def test_invalid_string(self):
        calc = Calculator()
        calc.expr = "a,b"

        try:
            result = calc.add()
        finally:
            result = -1

        self.assertEqual(result, -1)
 def test_instantiate_and_change_expr(self):
     calc = Calculator("1,2")
     new_value = "3,4"
     calc.expr = new_value
     self.assertEqual(calc.expr, new_value)
from calculator import Calculator

calc = Calculator("1,2")

e = calc.expr()

if e == '1,2':
    print "1. Success!"
else:
    print "1. Fail!"

calc.expr:x("4,5")
e = calc.expr()

if e == '4,5':
    print "2. Success!"
else:
    print "2. Fail!"

try:
    calc = Calculator()
    print "3. Fail!"
except:
    print "3. Success!"

try:
    calc = Calculator("a,b")
    print "4. Fail!"
except:
    print "4: Success!"