How do I unit-test a specific case of a home-rolled exception ?

[Ken Starks]

Peter Otten wrote:
> Ken Starks wrote:
> 
>> I have an exception class, and I want to check that
>> a particular instance of it has been raised; or
>> more accurately that one is raised that is equal to
>> an instance I specify.
>>
>> In the example below, I can check that a
>> 'LongRationalError' is raised, but I want
>> to check that it is specifically a
>> 'LongrationalError('1') or a 'LongRationalError('2')
>>
>> How do I do that?
> 
> (all untested)
> 
> try:
>     LongRational(1, "7")
> except LongRationalError, e:
>     self.assertEquals(e.code, "2")
> else:
>     self.fail()
> 
> Alternatively you can subclass LongRationalError...
> 
> class LongRationalError(Exception):
>     pass
> 
> class LongRationalDenominatorError(LongRationalError):
>     pass
> 
> class LongRationalNumeratorError(LongRationalError):
>     pass
> 
> ...and then check for the specialized exception:
> 
> self.assertRaises(LongRationalDenominatorError, LongRational, 1, "7")
> 
> Personally, I'd probably throw a plain old TypeError for incompatible types
> of both numerator and denominator.
> 
> Peter

Thanks Peter, that answers my question nicely. I rather thought
I would need a try .. except structure in my unit-test itself.