Exceptions Are Messages

Every error you have met so far, the ValueError from int, the KeyError from a dictionary, the FileNotFoundError from open, is an exception. It is Python's way of saying: I cannot carry on with this instruction, and here is exactly why. Once you see exceptions as messages rather than crashes, you can decide what your program should say in reply.

Think of an employee who cannot finish a task. They write a note saying what went wrong and pass it to their manager. If the manager cannot deal with it, the note goes up again, until it reaches someone who can decide, or the top of the company, where work stops. An exception travels the same way through a program.

What an Exception Is

When an instruction fails, Python creates an exception: an object with a type, such as ValueError, and a message, the sentence after the colon. Python then stops the current line and hands the exception to the code that called it, and then to the code that called that, looking for something that will deal with it. If nothing does, the program stops and prints the traceback. The next topic shows how to be the something that deals with it.

The Family

You have already met most of the everyday exceptions. Each name tells you the kind of problem before you read the message.

A Traceback Through Several Functions

With functions calling functions, a traceback lists the whole chain of calls, oldest at the top and the failing line at the bottom. The program below has main, which calls average, which divides by the number of values. Given an empty list, the division fails. Read it from the bottom, as Chapter 3 taught: a ZeroDivisionError, division by zero, on the line in average that divides. Then read upward to see how the program got there: main called average, and the file called main.

def average(values):
    return sum(values) / len(values)


def main():
    readings = []
    print(average(readings))


main()

Traceback (most recent call last):
  File "/Users/you/python-course/chain.py", line 10, in <module>
    main()
    ~~~~^^
  File "/Users/you/python-course/chain.py", line 7, in main
    print(average(readings))
          ~~~~~~~^^^^^^^^^^
  File "/Users/you/python-course/chain.py", line 2, in average
    return sum(values) / len(values)
           ~~~~~~~~~~~~^~~~~~~~~~~~~
ZeroDivisionError: division by zero

The chain is valuable. It tells you not only where the program failed but how it got there, and the fix often belongs higher up than the line that failed. Here, the real question is why main passed an empty list.

Crashing Is Sometimes Right

It is tempting to think a good program never shows a traceback. It is better to think a good program never carries on with wrong data. A program that stops loudly on a real problem is doing its job; one that quietly continues and prints a wrong total is not. The rest of this chapter is about telling those cases apart.