parser returns None if an error is seen

parser/Parser.py

@@ -9,9 +9,13 @@ class Parser(object):
         self.lexer = Lexer()
         self.tokens = self.lexer.tokens
         self.parser = yacc.yacc(module = self, outputdir = 'parser')
+        self.saw_error = False
 
     def p_unit(self, p):
         'unit : unit_items'
+        if self.saw_error:
+            p[0] = None
+        else:
             p[0] = UnitNode([p[1]])
 
     def p_unit_items(self, p):
@@ -42,5 +46,8 @@ class Parser(object):
         'empty :'
         p[0] = Node()
 
+    def p_error(self, p):
+        self.saw_error = True
+
     def parse(self):
         return self.parser.parse(self.input, lexer = self.lexer)