added test for returning false if math-function is no string

src/test/py/test_read_math_function.py

@@ -9,6 +9,8 @@ class read_function(unittest.TestCase):
         self.assertEqual(read_constant("e", "42*e^x"), False)
     def test_constant_isstring_true(self):
         self.assertEqual(is_string("42*e^x"), True)
+    def test_constant_isstring_false(self):
+        self.assertEqual(is_string(42), False)
 
 if __name__ == '__main__':
     unittest.main()