Example: Default Behavior

Because the control flow through a program is easier to capture in Python than in English, let’s run some examples that further illustrate exception basics. I’ve mentioned that exceptions not caught by try statements percolate up to the top level of the Python process and run Python’s default exception-handling logic (i.e., Python terminates the running program and prints a standard error message). Let’s look at an example. Running the following module file, bad.py, generates a divide-by-zero exception:

def gobad(x, y):
    return x / y

def gosouth(x):
    print(gobad(x, 0))

gosouth(1)

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Because the program ignores the exception it triggers, Python kills the program and prints a message:

% python bad.py
Traceback (most recent call last):
  File "bad.py", line 7, in <module>
    gosouth(1)
  File "bad.py", line 5, in gosouth
    print(gobad(x, 0))
  File "bad.py", line 2, in gobad
    return x / y
ZeroDivisionError: int division or modulo by zero

I ran this in a shell widow with Python 3.0. The message consists of a stack trace (“Traceback”) and the name of and details about the exception that was raised. The stack trace lists all lines active when the exception occurred, from oldest to newest. Note that because we’re not working at the interactive prompt, in this case the file and line number information is more useful. For example, here we can see that the bad divide happens at the last entry in the trace—line 2 of the file bad.py, a return statement.[74]

Because Python detects and reports all errors at runtime by raising exceptions, exceptions are intimately bound up with the ideas of error handling and debugging in general. If you’ve worked through this book’s examples, you’ve undoubtedly seen an exception or two along the way—even typos usually generate a SyntaxError or other exception when a file is imported or executed (that’s when the compiler is run). By default, you get a useful error display like the one just shown, which helps you track down the problem.

Often, this standard error message is all you need to resolve problems in your code. For more heavy-duty debugging jobs, you can catch exceptions with try statements, or use one of the debugging tools that I introduced in Chapter 3 and will summarize again in Chapter 35 (such as the pdb standard library module).