diff --git a/pymath/calculus/test/test_expression.py b/pymath/calculus/test/test_expression.py
index 90a7f4f..cc34e0f 100644
--- a/pymath/calculus/test/test_expression.py
+++ b/pymath/calculus/test/test_expression.py
@@ -8,11 +8,12 @@ from pymath.calculus.expression import Expression
 from pymath.calculus.fraction import Fraction
 from pymath.calculus.generic import first_elem
 from pymath.calculus.render import txt, tex
+from pymath.calculus.operator import op
 
 
 def test_init_from_str():
     exp = Expression("2 + 3")
-    assert exp.postfix_tokens == [2, 3, "+"]
+    assert exp.postfix_tokens == [2, 3, op.add]
 
 
 def test_init_from_exp():
@@ -21,7 +22,7 @@ def test_init_from_exp():
 
 def test_init_list():
     exp = Expression([2, 3, "+"])
-    assert exp.postfix_tokens == [2, 3, "+"]
+    assert exp.postfix_tokens == [2, 3, op.add]
 
 
 def test_init_one_element_int_from_str():
@@ -66,20 +67,20 @@ def test_add_exp():
     e = Expression("12- 4")
     f = Expression("4 + 1")
     g = e + f
-    assert g.postfix_tokens == [12, 4, '-', 4, 1, "+", "+"]
+    assert g.postfix_tokens == [12, 4, op.sub, 4, 1, op.add, op.add]
 
 
 def test_mul_exp():
     e = Expression("12- 4")
     f = Expression("4 + 1")
     g = e * f
-    assert g.postfix_tokens == [12, 4, '-', 4, 1, "+", "*"]
+    assert g.postfix_tokens == [12, 4, op.sub, 4, 1, op.add, op.mul]
 
 
 def test_neg_exp():
     e = Expression("12- 4")
     g = -e
-    assert g.postfix_tokens == [12, 4, '-', '-']
+    assert g.postfix_tokens == [12, 4, op.sub, op.sub1]
 
 
 # -----------------------------
diff --git a/pymath/calculus/test/test_str2tokens.py b/pymath/calculus/test/test_str2tokens.py
index d880ff0..213bd2f 100644
--- a/pymath/calculus/test/test_str2tokens.py
+++ b/pymath/calculus/test/test_str2tokens.py
@@ -6,12 +6,13 @@ import unittest
 
 from pymath.calculus.str2tokens import str2tokens, str2in_tokens, in2post_fix
 from pymath.calculus.polynom import Polynom
+from pymath.calculus.operator import op
 
 
 class TestStr2tokens(unittest.TestCase):
     """Testing functions from pymath.calculus.str2tokens"""
 
-    def test_str2intokens(self):
+    def test_str2in_tokens(self):
         ans = str2in_tokens("2+3*4")
         self.assertEqual(ans, [2, "+", 3, "*", 4])
 
@@ -25,11 +26,11 @@ class TestStr2tokens(unittest.TestCase):
     def test_in2post_fix(self):
         in_tokens = str2in_tokens("2+3*4")
         ans = in2post_fix(in_tokens)
-        self.assertEqual(ans, [2, 3, 4, "*", "+"])
+        self.assertEqual(ans, [2, 3, 4, op.mul, op.add])
 
         in_tokens = str2in_tokens("2*3+4")
         ans = in2post_fix(in_tokens)
-        self.assertEqual(ans, [2, 3, "*", 4, "+"])
+        self.assertEqual(ans, [2, 3, op.mul, 4, op.add])
 
     def test_str2in_tokens_big_num(self):
         exp = "123 + 3"