diff --git a/nac3core/src/codegen/concrete_type.rs b/nac3core/src/codegen/concrete_type.rs
index 140142ef..fd13afac 100644
--- a/nac3core/src/codegen/concrete_type.rs
+++ b/nac3core/src/codegen/concrete_type.rs
@@ -31,6 +31,9 @@ pub enum Primitive {
     Float,
     Bool,
     None,
+    Range,
+    Str,
+    Exception
 }
 
 #[derive(Debug)]
@@ -66,6 +69,9 @@ impl ConcreteTypeStore {
                 ConcreteTypeEnum::TPrimitive(Primitive::Float),
                 ConcreteTypeEnum::TPrimitive(Primitive::Bool),
                 ConcreteTypeEnum::TPrimitive(Primitive::None),
+                ConcreteTypeEnum::TPrimitive(Primitive::Range),
+                ConcreteTypeEnum::TPrimitive(Primitive::Str),
+                ConcreteTypeEnum::TPrimitive(Primitive::Exception),
             ],
         }
     }
@@ -118,6 +124,12 @@ impl ConcreteTypeStore {
             ConcreteType(3)
         } else if unifier.unioned(ty, primitives.none) {
             ConcreteType(4)
+        } else if unifier.unioned(ty, primitives.range) {
+            ConcreteType(5)
+        } else if unifier.unioned(ty, primitives.str) {
+            ConcreteType(6)
+        } else if unifier.unioned(ty, primitives.exception) {
+            ConcreteType(7)
         } else if let Some(cty) = cache.get(&ty) {
             if let Some(cty) = cty {
                 *cty
@@ -211,6 +223,9 @@ impl ConcreteTypeStore {
                     Primitive::Float => primitives.float,
                     Primitive::Bool => primitives.bool,
                     Primitive::None => primitives.none,
+                    Primitive::Range => primitives.range,
+                    Primitive::Str => primitives.str,
+                    Primitive::Exception => primitives.exception,
                 };
                 *cache.get_mut(&cty).unwrap() = Some(ty);
                 return ty;
diff --git a/nac3core/src/codegen/expr.rs b/nac3core/src/codegen/expr.rs
index c20be479..b1368ba2 100644
--- a/nac3core/src/codegen/expr.rs
+++ b/nac3core/src/codegen/expr.rs
@@ -76,14 +76,20 @@ impl<'ctx, 'a> CodeGenContext<'ctx, 'a> {
         index
     }
 
-    fn gen_symbol_val(&mut self, val: &SymbolValue) -> BasicValueEnum<'ctx> {
+    pub fn gen_symbol_val(&mut self, generator: &mut dyn CodeGenerator, val: &SymbolValue) -> BasicValueEnum<'ctx> {
         match val {
             SymbolValue::I32(v) => self.ctx.i32_type().const_int(*v as u64, true).into(),
             SymbolValue::I64(v) => self.ctx.i64_type().const_int(*v as u64, true).into(),
             SymbolValue::Bool(v) => self.ctx.bool_type().const_int(*v as u64, true).into(),
             SymbolValue::Double(v) => self.ctx.f64_type().const_float(*v).into(),
+            SymbolValue::Str(v) => {
+                let str_ptr = self.builder.build_global_string_ptr(v, "const").as_pointer_value().into();
+                let size = generator.get_size_type(self.ctx).const_int(v.len() as u64, false);
+                let ty = self.get_llvm_type(generator, self.primitives.str).into_struct_type();
+                ty.const_named_struct(&[str_ptr, size.into()]).into()
+            }
             SymbolValue::Tuple(ls) => {
-                let vals = ls.iter().map(|v| self.gen_symbol_val(v)).collect_vec();
+                let vals = ls.iter().map(|v| self.gen_symbol_val(generator, v)).collect_vec();
                 let fields = vals.iter().map(|v| v.get_type()).collect_vec();
                 let ty = self.ctx.struct_type(&fields, false);
                 let ptr = self.builder.build_alloca(ty, "tuple");
@@ -118,7 +124,7 @@ impl<'ctx, 'a> CodeGenContext<'ctx, 'a> {
         )
     }
 
