avoiding nested try/excepts

[Steven Bethard]

So I have code that looks something like this:

def f(xs):
for x in xs:
y = g(x) # can raise exception AnException
for z in h(y):
k(z) # can raise a variety of exceptions

Now, if g(x) raises AnException, I want to log something and skip the
inner loop (but continue with the outer loop). If k(z) raises an
exception, I want to log something and then re-raise the exception to
exit all the way out of f. Currently I do something like:

def f(xs):
for x in xs:
try:
y = g(x)
for z in h(y):
k(z)
except AnException, e:
log(x, e)
except:
log(x)
raise

I've heard before that it's generally a good idea to try to localize
your try-except blocks as much as possible. This would lead me to do
something like:

def f(xs):
for x in xs:
try:
y = g(x)
except AnException, e:
log(x, e)
else:
for z in h(y):
try:
k(z)
except:
log(x)
raise

but now I have another level of nesting and things get kinda hard for me
to read. So I have two questions:

(1) Should I really worry about localizing try-except blocks? and, if so
(2) Is there a cleaner way to do this kind of thing?

Note that I can't edit the g or k functions, so I can't move the
try-except blocks inside.

Thanks,

Steve

[Andrew Clover]

Re: avoiding nested try/excepts

Steven Bethard <steven.bethard @gmail.com> wrote:
[color=blue]
> except:
> log(x)
> raise[/color]

(Of course 'except:' is rarely a good idea! You probably want to
exclude at least MemoryError - as log() might then fail - and possibly
also KeyboardInterru pt, SystemExit depending on what you're doing.)
[color=blue]
> but now I have another level of nesting and things get kinda hard for me
> to read.[/color]

I find it fine - I prefer it to the shorter version as it's clearer
where AnException is expected to be raised.

In general, localising exception handling is a good move, and I'd
personally do so in the case of your example. If g() is the only
method that can possibly raise AnException the shorter version is
safe, but otherwise botched exception handling can be a real pain to
track down.

If you find the nesting level a bit much you could always use fewer
whitespaces. :-)

--
Andrew Clover
mailto:and@doxd esk.com

http://www.doxdesk.com/

[Peter Otten]

Re: avoiding nested try/excepts

Steven Bethard wrote:
[color=blue]
> (1) Should I really worry about localizing try-except blocks? and, if so[/color]

Not when the error-handling doesn't take advantage of the greater locality.
[color=blue]
> (2) Is there a cleaner way to do this kind of thing?[/color]

I've always thought that catching an exception was the clean way that
replaces dealing with functions returning error codes.
[color=blue]
> Note that I can't edit the g or k functions, so I can't move the
> try-except blocks inside.[/color]

If you cannot modify, you can still wrap them:
[color=blue][color=green][color=darkred]
>>> def catch(*exceptio ns):[/color][/color][/color]
.... def make_catcher(f) :
.... def catcher(*args):
.... try:
.... return f(*args)
.... except exceptions:
.... print "caught it"
.... return catcher
.... return make_catcher
....[color=blue][color=green][color=darkred]
>>> @catch(ZeroDivi sionError)[/color][/color][/color]
.... def f(a, b): return a / b
....[color=blue][color=green][color=darkred]
>>> f(1, 2)[/color][/color][/color]
0[color=blue][color=green][color=darkred]
>>> f(1, 0)[/color][/color][/color]
caught it[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

You may guess at what line in the above it dawned on me that you'd be better
off with individual wrappers than a generalized decorator...

Peter

[Steven Bethard]

Re: avoiding nested try/excepts

Andrew Clover wrote:[color=blue]
> Steven Bethard <steven.bethard @gmail.com> wrote:
>[color=green]
>> except:
>> log(x)
>> raise[/color]
>
>
> (Of course 'except:' is rarely a good idea! You probably want to
> exclude at least MemoryError - as log() might then fail - and possibly
> also KeyboardInterru pt, SystemExit depending on what you're doing.)[/color]

So what's the idiom for catching all exceptions *except*
MemoryError/KeyboardInterru pt/SystemExit? Too bad we won't get our new
Exceptions hierarchy until Python 3000...

Steve