try: except <never>:

[Hallvard B Furuseth]

I'd like an 'except <exception which is never raised>' statement
Is there a defined way to do that, for Python 2.2 and above?
'except None:' works for now, but I don't know if that's safe:

for ex in ZeroDivisionErr or, None:
try:
1/0
except ex:
print "Ignored first exception."

I could just use
except ZeroDivisionErr or:
if not <first iteration>:
raise
print "Ignored first exception."
but the variant above gets a bit neater.

--
Hallvard

[Paul Rubin]

Re: try: except &lt;never&gt ;:

Hallvard B Furuseth <h.b.furuseth@u sit.uio.no> writes:[color=blue]
> 'except None:' works for now, but I don't know if that's safe:
>
> for ex in ZeroDivisionErr or, None:
> try:
> 1/0
> except ex:
> print "Ignored first exception."[/color]

class NeverRaised(Exc eption): pass

for ex in ZeroDivisionErr or, NeverRaised:
...

[Hallvard B Furuseth]

Re: try: except &lt;never&gt ;:

Paul Rubin writes:[color=blue]
>Hallvard B Furuseth <h.b.furuseth@u sit.uio.no> writes:[color=green]
>> 'except None:' works for now, but I don't know if that's safe:
>>
>> for ex in ZeroDivisionErr or, None:
>> try:
>> 1/0
>> except ex:
>> print "Ignored first exception."[/color]
>
> class NeverRaised(Exc eption): pass
>
> for ex in ZeroDivisionErr or, NeverRaised:[/color]

Heh. Simple enough. Unless some obstinate person raises it anyway...

--
Hallvard

[Paul Rubin]

Re: try: except &lt;never&gt ;:

Hallvard B Furuseth <h.b.furuseth@u sit.uio.no> writes:[color=blue][color=green]
> > class NeverRaised(Exc eption): pass
> > for ex in ZeroDivisionErr or, NeverRaised:[/color]
>
> Heh. Simple enough. Unless some obstinate person raises it anyway...[/color]

Hmm, ok, how's this?:

def NeverRaised():
class blorp(Exception ): pass
return blorp
for ex in ZeroDivisionErr or, NeverRaised():
...

[Duncan Booth]

Re: try: except &lt;never&gt ;:

Paul Rubin wrote:
[color=blue]
> Hallvard B Furuseth <h.b.furuseth@u sit.uio.no> writes:[color=green][color=darkred]
>> > class NeverRaised(Exc eption): pass
>> > for ex in ZeroDivisionErr or, NeverRaised:[/color]
>>
>> Heh. Simple enough. Unless some obstinate person raises it anyway...[/color]
>
> Hmm, ok, how's this?:
>
> def NeverRaised():
> class blorp(Exception ): pass
> return blorp
> for ex in ZeroDivisionErr or, NeverRaised():
> ...[/color]

Or you can create an unraisable exception:
[color=blue][color=green][color=darkred]
>>> \[/color][/color][/color]
class NeverRaised(Exc eption):
def __init__(self, *args):
raise RuntimeError('N everRaised should never be raised')
[color=blue][color=green][color=darkred]
>>> \[/color][/color][/color]
try:
raise NeverRaised
except NeverRaised:
print "caught it"



Traceback (most recent call last):
File "<pyshell#4 7>", line 4, in __init__
raise RuntimeError('N everRaised should never be raised')
RuntimeError: NeverRaised should never be raised

[Tom Anderson]

Re: try: except &lt;never&gt ;:

On Tue, 10 Jan 2006, Duncan Booth wrote:
[color=blue]
> Paul Rubin wrote:
>[color=green]
>> Hallvard B Furuseth <h.b.furuseth@u sit.uio.no> writes:[color=darkred]
>>>> class NeverRaised(Exc eption): pass
>>>> for ex in ZeroDivisionErr or, NeverRaised:
>>>
>>> Heh. Simple enough. Unless some obstinate person raises it anyway...[/color]
>>
>> Hmm, ok, how's this?:
>>
>> def NeverRaised():
>> class blorp(Exception ): pass
>> return blorp
>> for ex in ZeroDivisionErr or, NeverRaised():
>> ...[/color][/color]

Nice.
[color=blue]
> Or you can create an unraisable exception:
>[color=green][color=darkred]
>>>> \[/color][/color]
> class NeverRaised(Exc eption):
> def __init__(self, *args):
> raise RuntimeError('N everRaised should never be raised')[/color]

Briliant! Although i'd be tempted to define an UnraisableExcep tionError to
signal what's happened. Or ...

class ImpossibleExcep tion(Exception) :
def __init__(self, *args):
raise ImpossibleExcep tion, args

Although crashing the interpreter is probably overkill.

tom

--
Like Kurosawa i make mad films; okay, i don't make films, but if i did
they'd have a samurai.

[Duncan Booth]

Re: try: except &lt;never&gt ;:

Tom Anderson wrote:
[color=blue][color=green]
>> class NeverRaised(Exc eption):
>> def __init__(self, *args):
>> raise RuntimeError('N everRaised should never be raised')[/color]
>
> Briliant! Although i'd be tempted to define an
> UnraisableExcep tionError to signal what's happened. Or ...
>
> class ImpossibleExcep tion(Exception) :
> def __init__(self, *args):
> raise ImpossibleExcep tion, args
>
> Although crashing the interpreter is probably overkill.[/color]

Crashng the interpreter would be, but what you just wrote is simply a more
obscure way of raising RuntimeError :-)
[color=blue][color=green][color=darkred]
>>> class ImpossibleExcep tion(Exception) :[/color][/color][/color]
.... def __init__(self, *args):
.... raise ImpossibleExcep tion, args
....[color=blue][color=green][color=darkred]
>>> raise ImpossibleExcep tion[/color][/color][/color]
Traceback (most recent call last):
File "<stdin>", line 3, in __init__
RuntimeError: maximum recursion depth exceeded

[Hallvard B Furuseth]

Re: try: except &lt;never&gt ;:

Thanks for the help.

Tom Anderson writes:[color=blue][color=green]
>> class NeverRaised(Exc eption):
>> def __init__(self, *args):
>> raise RuntimeError('N everRaised should never be raised')[/color]
>
> Briliant! Although i'd be tempted to define an UnraisableExcep tionError
> to signal what's happened. Or ...[/color]

A package we are using has ProgrammingErro r (like AssertionError except
__debug__ doesn't disable it), and RealityError when something *really*
can't happen:-)

--
Hallvard