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:
    <statements>               # Run this action first
finally:
    <statements>               # Always run this code on the way out

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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.

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.