D:\work>python error.py
Traceback (most recent call last):
  File "error.py", line 9, in <module>
    main()
  File "error.py", line 7, in main
    foo('0')
  File "error.py", line 3, in foo
    assert n != 0, 'n is zero!'
AssertionError: n is zero!

D:\work>python -O error.py
Traceback (most recent call last):
  File "error.py", line 9, in <module>
    main()
  File "error.py", line 7, in main
    foo('0')
  File "error.py", line 4, in foo
    return 10 / n
ZeroDivisionError: division by zero