Python Exception Handling: Cleanup and Reraise

cleanup_and_reraise.py

#!/usr/bin/python
#
# This code sample shows (what I hope to be) the right way to reraise an exception in the
# situation where you have to perform some cleanup before reraising.
#

cleaned_up = False

def raiser():
    raise RuntimeError("this should be reported at line 10")

def cleanup_raises():
    global cleaned_up
    cleaned_up = True
    raise RuntimeError("you should not see this!")

def reraiser1():
    try:
        raiser()
    except RuntimeError, e:
        try:
            cleanup_raises()
        except RuntimeError:
            pass
        # Wrong: stack trace reports this as the original scene of the
        # crime. Location information should say line 10 but it doesn't
        raise e

def reraiser2():
    try:
        raiser()
    except RuntimeError, e:
        try:
            cleanup_raises()
        except RuntimeError:
            pass
        # Wrong: stack trace reports cleanup_raises as the culprit. We
        # want to know about raiser.
        raise

def reraiser3():
    try:
        raiser()
    except RuntimeError, e:
        def cleanup():
            try:
                cleanup_raises()
            except RuntimeError:
                pass
        cleanup()
        # Right: Need to make sure that exceptions from cleanup_raises
        # are handled in a separate scope. This reports the original
        # exception location of raiser at line 10.
        raise

def reraiser4():
    try:
        raiser()
    except RuntimeError, e:
        (_, _, traceback) = sys.exc_info()

        try:
            cleanup_raises()
        except RuntimeError:
            pass
        # Right: Raise exception but provide original traceback
        # info. (Note: must call sys.exc_info() before calling any
        # other possible raiser.) This also reports raiser at line 10.
        raise e, None, traceback

def reraiser5():
    try:
        raiser()
    finally:
        try:
            cleanup_raises()
        except RuntimeError:
            pass
        # Right: Don't even catch the original exception -- just clean
        # up on the way out.
        assert(cleaned_up)

import sys
variant = sys.argv[1]
if variant == '1':
    reraiser1()
elif variant == '2':
    reraiser2()
elif variant == '3':
    reraiser3()
elif variant == '4':
    reraiser4()
elif variant == '5':
    reraiser5()