confusion around CustomException example in 'Python in a Nutshell'

[erick_bodine@comcast.net]

In Martinelli's Nutshell book in the Exceptions chapter there is an
example of a custom exception class (pg.112) that I am trying to
implement without success. The custom exception class example pulls
sys.exc_info() into an attribute and I am assuming that the attribute
would then contain the raised exception info in a tuple (admittedly I
could be assuming erroneously).

class LinuxDriverErro r(Exception):
def __init__(self, *args):
Exception.__ini t__(self, *args)
self.message = args[0]
self.wrapped_ex c = sys.exc_info()

try:
raise LinuxDriverErro r, "raising Cain!"
except CustomException , error:
print error.message
print error.wrapped_e xc

# just checking
print "sys.exc_info() : ", sys.exc_info()

If I run the above code I get the following output:

Just raising Cain!
wrapped_exc: (None, None, None)
sys.exc_info(): (<class __main__.LinuxD riverError at 0xf6f774dc>,
<__main__.Linux DriverError instance at 0xf6f74bec>, <traceback object
at 0xf6f7193c>)

I do not understand why the wrapped_exc attribute contains no objects.

I am running 2.3.4 on a fedora core 3 box.

thanks,

erick

[wittempj@hotmail.com]

Re: confusion around CustomException example in 'Python in a Nutshell'

I don't know the mentioned book, but if you rewrite like:[color=blue][color=green][color=darkred]
>>> import sys
>>> class LinuxDriverErro r(Exception):[/color][/color][/color]
.... def __init__(self, *args):
.... Exception.__ini t__(self, *args)
.... self.message = args[0]
....
.... def __str__(self):
.... return str(sys.exc_inf o())
....[color=blue][color=green][color=darkred]
>>> def test():[/color][/color][/color]
.... raise LinuxDriverErro r, "raising Cain!"
....[color=blue][color=green][color=darkred]
>>> try:[/color][/color][/color]
.... test()
.... except LinuxDriverErro r, error:
.... print error
....
(<class __main__.LinuxD riverError at 0x401e305c>,
<__main__.Linux DriverError instance at 0x401deb6c>, <traceback object
at 0x401df20c>)[color=blue][color=green][color=darkred]
>>> # just checking[/color][/color][/color]
.... print "sys.exc_info() : ", sys.exc_info()

This gets what you probably expect, the way you do it there is no
traceback object when you print it

[Peter Hansen]

Re: confusion around CustomException example in 'Python in a Nutshell'

erick_bodine@co mcast.net wrote:[color=blue]
> In Martinelli's Nutshell book in the Exceptions chapter there is an
> example of a custom exception class (pg.112) that I am trying to
> implement without success. The custom exception class example pulls
> sys.exc_info() into an attribute and I am assuming that the attribute
> would then contain the raised exception info in a tuple (admittedly I
> could be assuming erroneously).
>
> class LinuxDriverErro r(Exception):
> def __init__(self, *args):
> Exception.__ini t__(self, *args)
> self.message = args[0]
> self.wrapped_ex c = sys.exc_info()
>
> try:
> raise LinuxDriverErro r, "raising Cain!"
> except CustomException , error:
> print error.message
> print error.wrapped_e xc
>
> # just checking
> print "sys.exc_info() : ", sys.exc_info()
>
> If I run the above code I get the following output:
>
> Just raising Cain!
> wrapped_exc: (None, None, None)
> sys.exc_info(): (<class __main__.LinuxD riverError at 0xf6f774dc>,
> <__main__.Linux DriverError instance at 0xf6f74bec>, <traceback object
> at 0xf6f7193c>)
>
> I do not understand why the wrapped_exc attribute contains no objects.[/color]

Because at the time you create the exception there is
no "current exception" as seen from the point of view
of sys.exc_info().

Given the way this class was written, it is clearly
intended to be raised from within an "except" statement
as a result of another exception having been caught.

