Defensive Programming: using Exceptions

In the Calculator exercise we didn’t handle exceptional cases such as an invalid operator being passed to the calc function, or a division by zero.

We will address this now using some defensive programming concepts, where you make your code more robust by handling unexpected values or states.

In a file called defensive_exceptions.py, write a divide function that divides two numbers. If called like divide(8,4) it will return 2.

However, your divide function has some constraints:

If the second argument is less than zero, it raises a ValueError exception.
If the second argument is zero, it raises a ZeroDivisionError. That’s a default Python behavior, you don’t need to implement anything for that, but the test will check it.
If both arguments have the same value, it also raises a ValueError exception, with a message saying Arguments cannot be equal

To implement these constraints, you’ll need some if statements along with the raise statement and built-in Exceptions.

Mentoring topics

Discuss the benefits of using Exceptions as opposed to returning “result codes” to indicate problems.

To validate your code, download the test code below and run it as explained on the Hello, Python exercise page.