[Python-Dev] unittest missing assertNotRaises

[Chris Kaynor]

On Tue, Sep 27, 2011 at 4:43 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> But I can't see this being a useful test. As written, exceptions are still treated as errors, except for excClass, which is treated as a test failure. I can't see the use-case for that. assertRaises is useful:
>
> "IOError is allowed, but any other exception is a bug."
>
> makes perfect sense. assertNotRaises doesn't seem sensible or useful to me:
>
> "IOError is a failed test, but any other exception is a bug."
>
> What's the point? When would you use that?
>

I've run across a few cases where this is the correct behavior. The
most recent one that comes to mind is while testing some code which
has specific silencing options: specifically, writing a main file and
a backup file, where failure to write the backup is not an error, but
failure to write the main is. As such, the test suite should have the
following tests:
- Failure to write the main should assert that the code raises the
failure error. No error is a failure, any other error is an error,
that error is a success. (it may also check that the backup was
written)
- Failure to write the backup should assert that the code does not
raise the failure error. No error is a success, that error is a
failure, any other error is a error. (it may also check that the main
was written)
- Both succeeding should assert that the files were actually written,
and that no error was raised. Any other result is an error.

Now, the difference between a Failure and an Error is more or less a
mute point, however I would expect an Error to be any unexpected
result, while a Failure is a predicted (either via forethought or
prior tests) but incorrect result.