Dealing with the imperfect (or how to handle errors)

So you now have the perfect program, it runs flawlessly, except for one detail, it will crash on invalid user input. Have no fear, for Python has a special control structure for you. It's called try and it tries to do something. Here is an example of a program with a problem:

from __future__ import division, print_function
import sys
if sys.version_info.major == 2:
    input = raw_input
print("Type Control C or -1 to exit")
number = 1
while number != -1:
    number = int(input("Enter a number: "))
    print("You entered: ", number)

Notice how when you enter @#& it outputs something like:

Traceback (most recent call last):
  File "notry.py", line 8, in <module>
    number = int(input("Enter a number: "))
ValueError: invalid literal for int() with base 10: '@#&'

As you can see the int function is unhappy with the number @#& (as well it should be). The last line shows what the problem is; Python found a ValueError. How can our program deal with this? What we do is first: put the place where the errors occurs in a try block, and second: tell Python how we want ValueErrors handled. The following program does this:

from __future__ import division, print_function
import sys
if sys.version_info.major == 2:
    input = raw_input
print("Type Control C or -1 to exit")
number = 1
while number != -1:
    try:
        number = int(input("Enter a number: "))
        print("You entered: ", number)
    except ValueError:
        print("That was not a number")

Now when we run the new program and give it @#& it tells us “That was not a number.” and continues with what it was doing before.

When your program keeps having some error that you know how to handle, put code in a try block, and put the way to handle the error in the except block.

Exercises

Update at least the phone numbers program so it doesn't crash if a user doesn't enter any data at the menu.