Test error message return and return type

scripts/tests/test_validate_format.py

 # -*- coding: utf-8 -*-
 
+from email import message
 import unittest
 
+from validate.format import error_message
+
 
 class TestValidadeFormat(unittest.TestCase):
-    ...
+    
+    def test_error_message_return_and_return_type(self):
+        line_num_unity = 1
+        line_num_ten = 10
+        line_num_hundred = 100
+        line_num_thousand = 1000
+
+        msg = 'This is a unit test'
+
+        err_msg_unity = error_message(line_num_unity, msg)
+        err_msg_ten = error_message(line_num_ten, msg)
+        err_msg_hundred = error_message(line_num_hundred, msg)
+        err_msg_thousand = error_message(line_num_thousand, msg)
+
+        self.assertIsInstance(err_msg_unity, str)
+        self.assertIsInstance(err_msg_ten, str)
+        self.assertIsInstance(err_msg_hundred, str)
+        self.assertIsInstance(err_msg_thousand, str)
+
+        self.assertEqual(err_msg_unity, '(L002) This is a unit test')
+        self.assertEqual(err_msg_ten, '(L011) This is a unit test')
+        self.assertEqual(err_msg_hundred, '(L101) This is a unit test')
+        self.assertEqual(err_msg_thousand, '(L1001) This is a unit test')