[Tutor] Exception Handling and Stack traces

[Michael Powe]

On Fri, Sep 10, 2010 at 04:42:35PM +0200, Peter Otten wrote:
> Michael Powe wrote:
 
> > On Fri, Sep 10, 2010 at 03:56:51PM +0200, Peter Otten wrote:
> >> Michael Powe wrote:

> >> > I can't work out how to suppress stacktrace printing when exceptions
> >> > are thrown.
 
> >> WRONG:
> >> 
> >> >>> try:
> >> ...     1/0
> >> ... except ZeroDivisionError("whatever"):
> >> ...     print "caught"
> >> ...
> >> Traceback (most recent call last):
> >>   File "<stdin>", line 2, in <module>
> >> ZeroDivisionError: integer division or modulo by zero
> >> 
> >> CORRECT:
> >> 
> >> >>> try:
> >> ...     1/0
> >> ... except ZeroDivisionError as e:
> >> ...     print "caught", e
> >> ...
> >> caught integer division or modulo by zero
 
> > Note that in section 8.3 of that article, the statement is made that
> > if the exception matches the the exception type in the following
> > format, the statements within the except clause are executed.
> > 
> > except URLError :
> > # do something
> > 
> > That in fact, seems to me to be incorrect.  It is not my experience
> > (e.g., print statements are not executed in the example I gave and the
> > sys.exit() is not called).
 
> Sorry, the as-clause is *not* necessary. The relevant difference between the 
> correct and the wrong approach is that you must not instantiate the 
> exception:
 
> WRONG:
> 
> >>> try:
> ...     1/0
> ... except ZeroDivisionError("whatever"):
> ...     print "caught"
> ...
> Traceback (most recent call last):
>   File "<stdin>", line 2, in <module>
> ZeroDivisionError: integer division or modulo by zero
 
> CORRECT:
> 
> >>> try:
> ...     1/0
> ... except ZeroDivisionError:
> ...     print "caught"
> ...
> caught
> 
> I just put in the as-clause to show an easy way to print the exception. I 
> did not anticipate that it would obscure the message.

Hello,

No problem, I am working on getting this sorted out.  The
documentation seems to be written as reminder for people who already
know how this stuff works, rather than as a clear explanation for
anybody working with it.

Eventually, I arrived at a workable conclusion by wrapping only the
call in main and using your suggested 'as' clause.  This successfully
suppresses the traceback and gives a useable error message.  Although,
in the case of URLError, 'getaddrinfo failed' may not actually mean
much to the end user, it'll have to do.

I don't like the fact that I cannot locate my thrown exception at the
point of throwing -- i.e., I don't necessarily mind catching the
exception in main but I would like to be able to print out exactly
where the exception occurred.  This is more useful when
troubleshooting. However, an entire stacktrace is unacceptably
verbose.

Thanks.

mp

-- 
Michael Powe		michael at trollope.org		Naugatuck CT USA


It's easier to fight for one's principles than to live up to them.
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