Operation Manual

Experiments in Python
90
Notes:
The main part of the Roman Numerals program also introduces the idea of
“exceptions”. When a function you write detects something wrong, you need to
let the callers of your code know. You can do this by raising an error.
if number < 1 or number > 4999:
raise ValueError
“ValueError” is raised by other parts of Python when you attempt an operation on
a variable that cannot be done, so we’ve used it here. The program catches the
exception and prints a message.
except ValueError:
print("Try again")
Troubleshooting those bugs
A “bug” is computer jargon for a mistake in your program. I have
not intentionally put bugs in the listings, so if you find you cannot
run a program or it gives a weird response, you have somehow
introduced a bug. The term has been around since the time of
Thomas Edison (c. 1878) but the often-reported case of a moth
causing a problem inside a computer around 1947 cemented the
term in the world of computers.
The ability to track down bugs is an invaluable skill for all
programmers. With all the best intentions and planning, mistakes
can be made. When starting to program, it is better to type in the
examples listed here. It might take longer but you are in training.
You are training to type and how to avoid making typing errors.
Your ability to proof-read code will increase as you work through
these examples.
When you find a bug, don’t panic and start changing code at
random. Look at the listing, look at your code, and work through
each part logically to find out what you should be seeing, compared
to what you are actually seeing. The errors given by Python will be
confusing but it is only by following the instructions in your code
and telling you why it cannot continue.
At some point, you will discover that computer programmers use
another program called a debugger to analyse and find bugs in
their code as it runs. You won’t need the debugger just yet but
there is an alternative method. The easiest method is to add
“print()” commands to your code to find out what is going on in the
variables and how far Python is through your program. Think of it
as inserting a probe into the code to tell you what is happening.
When you have removed all the bugs and your program runs
properly, you can then remove all those print commands to clean
up the results.