How to test that an exception is raised ?

[Dan Perl]

"StepH" <stephane.bronsart at bea.be> wrote in message 
news:41fa0da9$0$2018$6c56d894 at feed0.news.be.easynet.net...
> But i've prob with the 1st test : test_badFile.
> When I run the test, unittest say that no error is Raised.
> But when I run the mps2xml module with a bad file as arg., the exception 
> is
> well Raised.

I assume you don't actually see the exception (you don't see the stack 
traceback printed as an output) but you just see your own message 
"FileError".  That means that the exception was raised and caught by your 
try-except statement.

> What i'm doing wrong ?
>
> It is because the exception in catched in the mps2xml module ?

With the above mentioned assumption, that must be it.  Once the exception is 
caught, it is caught and no other code invoking mps2xml.mps2xml (including 
the assertRaises in your test) will see the exception.

Your mps2xml.mps2xml function should return a value indicating success or 
failure or should re-raise the exception.  You need a mechanism to let 
invoking code know that it was successful or it failed.  With a return code, 
you should test that and not the exception.  The assert you use should work 
if you re-raise the exception.

And BTW, it is a bad idea to use a generic "except" statement like you do. 
That catches ALL the exceptions and you will never know if a different 
exception than IOError was raised.  You should catch only IOError 
specifically or any other exception that you may expect.  You can add then 
an extra "except" statement catching all the other exceptions, but only if 
it is essential for your application to handle all exceptions gracefully 
instead of just exiting with a stack traceback.

> Thanks for your helps.
>
> StepH.
>
>