added test for is_allowed_string returning true if it has only digits and allowed characters as well as corresponding functionality

src/main/py/read_math_function.py

@@ -6,3 +6,9 @@ def read_constant(type, value):
 
 def is_string(value):
     return isinstance(value, str)
+
+
+def is_allowed_string(type, value):
+    allowed = "+-*/()^"
+    result = all(character.isdigit() or character in allowed for character in value)
+    return result

src/test/py/test_read_math_function.py

@@ -11,6 +11,8 @@ class read_function(unittest.TestCase):
         self.assertEqual(is_string("42*e^x"), True)
     def test_constant_isstring_false(self):
         self.assertEqual(is_string(42), False)
+    def test_constant_isallowed_string_true(self):
+        self.assertEqual(is_allowed_string("c", "42^(2)+1-3*4"), True)
 
 if __name__ == '__main__':
     unittest.main()