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.

Did you actually check your statement "isinstance(SystemExit(1), Exception) evals to True" on Python 2.6? — Anthon
– Anthon, Commented May 22, 2014 at 7:13

Roman Bodnarchuk · Accepted Answer · 2011-04-19 10:30:21Z

13

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

edited Apr 19, 2011 at 10:30

answered Apr 19, 2011 at 10:17

Roman Bodnarchuk

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2 Comments

ncoghlan Over a year ago

PEP 352 provides the rationale: python.org/dev/peps/pep-0352/#exception-hierarchy-changes

ncoghlan Over a year ago

Oh, and GeneratorExit was subsequently moved in 2.6 to inherit directly from BaseException since it is a "system-exiting" exception in some styles of generator-based programming.

Noufal Ibrahim · Accepted Answer · 2011-04-19 10:01:11Z

11

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.

answered Apr 19, 2011 at 10:01

Noufal Ibrahim

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Comments

Sven Marnach · Accepted Answer · 2011-04-19 09:51:41Z

8

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

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

SystemExit is not a subclass of Exception.

answered Apr 19, 2011 at 9:51

Sven Marnach

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1 Comment

Gary Shi Over a year ago

Thanks, I found the real error, is in Python 2.3, isinstance(SystemExit(1), Exception) is True. And with Python 2.3, the testing code prints "caught by except Exception". For Python 2.6, this is correct.

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