If you don’t already know what the correct answer should be, it is hard to tell that this program does not work correctly. How do we know what’s really going on in this program? If we had written it, what could we do to verify it?
Verify the algorithm on a smaller set of data that we can check by hand. Change the program to use range(0, 10, 2) and try to sum up the even numbers from 0-10. This answer is small enough to be checked by hand.
Try running the program using Codelens one step at a time to check what it is doing. This is much easier if you have already modified the program to work on a smaller range of data.
Try printing out each number that we iterate through. Try adding this line to the start of the loop body (line 5): print("number is", number). This works with the full original range, but the output will be easier to read if you work with a smaller range of data.
Whenever possible, debug on as small a problem as possible. Debugging is hard. You can make it easier by limiting your debugging to small chunks of code (by testing parts of your program as you go) and debugging on small sets of data (testing your algorithm on just a few values instead of on the large program you want to solve.)
