Writing a unit test for Python logger formatted output

Question

How could you write a unittest in Python that would test that the output from a logger is indeed in the format you expect (i.e. set through a call to logging.basicConfig())? I'm thinking along the lines of a custom StreamHandler and use of the 're' library but it doesn't look like LogRecord passed to StreamHandler.emit() can give me the string that will be output.

Answer 1

From the documentation (http://packages.python.org/testfixtures/logging.html):

To help with this, TestFixtures allows you to easily capture the output of calls to Python’s logging framework and make sure they were as expected. There are three different techniques, depending on the type of test you are writing.

1. The context manager
2. The decorator
3. The manual usage

The examples are included in the documentation. The shortened version is below.

The context manager

>>> import logging
>>> from testfixtures import LogCapture
>>> with LogCapture() as l:
...     logger = logging.getLogger()
...     logger.info('a message')
...     logger.error('an error')

And after that you can check the logs for equality:

>>> l.check(
...     ('root', 'INFO', 'a message'),
...     ('root', 'ERROR', 'another error'),
...     )
Traceback (most recent call last):
 ...
AssertionError: Sequence not as expected:

same:
(('root', 'INFO', 'a message'),)

first:
(('root', 'ERROR', 'another error'),)

second:
(('root', 'ERROR', 'an error'),)

The decorator

Similar to the previous, but applied to specific function:

from testfixtures import log_capture

@log_capture()
def test_function(l):
    logger = logging.getLogger()
    logger.info('a message')
    logger.error('an error')

    l.check(
        ('root', 'INFO', 'a message'),
        ('root', 'ERROR', 'an error'),
        )

Manual usage

>>> from testfixtures import LogCapture
>>> l = LogCapture()

After which you can also "check" the logs:

>>> l.check(('root', 'INFO', 'a message'))
<...>

EDIT: To access specific logs and analyze them in a custom way, you can just iterate through l.records (where l is just LogCapture's instance) and access some properties of each of them (eg. msg contains message sent to logger, levelname contains codename of the level, and there are plenty other properties).