6.12: 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:

Code 6.12.1 (Python)

while True:
    line = input('> ')
    if line[0] == '#':
        continue
    if line == 'done':
        break
    print(line)
print('Done!')

# Code: http://www.py4e.com/code3/copytildone2.py

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 an empty line. Then there is no zero-th character, so we get a traceback. There are two solutions to this 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] == '#':