unittest: assertRaises() with an instance instead of a type

[Ethan Furman]

Steven D'Aprano wrote:
> To the degree that the decision of how finely to slice tests is a matter 
> of personal judgement and/or taste, I was wrong to say "that is not the 
> right way". I should have said "that is not how I would do that test".
> 
> I believe that a single test is too coarse, and three or more tests is 
> too fine, but two tests is just right. Let me explain how I come to that 
> judgement.
> 
> If you take a test-driven development approach, the right way to test 
> this is to write testFooWillFail once you decide that foo() should raise 
> MyException but before foo() actually does so. You would write the test, 
> the test would fail, and you would fix foo() to ensure it raises the 
> exception. Then you leave the now passing test in place to detect 
> regressions.
> 
> Then you do the same for the errorcode. Hence two tests.

[snip]

> So: never remove tests just because they are redundant. Only remove them 
> when they are obsolete due to changes in the code being tested.

Very persuasive argument -- I now find myself disposed to writing two 
tests (not three, nor five ;).

~Ethan~