adding a simulation mode

[Hans Mulder]

On 13/07/12 04:16:53, Steven D'Aprano wrote:
> On Thu, 12 Jul 2012 16:37:42 +0100, andrea crotti wrote:
> 
>> 2012/7/12 John Gordon <gordon at panix.com>:
>>> In <mailman.2043.1342102625.4697.python-list at python.org> andrea crotti
>>> <andrea.crotti.0 at gmail.com> writes:
>>>
>>>> Well that's what I thought, but I can't find any explicit exit
>>>> anywhere in shutil, so what's going on there?
>>>
>>> Try catching SystemExit specifically (it doesn't inherit from
>>> Exception, so "except Exception" won't catch it.)
>>>
>>
>> Ah yes that actually works, but I think is quite dodgy, why was it done
>> like this?

It may be that the function you're calling found a problem that the
author thinks is so grave that they shouldn't give you an opportunity
to deal with it.

If that's the case, I would be inclined to think that they are wrong.

> Built-in exceptions SystemExit, KeyboardInterrupt and GeneratorExit 
> deliberately do not inherit from Exception since they are not meant to be 
> caught by "catch-all" try...except Exception clauses.
> 
> You can see the exception hierarchy here:
> 
> http://docs.python.org/library/exceptions.html#exception-hierarchy
> 
> Please do NOT catch BaseException, since that is the wrong thing to do. 

I would agree if you had said "in production code".

If you are investigating why a third-party function is stopping your
interpreter, then catching BaseException may tell you that the code
is raising the wrong kind of Exception.  Once you know what kind the
function is raising, you should catch only that particular excpetion
subclass.


> If you must catch SystemExit, KeyboardInterrupt, etc. they you should do 
> so as separate catch clauses:
> 
> try:
>     main()
> except SystemExit as e:
>     print(e)  # see if we can find out who is raising this

If you want to find out who is raising the exception, you could
try this:

except SystemExit:
    import traceback
    traceback.print_exc()

That will print a complete stack trace.

If you only need to know what kind of exception you have,
you can do:

    print(repr(e))

A simple print(e) will print str(e), which in the case of
SystemExit, is an empty string.  That's not very informative.

> except KeyboardInterrupt:
>     print("Mwahahaha my pretty, you cannot cancel this!!!")
>     print("...er, now what do I do?")
> except Exception:
>     print("why am I catching exceptions I can't recover from?")


Hope this helps,

-- HansM