1 What is an Exception?

As exception can interupt the execution flow of a program. In the case of a try-except block…

try:
    raw_string = input("Enter a year: ")
    year = int(raw_string)

except ValueError as _err:
    print(f'"{raw_string}" is not a valid year')

we essentially have an if-else analogue. The try block is the good path and the except block is the bad path.

2 Why so Specific?

You may have noticed that the try-catch block only handles the ValueError exception. Let us rewrite the code with a bare exception.

try:
    raw_string = input("Enter a year: ")
    year = int(raw_string)

except:
    print(f'"{raw_string}" is not a valid year')

Run the code and press ctrl-c to force exit. You will see the following…

Enter a year. ^CTraceback (most recent call last):
  File "/home/tkennedy/Courses/Reviews/cs263/Module-07/exception_ex_1.py", line 4, in main
    raw_string = input("Enter a year. ")

KeyboardInterrupt

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/tkennedy/Courses/Reviews/cs263/Module-07/exception_ex_1.py", line 12, in <module>
    main()
  File "/home/tkennedy/Courses/Reviews/cs263/Module-07/exception_ex_1.py", line 8, in main
    print(f'"{raw_string}" is not a valid year')

UnboundLocalError: cannot access local variable 'raw_string' where it is not associated with a value

We caught the KeyboardInterrupt. Our intention was to handled malformed year input, but we wrote code that handles every possible exception that can occur. Let us only handle ValueErrors…

try:
    raw_string = input("Enter a year: ")
    year = int(raw_string)

except ValueError:
    print(f'"{raw_string}" is not a valid year')

Our output is now…

Enter a year: ^CTraceback (most recent call last):
  File "/home/tkennedy/Courses/Reviews/cs263/Module-07/exception_ex_2.py", line 12, in <module>
    main()
  File "/home/tkennedy/Courses/Reviews/cs263/Module-07/exception_ex_2.py", line 4, in main
    raw_string = input("Enter a year: ")

KeyboardInterrupt

While this example might demonstrate a small inconvenience… imagine debugging that in a larger program within function that is multiple function calls deep.

3 What is the “as” For?

That leaves the as _err. Sometimes you want to perform further handling of an error. For the moment… let us acknowledge that additional handling is possible while deferring further discussion.

def main():

    try:
        raw_string = input("Enter a year: ")
        year = int(raw_string)

    except ValueError as _err:
        print(f'"{raw_string}" is not a valid year')


if __name__ == "__main__":
    main()

The underscore indicates that, by convention the _err variable while named… is not used. In general a variable that starts with an underscore is understood to be needed syntacticly, but is not used in the code. An example of this is shown in the following code snippet.

for _ in range(1, 4):
    print("Hello!)

The loop prints "Hello!" four (4) times. While a variable is need to control the loop, the actual loop count is never used.