Problem with Unittest:

[Fredrik Lundh]

Francis Avila wrote:

> When you raise an exception, you raise the exception CLASS, not an instance
> of the exception:

"raise" takes either an instance or a class; in the latter case, it creates
the instance for you.  in modern Python, the actual exception (exc_value)
is *always* an instance of a class:

    http://www.python.org/doc/current/ref/raise.html

>     def duffer():
> -       raise ValueError()
> +       raise ValueError
>
>
>     class dufferTest(unittest.TestCase):
>         def test_duffer(self):
> -           self.assertRaises(ValueError(), duffer())
> +           self.assertRaises(ValueError, duffer())
>
> My guess is that somewhere in the bowels of assertRaises there's a test like
> this:
>
> if (<ExceptionClass in 1st argument> is <ExceptionClass received by
> executing the 2nd argument>):
>
> This test will fail, because in your case it's comparing two different
> instances: (ValueError() is ValueError()) will *always* be false, whereas
> (ValueError is ValueError) will *always* be true, because its the same
> class, and the same entity in memory.

the exception machinery uses "isinstance", not "is".  for details, see:

    http://www.python.org/doc/current/ref/try.html

> (Just a little time-saving tip: instead of setting up a suite, for simple
> unittest files you can do unittest.main(), which will run every test in the
> file whose name conforms to some lexical rules I do not now remember....)

to save even more time, run modified code at least once before you
ship the product ;-)

</F>