Checkpoint 7.12.1.
11-9-1: What kind of error will come from trying to access a nonexistent element or one that is out of range?
Section 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('#'):
Checkpoint 7.12.2.
11-9-2: What string method is used to prevent errors?
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] == '#':
Checkpoint 7.12.3.
11-9-3: What kind of pattern is used to prevent errors?
