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

[Peter Otten]

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