Python logging to StringIO handler

Question

I have a python test in which I want to test if the logging works properly. For example I have a function that creates a user and at the end the logging writes to log file the response.

logger = logging.getLogger('mylogger')
logger.setLevel(logging.DEBUG)
handler = logging.handlers.WatchedFileHandler('mylogfile.log')
formatter = logging.Formatter('%(asctime)s: %(message)s',
                              '%d/%b/%Y:%H:%M:%S %z')
handler.setFormatter(formatter)
logger.addHandler(handler)
logger.info('Some log text')

In my test case I want to send the log output to the StringIO.

class MyTest(unittest.TestCase):
    def setUp(self):
        stream = StringIO()
        self.handler = logging.StreamHandler(stream)
        log = logging.getLogger('mylogger')
        log.removeHandler(log.handlers[0])
        log.addHandler(self.handler)

    def tearDown(self):
        log = logging.getLogger('mylogger')
        log.removeHandler(self.handler)
        self.handler.close()

The problem is that I'm not sure how should I test wheter or not my logger is working.

Sirreverence

Sorry, this may just be beyond me, but wouldn't you just do stream.getvalue()? – Aegaeon

related: PyDev unittesting: How to capture text logged to a logging.Logger in "Captured Output" – Katzir

Answer 1

Here is an example that works, make sure you set the level of your log, and flush the buffer.

class MyTest(unittest.TestCase):
    def setUp(self):
        self.stream = StringIO()
        self.handler = logging.StreamHandler(self.stream)
        self.log = logging.getLogger('mylogger')
        self.log.setLevel(logging.INFO)
        for handler in self.log.handlers: 
            self.log.removeHandler(handler)
        self.log.addHandler(self.handler)
    def testLog(self):
        self.log.info("test")
        self.handler.flush()
        print '[', self.stream.getvalue(), ']'
        self.assertEqual(self.stream.getvalue(), 'test')

    def tearDown(self):
        self.log.removeHandler(self.handler)
        self.handler.close()

if __name__=='__main__':
    unittest.main()

Further reading, here is an example Temporily Capturing Python Logging to a string buffer that show how you could also do formatting.

Kindless

In python 3, the default behavior for loggers with no handlers attached is to throw the message to stdout. Add this after removing the handlers: self.log.addHandler(logging.FileHandler('/dev/null/')) – Agatha

Also in Python3, I had to do self.stream = StringIO('') to make it work with Python3 strings. – Revanchism