Why "except Exception" doesn't catch SystemExit?

Question

isinstance(SystemExit(1), Exception) evals to True, but this snippet prints "caught by bare except SystemExit(1,)".

try:
    sys.exit(0)
except Exception, e:
    print 'caught by except Exception', str(e)
except:
    print 'caught by bare except', repr(sys.exc_info()[1])

My testing environment is Python 2.6.

Answer 1

isinstance(SystemExit(1), Exception) is False on Python 2.6. Exception hierarchy in this version of Python was changed since Python 2.4.

E.g. KeyboardInterrupt is not subclass of Exception any more.

See more info http://docs.python.org/release/2.6.6/library/exceptions.html#exception-hierarchy

Answer 2

SystemExit derives from BaseException directly rather than from Exception.

Exception is the parent "All built-in, non-system-exiting exceptions"

SystemExit is a "system exiting exception" (by definition) and therefore doesn't derive from Exception. In your example, if you used BaseException, it would work as per your original assumptions.

Answer 3

Your error is in the very first sentence of your question:

>>> isinstance(SystemExit(1), Exception)
False

SystemExit is not a subclass of Exception.