-    fn gen_const(&mut self, value: &Constant, ty: Type) -> BasicValueEnum<'ctx> {
+    pub fn gen_const(&mut self, generator: &mut dyn CodeGenerator, value: &Constant, ty: Type) -> BasicValueEnum<'ctx> {
         match value {
             Constant::Bool(v) => {
                 assert!(self.unifier.unioned(ty, self.primitives.bool));
@@ -145,7 +151,7 @@ impl<'ctx, 'a> CodeGenContext<'ctx, 'a> {
                 let types =
                     if let TypeEnum::TTuple { ty } = &*ty { ty.clone() } else { unreachable!() };
                 let values = zip(types.into_iter(), v.iter())
-                    .map(|(ty, v)| self.gen_const(v, ty))
+                    .map(|(ty, v)| self.gen_const(generator, v, ty))
                     .collect_vec();
                 let types = values.iter().map(BasicValueEnum::get_type).collect_vec();
                 let ty = self.ctx.struct_type(&types, false);
@@ -153,7 +159,16 @@ impl<'ctx, 'a> CodeGenContext<'ctx, 'a> {
             }
             Constant::Str(v) => {
                 assert!(self.unifier.unioned(ty, self.primitives.str));
-                self.builder.build_global_string_ptr(v, "const").as_pointer_value().into()
+                if let Some(v) = self.const_strings.get(v) {
+                    *v
+                } else {
+                    let str_ptr = self.builder.build_global_string_ptr(v, "const").as_pointer_value().into();
+                    let size = generator.get_size_type(self.ctx).const_int(v.len() as u64, false);
+                    let ty = self.get_llvm_type(generator, self.primitives.str);
+                    let val = ty.into_struct_type().const_named_struct(&[str_ptr, size.into()]).into();
+                    self.const_strings.insert(v.to_string(), val);
+                    val
+                }
             }
             _ => unreachable!(),
         }
@@ -241,6 +256,14 @@ impl<'ctx, 'a> CodeGenContext<'ctx, 'a> {
             _ => unimplemented!(),
         }
     }
+    pub fn gen_string<G: CodeGenerator, S: Into<String>>(
+        &mut self,
+        generator: &mut G,
+        s: S
+    ) -> BasicValueEnum<'ctx> {
+        self.gen_const(generator, &nac3parser::ast::Constant::Str(s.into()), self.primitives.str)
+    }
+
 }
 
 pub fn gen_constructor<'ctx, 'a, G: CodeGenerator>(
diff --git a/nac3core/src/codegen/mod.rs b/nac3core/src/codegen/mod.rs
index ead7b0c2..1601a3e7 100644
--- a/nac3core/src/codegen/mod.rs
+++ b/nac3core/src/codegen/mod.rs
@@ -314,25 +314,50 @@ pub fn gen_func<'ctx, G: CodeGenerator>(
         none: unifier.get_representative(primitives.none),
         range: unifier.get_representative(primitives.range),
         str: unifier.get_representative(primitives.str),
+        exception: unifier.get_representative(primitives.exception),
     };
 
     let mut type_cache: HashMap<_, _> = [
-        (unifier.get_representative(primitives.int32), context.i32_type().into()),
-        (unifier.get_representative(primitives.int64), context.i64_type().into()),
-        (unifier.get_representative(primitives.float), context.f64_type().into()),
-        (unifier.get_representative(primitives.bool), context.bool_type().into()),
+        (primitives.int32, context.i32_type().into()),
+        (primitives.int64, context.i64_type().into()),
+        (primitives.float, context.f64_type().into()),
+        (primitives.bool, context.bool_type().into()),
+        (primitives.str, {
+            let str_type = context.opaque_struct_type("str");
+            let fields = [
+                context.i8_type().ptr_type(AddressSpace::Generic).into(),
+                generator.get_size_type(context).into(),
+            ];
+            str_type.set_body(&fields, false);
+            str_type.into()
+        }),
         (
-            unifier.get_representative(primitives.str),
-            context.i8_type().ptr_type(AddressSpace::Generic).into(),
-        ),
-        (
-            unifier.get_representative(primitives.range),
-            context.i32_type().array_type(3).ptr_type(AddressSpace::Generic).into()
+            primitives.range,
+            context.i32_type().array_type(3).ptr_type(AddressSpace::Generic).into(),
         ),
     ]
     .iter()
     .cloned()
     .collect();
+    type_cache.insert(primitives.exception, {
+        let exception = context.opaque_struct_type("Exception");
+        let int32 = context.i32_type().into();
+        let int64 = context.i64_type().into();
+        let str_ty = *type_cache.get(&primitives.str).unwrap();
+        let fields = [
+            int32,
+            str_ty,
+            int32,
+            int32,
+            str_ty,
+            str_ty,
+            int64,
+            int64,
+            int64
+        ];
+        exception.set_body(&fields, false);
+        exception.ptr_type(AddressSpace::Generic).into()
+    });
 
