== A Python surprise: exiting is an exception

Once upon a time I wrote a program to scan incoming mail messages during
the SMTP conversation for signs of spam. Because this was running as
part of our mailer, reliability was very important; unhandled errors
could cause us to lose mail.

As part of reliability, I decided I wanted to catch any unhandled
exception (which would normally abort the program with a backtrace),
log the backtrace, save a copy of the message that caused the bug, and
so on. So I wrote code that went like this:

>  def catchall(M, routine):
>    try:
>      routine(M)
>    except:
>      # log backtrace, etc

The mail checking routine told the mailer whether to accept or reject
the message through the program's exit status code, so the routine
calls _sys.exit()_ when it's done.

Then I actually ran this code, and the first time through it dutifully
spewed a backtrace into the logs about the code raising an unhandled
exception called _SystemExit_.

Naievely (without paying attention to the documentation for the
[[sys module|http://docs.python.org/lib/module-sys.html]]) I had
expected _sys.exit()_ to mostly just call the C library _exit()_
function. As the [[sys module]] documentation makes clear, this is not
what happens: _sys.exit()_ raises _SystemExit_, the whole chain of
exception handling happens (in part so that _finally_ clauses get
executed), and at the top of the interpreter you finally exit.

Unless, of course, you accidentally catch _SystemExit_. Then very odd
things can happen.

(This is another example of why
[[broad _except_s are dangerous|BroadTrys]].)