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

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'])

Fortunately this is not something that you have to write yourself; the testfixtures package provides a context manager that captures all logging output that occurs in the body of the with statement. You can find the package here:

http://pypi.python.org/pypi/testfixtures

And here are its docs about how to test logging:

http://testfixtures.readthedocs.org/en/latest/logging.html

UPDATE: No longer any need for the answer below. Use the built-in Python way instead!

This answer extends the work done in https://stackoverflow.com/a/1049375/1286628. The handler is largely the same (the constructor is more idiomatic, using super). Further, I add a demonstration of how to use the handler with the standard library's unittest.

class MockLoggingHandler(logging.Handler):    """Mock logging handler to check for expected logs.    Messages are available from an instance's ``messages`` dict, in order, indexed by    a lowercase log level string (e.g., 'debug', 'info', etc.).    """    def __init__(self, *args, **kwargs):        self.messages = {'debug': [], 'info': [], 'warning': [], 'error': [],                         'critical': []}        super(MockLoggingHandler, self).__init__(*args, **kwargs)    def emit(self, record):        "Store a message from ``record`` in the instance's ``messages`` dict."        try:            self.messages[record.levelname.lower()].append(record.getMessage())        except Exception:            self.handleError(record)    def reset(self):        self.acquire()        try:            for message_list in self.messages.values():                message_list.clear()        finally:            self.release()

Then you can use the handler in a standard-library unittest.TestCase like so:

import unittestimport loggingimport fooclass TestFoo(unittest.TestCase):    @classmethod    def setUpClass(cls):        super(TestFoo, cls).setUpClass()        # Assuming you follow Python's logging module's documentation's        # recommendation about naming your module's logs after the module's        # __name__,the following getLogger call should fetch the same logger        # you use in the foo module        foo_log = logging.getLogger(foo.__name__)        cls._foo_log_handler = MockLoggingHandler(level='DEBUG')        foo_log.addHandler(cls._foo_log_handler)        cls.foo_log_messages = cls._foo_log_handler.messages    def setUp(self):        super(TestFoo, self).setUp()        self._foo_log_handler.reset() # So each test is independent    def test_foo_objects_fromble_nicely(self):        # Do a bunch of frombling with foo objects        # Now check that they've logged 5 frombling messages at the INFO level        self.assertEqual(len(self.foo_log_messages['info']), 5)        for info_message in self.foo_log_messages['info']:            self.assertIn('fromble', info_message)