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[74] As mentioned in the prior chapter, the text of error messages and stack traces tends to vary slightly over time and shells. Don’t be alarmed if your error messages don’t exactly match mine. When I ran this example in Python 3.0’s IDLE GUI, for instance, its error message text showed filenames with full absolute directory paths.

The try/finally Statement

The other flavor of the try statement is a specialization that has to do with finalization actions. If a finally clause is included in a try, Python will always run its block of statements “on the way out” of the try statement, whether an exception occurred while the try block was running or not. Its general form is:

try:

# Run this action first

finally:

# Always run this code on the way out

With this variant, Python begins by running the statement block associated with the try header line. What happens next depends on whether an exception occurs during the try block:

If no exception occurs while the try block is running, Python jumps back to run the finally block and then continues execution past below the try statement.

If an exception does occur during the try block’s run, Python still comes back and runs the finally block, but it then propagates the exception up to a higher try or the top-level default handler; the program does not resume execution below the try statement. That is, the finally block is run even if an exception is raised, but unlike an except, the finally does not terminate the exception—it continues being raised after the finally block runs.

The try/finally form is useful when you want to be completely sure that an action will happen after some code runs, regardless of the exception behavior of the program. In practice, it allows you to specify cleanup actions that always must occur, such as file closes and server disconnects.

Note that the finally clause cannot be used in the same try statement as except and else in Python 2.4 and earlier, so the try/finally is best thought of as a distinct statement form if you are using an older release. In Python 2.5, and later, however, finally can appear in the same statement as except and else, so today there is really a single try statement with many optional clauses (more about this shortly). Whichever version you use, though, the finally clause still serves the same purpose—to specify “cleanup” actions that must always be run, regardless of any exceptions.

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Note

As we’ll also see later in this chapter, in Python 2.6 and 3.0, the new with statement and its context managers provide an object-based way to do similar work for exit actions. Unlike finally, this new statement also supports entry actions, but it is limited in scope to objects that implement the context manager protocol.

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Example: Coding Termination Actions with try/finally

We saw some simple try/finally examples in the prior chapter. Here’s a more realistic example that illustrates a typical role for this statement:

class MyError(Exception): pass

def stuff(file):

raise MyError()

file = open('data', 'w') # Open an output file

try:

stuff(file) # Raises exception

finally:

file.close() # Always close file to flush output buffers

print('not reached') # Continue here only if no exception

In this code, we’ve wrapped a call to a file-processing function in a try with a finally clause to make sure that the file is always closed, and thus finalized, whether the function triggers an exception or not. This way, later code can be sure that the file’s output buffer’s content has been flushed from memory to disk. A similar code structure can guarantee that server connections are closed, and so on.

As we learned in Chapter 9, file objects are automatically closed on garbage collection; this is especially useful for temporary files that we don’t assign to variables. However, it’s not always easy to predict when garbage collection will occur, especially in larger