--- a/src/python_ast.rs
+++ b/src/python_ast.rs
@@ -103,7 +103,7 @@ pub enum expr {
     Num(String),
     Str(String),
     //Bytes(String),
-    NameConstant(String),
+    NameConstant(name_constant),
     Ellipsis,
 
     // the following expression can appear in assignment context
@@ -126,6 +126,13 @@ pub enum expr_context {
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub enum name_constant {
+    True,
+    False,
+    None_
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub enum slice {
     Slice(Option<expr>, Option<expr>, Option<expr>),
     ExtSlice(Vec<slice>),