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

Wed Mar 28 14:07:01 EDT 2012

On 2012年3月28日 14:28:08 +0200, Ulrich Eckhardt wrote:
> Hi!
>> I'm currently writing some tests for the error handling of some code. In
> this scenario, I must make sure that both the correct exception is
> raised and that the contained error code is correct:
>>> try:
> foo()
> self.fail('exception not raised')
> catch MyException as e:
> self.assertEqual(e.errorcode, SOME_FOO_ERROR)
> catch Exception:
> self.fail('unexpected exception raised')

First off, that is not Python code. "catch Exception" gives a syntax 
error.
Secondly, that is not the right way to do this unit test. You are testing 
two distinct things, so you should write it as two separate tests:
 def testFooRaisesException(self):
 # Test that foo() raises an exception.
 self.assertRaises(MyException, foo)
If foo does *not* raise an exception, the unittest framework will handle 
the failure for you. If it raises a different exception, the framework 
will also handle that too.
Then write a second test to check the exception code:
 def testFooExceptionCode(self):
 # Test that foo()'s exception has the right error code.
 try:
 foo()
 except MyException as err:
 self.assertEquals(err.errorcode, SOME_FOO_ERROR)

Again, let the framework handle any unexpected cases.
If you have lots of functions to test, write a helper function:
def catch(exception, func, *args, **kwargs):
 try:
 func(*args, **kwargs)
 except exception as err:
 return err
 raise RuntimeError('no error raised')
and then the test becomes:
 def testFooExceptionCode(self):
 # Test that foo()'s exception has the right error code.
 self.assertEquals(
 catch(MyException, foo).errorcode, SOME_FOO_ERROR
 )
(By the way, I have to question the design of an exception with error 
codes. That seems pretty poor design to me. Normally the exception *type* 
acts as equivalent to an error code.)
-- 
Steven