     let (args, ret) = if let ConcreteTypeEnum::TFunc { args, ret, .. } =
         task.store.get(task.signature)
diff --git a/nac3core/src/codegen/stmt.rs b/nac3core/src/codegen/stmt.rs
index d8559f28..f8f42d43 100644
--- a/nac3core/src/codegen/stmt.rs
+++ b/nac3core/src/codegen/stmt.rs
@@ -402,6 +402,71 @@ pub fn gen_if<'ctx, 'a, G: CodeGenerator>(
             }
         }
         then_exited && else_exited
+pub fn exn_constructor<'ctx, 'a>(
+    ctx: &mut CodeGenContext<'ctx, 'a>,
+    obj: Option<(Type, ValueEnum<'ctx>)>,
+    _fun: (&FunSignature, DefinitionId),
+    mut args: Vec<(Option<StrRef>, ValueEnum<'ctx>)>,
+    generator: &mut dyn CodeGenerator
+) -> Option<BasicValueEnum<'ctx>> {
+    let (zelf_ty, zelf) = obj.unwrap();
+    let zelf = zelf.to_basic_value_enum(ctx, generator).into_pointer_value();
+    let int32 = ctx.ctx.i32_type();
+    let zero = int32.const_zero();
+    let zelf_id = {
+        if let TypeEnum::TObj { obj_id, .. } = &*ctx.unifier.get_ty(zelf_ty) {
+            obj_id.0
+        } else {
+            unreachable!()
+        }
+    };
+    let defs = ctx.top_level.definitions.read();
+    let def = defs[zelf_id].read();
+    let zelf_name = if let TopLevelDef::Class { name, .. } = &*def {
+        *name
+    } else {
+        unreachable!()
+    };
+    let exception_name = format!("0:{}", zelf_name);
+    unsafe {
+        let id_ptr = ctx.builder.build_in_bounds_gep(zelf, &[zero, zero], "exn.id");
+        let id = ctx.resolver.get_string_id(&exception_name);
+        ctx.builder.build_store(id_ptr, int32.const_int(id as u64, false));
+        let empty_string = ctx.gen_const(generator, &Constant::Str("".into()), ctx.primitives.str);
+        let ptr = ctx.builder.build_in_bounds_gep(
+            zelf, &[zero, int32.const_int(5, false)], "exn.msg");
+        let msg = if !args.is_empty() {
+            args.remove(0).1.to_basic_value_enum(ctx, generator)
+        } else {
+            empty_string
+        };
+        ctx.builder.build_store(ptr, msg);
+        for i in [6, 7, 8].iter() {
+            let value = if !args.is_empty() {
+                args.remove(0).1.to_basic_value_enum(ctx, generator)
+            } else {
+                ctx.ctx.i64_type().const_zero().into()
+            };
+            let ptr = ctx.builder.build_in_bounds_gep(
+                zelf, &[zero, int32.const_int(*i, false)], "exn.param");
+            ctx.builder.build_store(ptr, value);
+        }
+        // set file, func to empty string
+        for i in [1, 4].iter() {
+            let ptr = ctx.builder.build_in_bounds_gep(
+                zelf, &[zero, int32.const_int(*i, false)], "exn.str");
+            ctx.builder.build_store(ptr, empty_string);
+        }
+        // set ints to zero
+        for i in [2, 3].iter() {
+            let ptr = ctx.builder.build_in_bounds_gep(
+                zelf, &[zero, int32.const_int(*i, false)], "exn.ints");
+            ctx.builder.build_store(ptr, zero);
+        }
+    }
+    Some(zelf.into())
+}
+
     } else {
         unreachable!()
     }
diff --git a/nac3core/src/symbol_resolver.rs b/nac3core/src/symbol_resolver.rs
index 88b00dd3..dcd3c378 100644
--- a/nac3core/src/symbol_resolver.rs
+++ b/nac3core/src/symbol_resolver.rs
@@ -2,14 +2,17 @@ use std::collections::HashMap;
 use std::fmt::Debug;
 use std::{cell::RefCell, sync::Arc};
 
