How should I verify a log message when testing Python code under nose?

Question

I\'m trying to write a simple unit test that will verify that, under a certain condition, a class in my application will log an error via the standard logging API. I can\'t

Answer 1

Found one answer since I posted this. Not bad.

Answer 2

From python 3.4 on, the standard unittest library offers a new test assertion context manager, assertLogs. From the docs:

with self.assertLogs('foo', level='INFO') as cm:
    logging.getLogger('foo').info('first message')
    logging.getLogger('foo.bar').error('second message')
    self.assertEqual(cm.output, ['INFO:foo:first message',
                                 'ERROR:foo.bar:second message'])

Answer 3

You should use mocking, as someday You might want to change Your logger to a, say, database one. You won't be happy if it'll try to connect to the database during nosetests.

Mocking will continue to work even if standard output will be suppressed.

I have used pyMox's stubs. Remember to unset the stubs after the test.

Answer 4

The ExpectLog class implemented in tornado is a great utility:

with ExpectLog('channel', 'message regex'):
    do_it()

http://tornado.readthedocs.org/en/latest/_modules/tornado/testing.html#ExpectLog

Answer 5

I used to mock loggers, but in this situation I found best to use logging handlers, so I wrote this one based on the document suggested by jkp(now dead, but cached on Internet Archive)

class MockLoggingHandler(logging.Handler):
    """Mock logging handler to check for expected logs."""

    def __init__(self, *args, **kwargs):
        self.reset()
        logging.Handler.__init__(self, *args, **kwargs)

    def emit(self, record):
        self.messages[record.levelname.lower()].append(record.getMessage())

    def reset(self):
        self.messages = {
            'debug': [],
            'info': [],
            'warning': [],
            'error': [],
            'critical': [],
        }

Answer 6

If you define a helper method like this:

import logging

def capture_logging():
    records = []

    class CaptureHandler(logging.Handler):
        def emit(self, record):
            records.append(record)

        def __enter__(self):
            logging.getLogger().addHandler(self)
            return records

        def __exit__(self, exc_type, exc_val, exc_tb):
            logging.getLogger().removeHandler(self)

    return CaptureHandler()

Then you can write test code like this:

    with capture_logging() as log:
        ... # trigger some logger warnings
    assert len(log) == ...
    assert log[0].getMessage() == ...