diff --git a/nac3core/src/toplevel/builtins.rs b/nac3core/src/toplevel/builtins.rs
index 5602617..295a400 100644
--- a/nac3core/src/toplevel/builtins.rs
+++ b/nac3core/src/toplevel/builtins.rs
@@ -87,6 +87,7 @@ pub fn get_exn_constructor(
         object_id: DefinitionId(class_id),
         type_vars: Vec::default(),
         fields: exception_fields,
+        attr: Vec::default(),
         methods: vec![("__init__".into(), signature, DefinitionId(cons_id))],
         ancestors: vec![
             TypeAnnotation::CustomClass { id: DefinitionId(class_id), params: Vec::default() },
@@ -444,6 +445,7 @@ pub fn get_builtins(unifier: &mut Unifier, primitives: &PrimitiveStore) -> Built
             object_id: PRIMITIVE_DEF_IDS.exception,
             type_vars: Vec::default(),
             fields: exception_fields,
+            attr: Vec::default(),
             methods: Vec::default(),
             ancestors: vec![],
             constructor: None,
@@ -470,6 +472,7 @@ pub fn get_builtins(unifier: &mut Unifier, primitives: &PrimitiveStore) -> Built
                 object_id: PRIMITIVE_DEF_IDS.option,
                 type_vars: vec![option_ty_var],
                 fields: vec![],
+                attr: Vec::default(),
                 methods: vec![
                     ("is_some".into(), is_some_ty.0, DefinitionId(PRIMITIVE_DEF_IDS.option.0 + 1)),
                     ("is_none".into(), is_some_ty.0, DefinitionId(PRIMITIVE_DEF_IDS.option.0 + 2)),
@@ -560,6 +563,7 @@ pub fn get_builtins(unifier: &mut Unifier, primitives: &PrimitiveStore) -> Built
             object_id: PRIMITIVE_DEF_IDS.ndarray,
             type_vars: vec![ndarray_dtype_ty, ndarray_ndims_ty],
             fields: Vec::default(),
+            attr: Vec::default(),
             methods: vec![
                 ("copy".into(), ndarray_copy_ty.0, DefinitionId(PRIMITIVE_DEF_IDS.ndarray.0 + 1)),
                 ("fill".into(), ndarray_fill_ty.0, DefinitionId(PRIMITIVE_DEF_IDS.ndarray.0 + 2)),
diff --git a/nac3core/src/toplevel/helper.rs b/nac3core/src/toplevel/helper.rs
index e212ac8..5e34052 100644
--- a/nac3core/src/toplevel/helper.rs
+++ b/nac3core/src/toplevel/helper.rs
@@ -280,6 +280,7 @@ impl TopLevelComposer {
             object_id: obj_id,
             type_vars: Vec::default(),
             fields: Vec::default(),
+            attr: Vec::default(),
             methods: Vec::default(),
             ancestors: Vec::default(),
             constructor,
diff --git a/nac3core/src/toplevel/mod.rs b/nac3core/src/toplevel/mod.rs
index e38207f..369c219 100644
--- a/nac3core/src/toplevel/mod.rs
+++ b/nac3core/src/toplevel/mod.rs
@@ -99,6 +99,10 @@ pub enum TopLevelDef {
         ///
         /// Name and type is mutable.
         fields: Vec<(StrRef, Type, bool)>,
+        /// Class Attributes
+        /// 
+        /// Same as fields, but are immutable
+        attr: Vec<(StrRef, Type, bool)>,
         /// Class methods, pointing to the corresponding function definition.
         methods: Vec<(StrRef, Type, DefinitionId)>,
         /// Ancestor classes, including itself.
diff --git a/nac3core/src/typecheck/type_inferencer/test.rs b/nac3core/src/typecheck/type_inferencer/test.rs
index e6fe800..f036403 100644
--- a/nac3core/src/typecheck/type_inferencer/test.rs
+++ b/nac3core/src/typecheck/type_inferencer/test.rs
@@ -289,6 +289,7 @@ impl TestEnvironment {
                     object_id: DefinitionId(i),
                     type_vars: Default::default(),
                     fields: Default::default(),
+                    attr: Default::default(),
                     methods: Default::default(),
                     ancestors: Default::default(),
                     resolver: None,
@@ -331,6 +332,7 @@ impl TestEnvironment {
                 object_id: DefinitionId(defs + 1),
                 type_vars: vec![v0],
                 fields: [("a".into(), v0, true)].into(),
+                attr: Default::default(),
                 methods: Default::default(),
                 ancestors: Default::default(),
                 resolver: None,
@@ -368,6 +370,7 @@ impl TestEnvironment {
                 object_id: DefinitionId(defs + 2),
                 type_vars: Default::default(),
                 fields: [("a".into(), int32, true), ("b".into(), fun, true)].into(),
+                attr: Default::default(),
                 methods: Default::default(),
                 ancestors: Default::default(),
                 resolver: None,
@@ -399,6 +402,7 @@ impl TestEnvironment {
                 object_id: DefinitionId(defs + 3),
                 type_vars: Default::default(),
                 fields: [("a".into(), bool, true), ("b".into(), fun, false)].into(),
+                attr: Default::default(),
                 methods: Default::default(),
                 ancestors: Default::default(),
                 resolver: None,