-use crate::{codegen::CodeGenerator, typecheck::{
-    type_inferencer::PrimitiveStore,
-    typedef::{Type, Unifier},
-}};
 use crate::{
     codegen::CodeGenContext,
     toplevel::{DefinitionId, TopLevelDef},
 };
+use crate::{
+    codegen::CodeGenerator,
+    typecheck::{
+        type_inferencer::PrimitiveStore,
+        typedef::{Type, Unifier},
+    },
+};
 use crate::{location::Location, typecheck::typedef::TypeEnum};
 use inkwell::values::{BasicValueEnum, FloatValue, IntValue, PointerValue};
 use itertools::{chain, izip};
@@ -20,6 +23,7 @@ use parking_lot::RwLock;
 pub enum SymbolValue {
     I32(i32),
     I64(i64),
+    Str(String),
     Double(f64),
     Bool(bool),
     Tuple(Vec<SymbolValue>),
@@ -109,7 +113,7 @@ pub trait SymbolResolver {
 }
 
 thread_local! {
-    static IDENTIFIER_ID: [StrRef; 8] = [
+    static IDENTIFIER_ID: [StrRef; 10] = [
         "int32".into(),
         "int64".into(),
         "float".into(),
@@ -117,7 +121,9 @@ thread_local! {
         "None".into(),
         "virtual".into(),
         "list".into(),
-        "tuple".into()
+        "tuple".into(),
+        "str".into(),
+        "Exception".into(),
     ];
 }
 
@@ -139,6 +145,8 @@ pub fn parse_type_annotation<T>(
     let virtual_id = ids[5];
     let list_id = ids[6];
     let tuple_id = ids[7];
+    let str_id = ids[8];
+    let exn_id = ids[9];
 
     let name_handling = |id: &StrRef, unifier: &mut Unifier| {
         if *id == int32_id {
@@ -151,6 +159,10 @@ pub fn parse_type_annotation<T>(
             Ok(primitives.bool)
         } else if *id == none_id {
             Ok(primitives.none)
+        } else if *id == str_id {
+            Ok(primitives.str)
+        } else if *id == exn_id {
+            Ok(primitives.exception)
         } else {
             let obj_id = resolver.get_identifier_def(*id);
             if let Some(obj_id) = obj_id {
@@ -179,8 +191,7 @@ pub fn parse_type_annotation<T>(
                 }
             } else {
                 // it could be a type variable
-                let ty = resolver
-                    .get_symbol_type(unifier, top_level_defs, primitives, *id)?;
+                let ty = resolver.get_symbol_type(unifier, top_level_defs, primitives, *id)?;
                 if let TypeEnum::TVar { .. } = &*unifier.get_ty(ty) {
                     Ok(ty)
                 } else {
@@ -192,35 +203,17 @@ pub fn parse_type_annotation<T>(
 
     let subscript_name_handle = |id: &StrRef, slice: &Expr<T>, unifier: &mut Unifier| {
         if *id == virtual_id {
-            let ty = parse_type_annotation(
-                resolver,
-                top_level_defs,
-                unifier,
-                primitives,
-                slice,
-            )?;
+            let ty = parse_type_annotation(resolver, top_level_defs, unifier, primitives, slice)?;
             Ok(unifier.add_ty(TypeEnum::TVirtual { ty }))
         } else if *id == list_id {
-            let ty = parse_type_annotation(
-                resolver,
-                top_level_defs,
-                unifier,
-                primitives,
-                slice,
-            )?;
+            let ty = parse_type_annotation(resolver, top_level_defs, unifier, primitives, slice)?;
             Ok(unifier.add_ty(TypeEnum::TList { ty }))
         } else if *id == tuple_id {
             if let Tuple { elts, .. } = &slice.node {
                 let ty = elts
                     .iter()
                     .map(|elt| {
-                        parse_type_annotation(
-                            resolver,
-                            top_level_defs,
-                            unifier,
-                            primitives,
-                            elt,
-                        )
+                        parse_type_annotation(resolver, top_level_defs, unifier, primitives, elt)
                     })
                     .collect::<Result<Vec<_>, _>>()?;
                 Ok(unifier.add_ty(TypeEnum::TTuple { ty }))
@@ -231,23 +224,11 @@ pub fn parse_type_annotation<T>(
             let types = if let Tuple { elts, .. } = &slice.node {
                 elts.iter()
                     .map(|v| {
-                        parse_type_annotation(
-                            resolver,
-                            top_level_defs,
-                            unifier,
-                            primitives,
-                            v,
-                        )
+                        parse_type_annotation(resolver, top_level_defs, unifier, primitives, v)
                     })
                     .collect::<Result<Vec<_>, _>>()?
             } else {
-                vec![parse_type_annotation(
-                    resolver,
-                    top_level_defs,
-                    unifier,
-                    primitives,
-                    slice,
-                )?]
+                vec![parse_type_annotation(resolver, top_level_defs, unifier, primitives, slice)?]
             };
 
