn_constructor(
+        "DivisionByZeroError",
+        12,
+        10,
+        &mut primitives.1,
+        &primitives.0,
+    );
+    let (index_err_fun, index_err_class, _, _) = get_exn_constructor(
+        "IndexError",
+        13,
+        11,
+        &mut primitives.1,
+        &primitives.0,
+    );
+
     let int32 = primitives.0.int32;
     let int64 = primitives.0.int64;
     let uint32 = primitives.0.uint32;
@@ -24,7 +109,6 @@ pub fn get_builtins(primitives: &mut (PrimitiveStore, Unifier)) -> BuiltinInfo {
         None,
     );
     let var_map: HashMap<_, _> = vec![(num_ty.1, num_ty.0)].into_iter().collect();
-
     let exception_fields = vec![
         ("__name__".into(), int32, true),
         ("__file__".into(), string, true),
@@ -36,36 +120,7 @@ pub fn get_builtins(primitives: &mut (PrimitiveStore, Unifier)) -> BuiltinInfo {
         ("__param1__".into(), int64, true),
         ("__param2__".into(), int64, true),
     ];
-    let div_by_zero = primitives.1.add_ty(TypeEnum::TObj {
-        obj_id: DefinitionId(12),
-        fields: exception_fields.iter().map(|(a, b, c)| (*a, (*b, *c))).collect(),
-        params: Default::default(),
-    });
-    let index_error = primitives.1.add_ty(TypeEnum::TObj {
-        obj_id: DefinitionId(13),
-        fields: exception_fields.iter().map(|(a, b, c)| (*a, (*b, *c))).collect(),
-        params: Default::default(),
-    });
-    let exn_cons_args = vec![
-        FuncArg {
-            name: "msg".into(),
-            ty: string,
-            default_value: Some(SymbolValue::Str("".into())),
-        },
-        FuncArg { name: "param0".into(), ty: int64, default_value: Some(SymbolValue::I64(0)) },
-        FuncArg { name: "param1".into(), ty: int64, default_value: Some(SymbolValue::I64(0)) },
-        FuncArg { name: "param2".into(), ty: int64, default_value: Some(SymbolValue::I64(0)) },
-    ];
-    let div_by_zero_signature = primitives.1.add_ty(TypeEnum::TFunc(FunSignature {
-        args: exn_cons_args.clone(),
-        ret: div_by_zero,
-        vars: Default::default(),
-    }));
-    let index_error_signature = primitives.1.add_ty(TypeEnum::TFunc(FunSignature {
-        args: exn_cons_args,
-        ret: index_error,
-        vars: Default::default(),
-    }));
+
     let top_level_def_list = vec![
         Arc::new(RwLock::new(TopLevelComposer::make_top_level_class_def(
             0,
@@ -120,7 +175,7 @@ pub fn get_builtins(primitives: &mut (PrimitiveStore, Unifier)) -> BuiltinInfo {
             name: "Exception".into(),
             object_id: DefinitionId(7),
             type_vars: Default::default(),
-            fields: exception_fields.clone(),
+            fields: exception_fields,
             methods: Default::default(),
             ancestors: vec![],
             constructor: None,
@@ -141,56 +196,10 @@ pub fn get_builtins(primitives: &mut (PrimitiveStore, Unifier)) -> BuiltinInfo {
             None,
             None,
         ))),
-        Arc::new(RwLock::new(TopLevelDef::Function {
-            name: "ZeroDivisionError.__init__".into(),
-            simple_name: "__init__".into(),
-            signature: div_by_zero_signature,
-            var_id: Default::default(),
-            instance_to_symbol: Default::default(),
-            instance_to_stmt: Default::default(),
-            resolver: None,
-            codegen_callback: Some(Arc::new(GenCall::new(Box::new(exn_constructor)))),
-            loc: None,
-        })),
-        Arc::new(RwLock::new(TopLevelDef::Function {
-            name: "IndexError.__init__".into(),
-            simple_name: "__init__".into(),
-            signature: index_error_signature,
-            var_id: Default::default(),
-            instance_to_symbol: Default::default(),
-            instance_to_stmt: Default::default(),
-            resolver: None,
-            codegen_callback: Some(Arc::new(GenCall::new(Box::new(exn_constructor)))),
-            loc: None,
-        })),
-        Arc::new(RwLock::new(TopLevelDef::Class {
-            name: "ZeroDivisionError".into(),
-            object_id: DefinitionId(12),
-            type_vars: Default::default(),
-            fields: exception_fields.clone(),
-            methods: vec![("__init__".into(), div_by_zero_signature, DefinitionId(8))],
-            ancestors: vec![
-                TypeAnnotation::CustomClass { id: DefinitionId(10), params: Default::default() },
-                TypeAnnotation::CustomClass { id: DefinitionId(7), params: Default::default() },
-            ],
-            constructor: Some(div_by_zero_signature),
-            resolver: None,
-            loc: None,
-        })),
-        Arc::new(RwLock::new(TopLevelDef::Class {
-            name: "IndexError".into(),
-            object_id: DefinitionId(13),
-            type_vars: Default::default(),
-            fields: exception_fields,
-            methods: vec![("__init__".into(), index_error_signature, DefinitionId(9))],
-            ancestors: vec![
-                TypeAnnotation::CustomClass { id: DefinitionId(11), params: Default::default() },
-                TypeAnnotation::CustomClass { id: DefinitionId(7), params: Default::default() },
-            ],
-            constructor: Some(index_error_signature),
-            resolver: None,
-            loc: None,
-        })),
+        Arc::new(RwLock::new(div_by_zero_fun)),
+        Arc::new(RwLock::new(index_err_fun)),
+        Arc::new(RwLock::new(div_by_zero_class)),
+        Arc::new(RwLock::new(index_err_class)),
         Arc::new(RwLock::new(TopLevelDef::Function {
             name: "int32".into(),
             simple_name: "int32".into(),
diff --git a/nac3core/src/toplevel/composer.rs b/nac3core/src/toplevel/composer.rs
index f65357233..f5897a2ed 100644
--- a/nac3core/src/toplevel/composer.rs
+++ b/nac3core/src/toplevel/composer.rs
@@ -461,6 +461,9 @@ impl TopLevelComposer {
         };
         let mut errors = HashSet::new();
         for (class_def, class_ast) in def_list.iter().skip(self.builtin_num) {
+            if class_ast.is_none() {
+                continue;
+            }
             if let Err(e) = analyze(class_def, class_ast) {
                 errors.insert(e);
             }
@@ -557,6 +560,9 @@ impl TopLevelComposer {
         // first, only push direct parent into the list
         let mut errors = HashSet::new();
         for (class_def, class_ast) in self.definition_ast_list.iter_mut().skip(self.builtin_num) {
+            if class_ast.is_none() {
+                continue;
+            }
             if let Err(e) = get_direct_parents(class_def, class_ast) {
                 errors.insert(e);
             }
@@ -587,7 +593,10 @@ impl TopLevelComposer {
             );
             Ok(())
         };
-        for (class_def, _) in self.definition_ast_list.iter().skip(self.builtin_num) {
+        for (class_def, ast) in self.definition_ast_list.iter().skip(self.builtin_num) {
+            if ast.is_none() {
+                continue;
+            }
             if let Err(e) = get_all_ancestors(class_def) {
                 errors.insert(e);
             }
@@ -598,6 +607,9 @@ impl TopLevelComposer {
 
