6 PEP 341: Unified try/except/finally

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6 PEP 341: Unified try/except/finally

Until Python 2.5, the `try` statement came in two flavours. You could use a `finally` block to ensure that code is always executed, or one or more `except` blocks to catch specific exceptions. You couldn't combine both `except` blocks and a `finally` block, because generating the right bytecode for the combined version was complicated and it wasn't clear what the semantics of the combined statement should be.

Guido van Rossum spent some time working with Java, which does support the equivalent of combining `except` blocks and a `finally` block, and this clarified what the statement should mean. In Python 2.5, you can now write:

    try:
        block-1 ...
    except Exception1:
        handler-1 ...
    except Exception2:
        handler-2 ...
    else:
        else-block
    finally:
        final-block

The code in block-1 is executed. If the code raises an exception, the various `except` blocks are tested: if the exception is of class `Exception1`, handler-1 is executed; otherwise if it's of class `Exception2`, handler-2 is executed, and so forth. If no exception is raised, the else-block is executed.

No matter what happened previously, the final-block is executed once the code block is complete and any raised exceptions handled. Even if there's an error in an exception handler or the else-block and a new exception is raised, the code in the final-block is still run.

See Also:

PEP 341, Unifying try-except and try-finally
  PEP written by Georg Brandl; implementation by Thomas Lee.