             let obj_id = resolver
diff --git a/nac3core/src/toplevel/builtins.rs b/nac3core/src/toplevel/builtins.rs
index 7a331fd4..e8c84d71 100644
--- a/nac3core/src/toplevel/builtins.rs
+++ b/nac3core/src/toplevel/builtins.rs
@@ -1,6 +1,6 @@
 use super::*;
 use crate::{
-    codegen::{expr::destructure_range, irrt::calculate_len_for_slice_range},
+    codegen::{expr::destructure_range, irrt::calculate_len_for_slice_range, stmt::exn_constructor},
     symbol_resolver::SymbolValue,
 };
 use inkwell::{FloatPredicate, IntPredicate};
@@ -21,6 +21,47 @@ pub fn get_builtins(primitives: &mut (PrimitiveStore, Unifier)) -> BuiltinInfo {
     let num_ty = primitives.1.get_fresh_var_with_range(&[int32, int64, float, boolean]);
     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),
+        ("__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 div_by_zero = primitives.1.add_ty(TypeEnum::TObj {
+        obj_id: DefinitionId(10),
+        fields: RefCell::new(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(11),
+        fields: RefCell::new(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(RefCell::new(FunSignature {
+        args: exn_cons_args.clone(),
+        ret: div_by_zero,
+        vars: Default::default()
+    })));
+    let index_error_signature = primitives.1.add_ty(TypeEnum::TFunc(RefCell::new(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,
@@ -49,6 +90,62 @@ pub fn get_builtins(primitives: &mut (PrimitiveStore, Unifier)) -> BuiltinInfo {
             None,
         ))),
         Arc::new(RwLock::new(TopLevelComposer::make_top_level_class_def(6, None, "str".into(), None))),
+        Arc::new(RwLock::new(TopLevelDef::Class {
+            name: "Exception".into(),
+            object_id: DefinitionId(7),
+            type_vars: Default::default(),
+            fields: exception_fields.clone(),
+            methods: Default::default(),
+            ancestors: vec![],
+            constructor: None,
+            resolver: 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))))
+        })),
+        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))))
+        })),
+        Arc::new(RwLock::new(TopLevelDef::Class {
+            name: "ZeroDivisionError".into(),
+            object_id: DefinitionId(10),
+            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,
+        })),
+        Arc::new(RwLock::new(TopLevelDef::Class {
+            name: "IndexError".into(),
+            object_id: DefinitionId(11),
+            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,
+        })),
         Arc::new(RwLock::new(TopLevelDef::Function {
             name: "int32".into(),
             simple_name: "int32".into(),
@@ -609,6 +706,8 @@ pub fn get_builtins(primitives: &mut (PrimitiveStore, Unifier)) -> BuiltinInfo {
     (
         izip!(top_level_def_list, ast_list).collect_vec(),
         &[
+            "ZeroDivisionError",
+            "IndexError",
             "int32",
             "int64",
             "float",
diff --git a/nac3core/src/toplevel/composer.rs b/nac3core/src/toplevel/composer.rs
index fb748fe2..762aaed0 100644
--- a/nac3core/src/toplevel/composer.rs
+++ b/nac3core/src/toplevel/composer.rs
@@ -4,7 +4,8 @@ use nac3parser::ast::fold::Fold;
 
 use crate::{
     typecheck::type_inferencer::{FunctionData, Inferencer},
-    codegen::expr::get_subst_key,
+    codegen::{expr::get_subst_key, stmt::exn_constructor},
+    symbol_resolver::SymbolValue,
 };
 
 use super::*;
@@ -90,8 +91,13 @@ impl TopLevelComposer {
                 assert!(name == *simple_name);
                 builtin_ty.insert(name, *signature);
                 builtin_id.insert(name, DefinitionId(id));
-            } else {
-                unreachable!()
+            } else if let TopLevelDef::Class { name, constructor, object_id, type_vars, .. } = &*def {
+                assert!(id == object_id.0);
+                assert!(type_vars.is_empty());
+                if let Some(constructor) = constructor {
+                    builtin_ty.insert(*name, *constructor);
+                }
+                builtin_id.insert(*name, DefinitionId(id));
             }
         }
 
