[Twisted-Python] Unit tests, inline callbacks, and exception handling

Patrick Hartling patrick at priority5.com
Thu Sep 24 13:04:07 EDT 2009

On Sep 24, 2009, at 11:59 AM, exarkun at twistedmatrix.com wrote:

> On 04:51 pm, patrick at priority5.com wrote:
>> We are using nose and the twisted.trial.unittest.TestCase class from
>> Twisted 8.2.0 to write tests that use inline callbacks. In one of the
>> tests, we are trying to validate that the code will behave correctly
>> by raising an exception if two clients with the same user name  
>> connect
>> to a server. The basic test code is similar to this:
>> @inlineCallbacks
>> def testDuplicateConnect(self):
>>   server_port = random.randint(10000, 40000)
>>   server = Server()
>>   server.listen(port = server_port)
>>   client1 = Client(username = "Test")
>>   yield client1.connect(port = server_port)
>>   try:
>>      client2 = Client(username = "Test")
>>      yield client2.connect(port = server_port)
>>      assert False
>>   except UsernameAlreadyExists, ex:
>>      pass
>>   yield server.close()
>>   yield client.close()
>> The test method executes and returns successfully. The exception is
>> raised and caught correctly, but because a
>> twisted.python.failure.Failure object was seen during deferred
>> processing, twisted.trial.unittest.TestCase._cleanUp() concludes that
>> an error occurred. That is perhaps not the most accurate way of
>> describing what happens, but the end result is that a caught  
>> exception
>> ends up being reported as an error.
>> My question is this: are there any tips for performing exception
>> handling in test methods that are decorated with @inlineCallbacks? I
>> have been tracing through a lot of code trying to find any extension
>> point, hook, etc. where I could indicate that the exception was  
>> caught
>> correctly, but I have not found anything yet. Adding the obvious
>> @raises decoration does not work because the method catches the
>> exception. The closest I have come is to call
>> self._observer._ignoreErrors() from the test method with the  
>> exception
>> type in question, but that does not strike me as being on the right
>> track.
> If the problem is because the server code uses  
> `twisted.python.log.err`
> to log the double client connection attempt, then you can make it so
> that this doesn't fail the test by flushing the error before the test
> returns, with something like this:
>    self.flushLoggedErrors(UsernameAlreadyExists)
> Or possibly a different exception type, depending on what exactly the
> server is logging.

That did it. Thanks!


> If the server isn't logging anything, then I can't really know why  
> your
> test is failing based on the example you've given.  If you can  
> construct
> a SSCCE (<http://sscce.org>), that would help a lot.
> Jean-Paul
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Patrick L. Hartling
Senior Software Engineer, Priority 5

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