         // insert the ancestors to the def list
         for (class_def, class_ast) in self.definition_ast_list.iter_mut().skip(self.builtin_num) {
+            if class_ast.is_none() {
+                continue;
+            }
             let mut class_def = class_def.write();
             let (class_ancestors, class_id, class_type_vars) = {
                 if let TopLevelDef::Class { ancestors, object_id, type_vars, .. } =
@@ -661,6 +673,9 @@ impl TopLevelComposer {
 
         let mut errors = HashSet::new();
         for (class_def, class_ast) in def_ast_list.iter().skip(self.builtin_num) {
+            if class_ast.is_none() {
+                continue;
+            }
             if matches!(&*class_def.read(), TopLevelDef::Class { .. }) {
                 if let Err(e) = Self::analyze_single_class_methods_fields(
                     class_def.clone(),
@@ -687,7 +702,10 @@ impl TopLevelComposer {
         loop {
             let mut finished = true;
 
-            for (class_def, _) in def_ast_list.iter().skip(self.builtin_num) {
+            for (class_def, class_ast) in def_ast_list.iter().skip(self.builtin_num) {
+                if class_ast.is_none() {
+                    continue;
+                }
                 let mut class_def = class_def.write();
                 if let TopLevelDef::Class { ancestors, .. } = class_def.deref() {
                     // if the length of the ancestor is equal to the current depth
@@ -950,6 +968,9 @@ impl TopLevelComposer {
             Ok(())
         };
         for (function_def, function_ast) in def_list.iter().skip(self.builtin_num) {
+            if function_ast.is_none() {
+                continue;
+            }
             if let Err(e) = analyze(function_def, function_ast) {
                 errors.insert(e);
             }
@@ -1553,6 +1574,9 @@ impl TopLevelComposer {
             Ok(())
         };
         for (i, (def, ast)) in definition_ast_list.iter().enumerate().skip(self.builtin_num) {
+            if ast.is_none() {
+                continue;
+            }
             if let Err(e) = analyze(i, def, ast) {
                 errors.insert(e);
             }
@@ -1849,6 +1873,9 @@ impl TopLevelComposer {
             Ok(())
         };
         for (id, (def, ast)) in self.definition_ast_list.iter().enumerate().skip(self.builtin_num) {
+            if ast.is_none() {
+                continue;
+            }
             if let Err(e) = analyze_2(id, def, ast) {
                 errors.insert(e);
             }