@@ -471,7 +477,6 @@ impl TopLevelComposer {
         let unifier = self.unifier.borrow_mut();
 
         // first, only push direct parent into the list
-        // skip 5 to skip analyzing the primitives
         for (class_def, class_ast) in self.definition_ast_list.iter_mut().skip(self.builtin_num) {
             let mut class_def = class_def.write();
             let (class_def_id, class_bases, class_ancestors, class_resolver, class_type_vars) = {
@@ -540,7 +545,6 @@ impl TopLevelComposer {
 
         // second, get all ancestors
         let mut ancestors_store: HashMap<DefinitionId, Vec<TypeAnnotation>> = Default::default();
-        // skip 5 to skip analyzing the primitives
         for (class_def, _) in self.definition_ast_list.iter().skip(self.builtin_num) {
             let class_def = class_def.read();
             let (class_ancestors, class_id) = {
@@ -562,8 +566,7 @@ impl TopLevelComposer {
         }
 
         // insert the ancestors to the def list
-        // skip 5 to skip analyzing the primitives
-        for (class_def, _) in self.definition_ast_list.iter_mut().skip(self.builtin_num) {
+        for (class_def, class_ast) in self.definition_ast_list.iter_mut().skip(self.builtin_num) {
             let mut class_def = class_def.write();
             let (class_ancestors, class_id, class_type_vars) = {
                 if let TopLevelDef::Class { ancestors, object_id, type_vars, .. } =
@@ -581,6 +584,26 @@ impl TopLevelComposer {
             // insert self type annotation to the front of the vector to maintain the order
             class_ancestors
                 .insert(0, make_self_type_annotation(class_type_vars.as_slice(), class_id));
+
+            // special case classes that inherit from Exception
+            if class_ancestors.iter().any(|ann| matches!(ann, TypeAnnotation::CustomClass { id, .. } if id.0 == 7)) {
+                // if inherited from Exception, the body should be a pass
+                if let ast::StmtKind::ClassDef { body, .. } = &class_ast.as_ref().unwrap().node {
+                    if body.len() != 1 || !matches!(body[0].node, ast::StmtKind::Pass { .. }) {
+                        return Err("Classes inherited from exception should have `pass` as body".into());
+                    }
+                } else {
+                    unreachable!()
+                }
+            }
+        }
+
+        // deal with ancestor of Exception object
+        if let TopLevelDef::Class { name, ancestors, object_id, .. } = &mut *self.definition_ast_list[7].0.write() {
+            assert_eq!(*name, "Exception".into());
+            ancestors.push(make_self_type_annotation(&[], *object_id));
+        } else {
+            unreachable!();
         }
 
         Ok(())
@@ -595,7 +618,6 @@ impl TopLevelComposer {
 
         let mut type_var_to_concrete_def: HashMap<Type, TypeAnnotation> = HashMap::new();
 
-        // skip 5 to skip analyzing the primitives
         for (class_def, class_ast) in def_ast_list.iter().skip(self.builtin_num) {
             if matches!(&*class_def.read(), TopLevelDef::Class { .. }) {
                 Self::analyze_single_class_methods_fields(
@@ -610,8 +632,6 @@ impl TopLevelComposer {
             }
         }
 
-        // println!("type_var_to_concrete_def1: {:?}", type_var_to_concrete_def);
-
         // handle the inheritanced methods and fields
         let mut current_ancestor_depth: usize = 2;
         loop {
@@ -646,19 +666,8 @@ impl TopLevelComposer {
             }
         }
 