Try this instead:

try:
try:
1/0
except:
raise LinuxDriverErro r, 'raising Cain!'
except Exception, ex:
print ex.wrapped_exc


By the way, the code you show is probably not what you
were actually running anyway... you caught a
"CustomExceptio n" but the LinuxDriverErro r is not
a subclass of that class so it wouldn't/shouldn't
have been caught.

-Peter

[erick_bodine@comcast.net]

Re: confusion around CustomException example in 'Python in a Nutshell'


Peter Hansen wrote:[color=blue]
> erick_bodine@co mcast.net wrote:[color=green]
> > In Martinelli's Nutshell book in the Exceptions chapter there is an
> > example of a custom exception class (pg.112) that I am trying to
> > implement without success. The custom exception class example[/color][/color]
pulls[color=blue][color=green]
> > sys.exc_info() into an attribute and I am assuming that the[/color][/color]
attribute[color=blue][color=green]
> > would then contain the raised exception info in a tuple (admittedly[/color][/color]
I[color=blue][color=green]
> > could be assuming erroneously).
> >
> > class LinuxDriverErro r(Exception):
> > def __init__(self, *args):
> > Exception.__ini t__(self, *args)
> > self.message = args[0]
> > self.wrapped_ex c = sys.exc_info()
> >
> > try:
> > raise LinuxDriverErro r, "raising Cain!"
> > except CustomException , error:
> > print error.message
> > print error.wrapped_e xc
> >
> > # just checking
> > print "sys.exc_info() : ", sys.exc_info()
> >
> > If I run the above code I get the following output:
> >
> > Just raising Cain!
> > wrapped_exc: (None, None, None)
> > sys.exc_info(): (<class __main__.LinuxD riverError at 0xf6f774dc>,
> > <__main__.Linux DriverError instance at 0xf6f74bec>, <traceback[/color][/color]
object[color=blue][color=green]
> > at 0xf6f7193c>)
> >
> > I do not understand why the wrapped_exc attribute contains no[/color][/color]
objects.[color=blue]
>
> Because at the time you create the exception there is
> no "current exception" as seen from the point of view
> of sys.exc_info().
>
> Given the way this class was written, it is clearly
> intended to be raised from within an "except" statement
> as a result of another exception having been caught.
>
> Try this instead:
>
> try:
> try:
> 1/0
> except:
> raise LinuxDriverErro r, 'raising Cain!'
> except Exception, ex:
> print ex.wrapped_exc
>[/color]
This clears the fog a little.[color=blue]
>
> By the way, the code you show is probably not what you
> were actually running anyway... you caught a
> "CustomExceptio n" but the LinuxDriverErro r is not
> a subclass of that class so it wouldn't/shouldn't
> have been caught.[/color]

<classic keyboard fumbling> What I am trying to do is get information
from a raised custom exception. I am catching the exception in a
main() function but it was actually raised in a separate module
function. It would be nice if I could print out where the exception
was raised from (module.functio n name + line number).

-------------------------
def some_module_fun ction():
if something_bad:
raise LinuxDriverErro r, "raising.."
---------------------------

def main():
try:
mymodule.some_m odule_function( )
except LinuxDriverErro r, error:
print error.message
print error.from_wher e



Thanks,

--Erick

[color=blue]
>
> -Peter[/color]

[Peter Hansen]

Re: confusion around CustomException example in 'Python in a Nutshell'

erick_bodine@co mcast.net wrote:[color=blue]
> <classic keyboard fumbling> What I am trying to do is get information
> from a raised custom exception. I am catching the exception in a
> main() function but it was actually raised in a separate module
> function. It would be nice if I could print out where the exception
> was raised from (module.functio n name + line number).[/color]

Ah, good. Exactly what the useful functions in the
'traceback' module are intended for, if I understand
your meaning...

If doing "traceback.prin t_exc()" is printing the
information you want (in a traceback that has much
more than what you say you want), then you can use
the other functions in that module, and methods in
the 'traceback' objects themselves (the third item
in the tuple returned by sys.exc_info()) , will
give you all you could ever want...

Google can help you find examples of usage as well,
in the archives for this group.

-Peter