7.12. Debugging¶
A skill that you should cultivate as you program is always asking yourself, “What could go wrong here?” or alternatively, “What crazy thing might our user do to crash our (seemingly) perfect program?”
For example, look at the program which we used to demonstrate the
while loop in the chapter on iteration:
while True:
line = input('> ')
if line[0] == '#':
continue
if line == 'done':
break
print(line)
print('Done!')
Look what happens when the user enters an empty line of input:
> hello there
hello there
> # don't print this
> print this!
print this!
>
Traceback (most recent call last):
File "copytildone.py", line 3, in <module>
if line[0] == '#':
IndexError: string index out of range
The code works fine until it is presented with an empty line. In that case, there is no zero-th character, so we get a traceback. There are two solutions to make line three “safe,” even if the line is empty.
One possibility is to simply use the startswith method,
which returns False if the string is empty.
if line.startswith('#'):
Another way is to safely write the if statement using the
guardian pattern and make sure the second logical
expression is evaluated only where there is at least one character in
the string:
if len(line) > 0 and line[0] == '#':