-        // println!("type_var_to_concrete_def3: {:?}\n", type_var_to_concrete_def);
-
         // unification of previously assigned typevar
         for (ty, def) in type_var_to_concrete_def {
-            // println!(
-            //     "{:?}_{} -> {:?}\n",
-            //     ty,
-            //     unifier.stringify(ty,
-            //         &mut |id| format!("class{}", id),
-            //         &mut |id| format!("tvar{}", id)
-            //     ),
-            //     def
-            // );
             let target_ty =
                 get_type_from_type_annotation_kinds(&temp_def_list, unifier, primitives, &def)?;
             unifier.unify(ty, target_ty)?;
@@ -946,7 +955,7 @@ impl TopLevelComposer {
                                 ))
                             }
                         }
-                        
+
                         if name == &"__init__".into() && !defined_paramter_name.contains(&zelf) {
                             return Err(format!("__init__ method must have a `self` parameter (at {})", b.location));
                         }
@@ -1301,10 +1310,14 @@ impl TopLevelComposer {
     fn analyze_function_instance(&mut self) -> Result<(), String> {
         // first get the class contructor type correct for the following type check in function body
         // also do class field instantiation check
-        for (def, ast) in self.definition_ast_list.iter().skip(self.builtin_num) {
+        let init_str_id = "__init__".into();
+        let mut definition_extension = Vec::new();
+        let mut constructors = Vec::new();
+        for (i, (def, ast)) in self.definition_ast_list.iter().enumerate().skip(self.builtin_num) {
             let class_def = def.read();
             if let TopLevelDef::Class {
                 constructor,
+                ancestors,
                 methods,
                 fields,
                 type_vars,
@@ -1314,13 +1327,54 @@ impl TopLevelComposer {
                 ..
             } = &*class_def
             {
+                let self_type = get_type_from_type_annotation_kinds(
+                    self.extract_def_list().as_slice(),
+                    &mut self.unifier,
+                    &self.primitives_ty,
+                    &make_self_type_annotation(type_vars, *object_id),
+                )?;
+                if ancestors.iter().any(|ann| matches!(ann, TypeAnnotation::CustomClass { id, .. } if id.0 == 7)) {
+                    // create constructor for these classes
+                    let string = self.primitives_ty.str;
+                    let int64 = self.primitives_ty.int64;
+                    let signature = self.unifier.add_ty(TypeEnum::TFunc(RefCell::new(FunSignature {
+                        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))},
+                        ],
+                        ret: self_type,
+                        vars: Default::default()
+                    })));
+                    let cons_fun = TopLevelDef::Function {
+                        name: format!("{}.{}", class_name, "__init__"),
+                        simple_name: init_str_id,
+                        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))))
+                    };
+                    constructors.push((i, signature, definition_extension.len()));
+                    definition_extension.push((Arc::new(RwLock::new(cons_fun)), None));
+                    self.unifier
+                        .unify(constructor.unwrap(), signature)
+                        .map_err(|old| format!("{} (at {})", old, ast.as_ref().unwrap().location))?;
+                    continue;
+                }
                 let mut init_id: Option<DefinitionId> = None;
                 // get the class contructor type correct
                 let (contor_args, contor_type_vars) = {
                     let mut constructor_args: Vec<FuncArg> = Vec::new();
                     let mut type_vars: HashMap<u32, Type> = HashMap::new();
                     for (name, func_sig, id) in methods {
-                        if name == &"__init__".into() {
+                        if *name == init_str_id {
                             init_id = Some(*id);
                             if let TypeEnum::TFunc(sig) = self.unifier.get_ty(*func_sig).as_ref() {
                                 let FunSignature { args, vars, .. } = &*sig.borrow();
@@ -1333,12 +1387,6 @@ impl TopLevelComposer {
                     }
                     (constructor_args, type_vars)
                 };
-                let self_type = get_type_from_type_annotation_kinds(
-                    self.extract_def_list().as_slice(),
-                    &mut self.unifier,
-                    &self.primitives_ty,
-                    &make_self_type_annotation(type_vars, *object_id),
-                )?;
                 let contor_type = self.unifier.add_ty(TypeEnum::TFunc(
                     FunSignature { args: contor_args, ret: self_type, vars: contor_type_vars }
                         .into(),
@@ -1352,7 +1400,7 @@ impl TopLevelComposer {
                     let init_ast =
                         self.definition_ast_list.get(init_id.0).unwrap().1.as_ref().unwrap();
                     if let ast::StmtKind::FunctionDef { name, body, .. } = &init_ast.node {
-                        if name != &"__init__".into() {
+                        if *name != init_str_id {
                             unreachable!("must be init function here")
                         }
                         let all_inited = Self::get_all_assigned_field(body.as_slice())?;
@@ -1370,11 +1418,23 @@ impl TopLevelComposer {
                 }
             }
         }
+        for (i, signature, id) in constructors.into_iter() {
+            if let TopLevelDef::Class { methods, .. } = &mut *self.definition_ast_list[i].0.write() {
+                methods.push((init_str_id, signature,
+                        DefinitionId(self.definition_ast_list.len() + id)));
+            } else {
+                unreachable!()
+            }
+        }
+        self.definition_ast_list.extend_from_slice(&definition_extension);
 
