Question

I have a script containing a section similar to this, on Python 2.6:

import sys

list_id='cow'
prev=[0,'cow']

try:
    if list_id==prev[1]:
        print '{0} is the same as {1}'.format(list_id,prev[1])
        sys.exit(0)
except:
    print 'exception occurred, exiting with error'
    sys.exit(1)

I noticed that though it was printing the ‘is the same’ line, it also logs the exception!

If you remove the try/except block, the interpreter shows no error. If you catch a specific error like ValueError, the except block is not executed.

import sys

list_id='cow'
prev=[0,'cow']

try:
    if list_id==prev[1]:
        print '{0} is the same as {1}'.format(list_id,prev[1])
        sys.exit(0)
except Exception as k:
    print 'exception occurred, exiting with error. Exception is:'
    print k.args
    sys.exit(1)

The except block is not executed, and the process finishes with return code 0. So, the exception is above Exception in the hierarchy?

import sys

list_id='cow'
prev=[0,'cow']

try:
    if list_id==prev[1]:
        print '{0} is the same as {1}'.format(list_id,prev[1])
        sys.exit(0)
except BaseException as k:
    print 'exception occurred, exiting with error. Exception is:'
    print k.args
    sys.exit(1)

produces

cow is the same as cow exception occurred, exiting with error.
Exception is: (0,)

And the process finishes with exit code 1.

Why is this Except block being executed at all?

Answer 1

sys.exit() raises SystemExit, which is what you’re seeing.

As to why it doesn’t inherit from Exception:

The exception inherits from BaseException instead of StandardError or
Exception so that it is not accidentally caught by code that catches
Exception. This allows the exception to properly propagate up and cause the interpreter to exit.