         let ctx = Arc::new(self.make_top_level_context());
         // type inference inside function body
         for (id, (def, ast)) in self.definition_ast_list.iter().enumerate().skip(self.builtin_num)
         {
+            if ast.is_none() {
+                continue;
+            }
             let mut function_def = def.write();
             if let TopLevelDef::Function {
                 instance_to_stmt,
diff --git a/nac3core/src/toplevel/helper.rs b/nac3core/src/toplevel/helper.rs
index dcc74dd1..6d33a339 100644
--- a/nac3core/src/toplevel/helper.rs
+++ b/nac3core/src/toplevel/helper.rs
@@ -90,7 +90,22 @@ impl TopLevelComposer {
             fields: HashMap::new().into(),
             params: HashMap::new().into(),
         });
-        let primitives = PrimitiveStore { int32, int64, float, bool, none, range, str };
+        let exception = unifier.add_ty(TypeEnum::TObj {
+            obj_id: DefinitionId(7),
+            fields: vec![
+                ("__name__".into(), (int32, true)),
+                ("__file__".into(), (int32, true)),
+                ("__line__".into(), (int32, true)),
+                ("__col__".into(), (int32, true)),
+                ("__func__".into(), (str, true)),
+                ("__message__".into(), (str, true)),
+                ("__param0__".into(), (int64, true)),
+                ("__param1__".into(), (int64, true)),
+                ("__param2__".into(), (int64, true)),
+            ].into_iter().collect::<HashMap<_, _>>().into(),
+            params: HashMap::new().into(),
+        });
+        let primitives = PrimitiveStore { int32, int64, float, bool, none, range, str, exception };
         crate::typecheck::magic_methods::set_primitives_magic_methods(&primitives, &mut unifier);
         (primitives, unifier)
     }
@@ -381,6 +396,13 @@ impl TopLevelComposer {
                     Some("int64".to_string())
                 }
             }
+            SymbolValue::Str(..) => {
+                if matches!(ty, TypeAnnotation::Primitive(t) if *t == primitive.str) {
+                    None
+                } else {
+                    Some("str".to_string())
+                }
+            }
             SymbolValue::Tuple(elts) => {
                 if let TypeAnnotation::Tuple(elts_ty) = ty {
                     for (e, t) in elts.iter().zip(elts_ty.iter()) {
diff --git a/nac3core/src/toplevel/type_annotation.rs b/nac3core/src/toplevel/type_annotation.rs
index 8fb6c7f9..72bc59c6 100644
--- a/nac3core/src/toplevel/type_annotation.rs
+++ b/nac3core/src/toplevel/type_annotation.rs
@@ -63,6 +63,8 @@ pub fn parse_ast_to_type_annotation_kinds<T>(
             Ok(TypeAnnotation::Primitive(primitives.none))
         } else if id == &"str".into() {
             Ok(TypeAnnotation::Primitive(primitives.str))
+        } else if id == &"Exception".into() {
+            Ok(TypeAnnotation::CustomClass { id: DefinitionId(7), params: Default::default() })
         } else if let Some(obj_id) = resolver.get_identifier_def(*id) {
             let type_vars = {
                 let def_read = top_level_defs[obj_id.0].try_read();
diff --git a/nac3core/src/typecheck/type_inferencer/mod.rs b/nac3core/src/typecheck/type_inferencer/mod.rs
index 9a303749..87bf8235 100644
--- a/nac3core/src/typecheck/type_inferencer/mod.rs
+++ b/nac3core/src/typecheck/type_inferencer/mod.rs
@@ -37,6 +37,7 @@ pub struct PrimitiveStore {
     pub none: Type,
     pub range: Type,
     pub str: Type,
+    pub exception: Type,
 }
 
 pub struct FunctionData {