Testing logging

Python includes an excellent logging package, however many people assume that logging calls do not need to be tested. They may also want to test logging calls but find the prospect too daunting. 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.

Note

The LogCapture class is useful for checking that your code logs the right messages. If you want to check that the configuration of your handlers is correct, please see the section below.

Methods of capture

There are three different techniques for capturing messages logged to the Python logging framework, depending on the type of test you are writing. They are all described in the sections below.

The context manager

If you’re using a version of Python where the with keyword is available, the context manager provided by TestFixtures can be used:

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

For the duration of the with block, log messages are captured. The context manager provides a check method that raises an exception if the logging wasn’t as you expected:

>>> 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'),)

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

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

It also has a string representation that allows you to see what has been logged, which is useful for doc tests:

>>> print(l)
root INFO
  a message
root ERROR
  an error

The decorator

If you are working in a traditional unittest environment and only want to capture logging for a particular test function, you may find the decorator suits your needs better:

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

If you want to capture logging for the duration of a doctest or in every test in a TestCase, then you can use the LogCapture manually.

The instantiation and replacement are done in the setUp function of the TestCase or passed to the DocTestSuite constructor:

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

You can then execute whatever will log the messages you want to test for:

>>> from logging import getLogger
>>> getLogger().info('a message')

At any point, you can check what has been logged using the check method:

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

Alternatively, you can use the string representation of the LogCapture:

>>> print(l)
root INFO
  a message

Then, in the tearDown function of the TestCase or passed to the DocTestSuite constructor, you should make sure you stop the capturing:

>>> l.uninstall()

If you have multiple LogCapture objects in use, you can easily uninstall them all:

>>> LogCapture.uninstall_all()

Checking captured log messages

Regardless of how you use the LogCapture to capture messages, there are three ways of checking that the messages captured were as expected.

The following example is useful for showing these:

from testfixtures import LogCapture
from logging import getLogger
logger = getLogger()

with LogCapture() as l:
    logger.info('start of block number %i', 1)
    try:
        raise RuntimeError('No code to run!')
    except:
        logger.error('error occurred', exc_info=True)

The check method

The LogCapture has a check() method that will compare the log messages captured with those you expect. Expected messages are expressed as three-element tuples where the first element is the name of the logger to which the message should have been logged, the second element is the string representation of the level at which the message should have been logged and the third element is the message that should have been logged after any parameter interpolation has taken place.

If things are as you expected, the method will not raise any exceptions:

>>> result = l.check(
...     ('root', 'INFO', 'start of block number 1'),
...     ('root', 'ERROR', 'error occurred'),
...     )

However, if the actual messages logged were different, you’ll get an AssertionError explaining what happened:

>>> l.check(('root', 'INFO', 'start of block number 1'))
Traceback (most recent call last):
 ...
AssertionError: sequence not as expected:

same:
(('root', 'INFO', 'start of block number 1'),)

expected:
()

actual:
(('root', 'ERROR', 'error occurred'),)

Printing

The LogCapture has a string representation that shows what messages it has captured. This can be useful in doc tests:

>>> print(l)
root INFO
  start of block number 1
root ERROR
  error occurred

This representation can also be used to check that no logging has occurred:

>>> empty = LogCapture()
>>> print(empty)
No logging captured

Inspecting

The LogCapture also keeps a list of the LogRecord instances it captures. This is useful when you want to check specifics of the captured logging that aren’t available from either the string representation or the check() method.

A common case of this is where you want to check that exception information was logged for certain messages:

>>> print(l.records[-1].exc_info)
(<... '...RuntimeError'>, RuntimeError('No code to run!',), <traceback object at ...>)

If you’re working in a unit test, the following code may be more appropriate:

from testfixtures import compare, Comparison as C

compare(C(RuntimeError('No code to run!')), l.records[-1].exc_info[1])

Only capturing specific logging

Some actions that you want to test may generate a lot of logging, only some of which you actually need to care about.

The logging you care about is often only that above a certain log level. If this is the case, you can configure LogCapture to only capture logging at or above a specific level.

If using the context manager, you would do this:

>>> with LogCapture(level=logging.INFO) as l:
...     logger = getLogger()
...     logger.debug('junk')
...     logger.info('something we care about')
...     logger.error('an error')
>>> print(l)
root INFO
  something we care about
root ERROR
  an error

If using the decorator, you would do this:

@log_capture(level=logging.INFO)
def test_function(l):
    logger= getLogger()
    logger.debug('junk')
    logger.info('what we care about')

    l.check(('root', 'INFO', 'what we care about'))

In other cases this problem can be alleviated by only capturing a specific logger.

If using the context manager, you would do this:

>>> with LogCapture('specific') as l:
...     getLogger('something').info('junk')
...     getLogger('specific').info('what we care about')
...     getLogger().info('more junk')
>>> print(l)
specific INFO
  what we care about

If using the decorator, you would do this:

@log_capture('specific')
def test_function(l):
    getLogger('something').info('junk')
    getLogger('specific').info('what we care about')
    getLogger().info('more junk')

    l.check(('specific', 'INFO', 'what we care about'))

However, it may be that while you don’t want to capture all logging, you do want to capture logging from multiple specific loggers.

You would do this with the context manager as follows:

>>> with LogCapture(('one','two')) as l:
...     getLogger('three').info('3')
...     getLogger('two').info('2')
...     getLogger('one').info('1')
>>> print(l)
two INFO
  2
one INFO
  1

Likewise, the same thing can be done with the decorator:

@log_capture('one','two')
def test_function(l):
    getLogger('three').info('3')
    getLogger('two').info('2')
    getLogger('one').info('1')

    l.check(
        ('two', 'INFO', '2'),
        ('one', 'INFO', '1')
        )

It may also be that the simplest thing to do is only capture logging for part of your test. This is particularly common with long doc tests. To make this easier, LogCapture supports manual installation and uninstallation as shown in the following example:

>>> l = LogCapture(install=False)
>>> getLogger().info('junk')
>>> l.install()
>>> getLogger().info('something we care about')
>>> l.uninstall()
>>> getLogger().info('more junk')
>>> l.install()
>>> getLogger().info('something else we care about')
>>> print(l)
root INFO
  something we care about
root INFO
  something else we care about

Checking the configuration of your log handlers

LogCapture is good for checking that your code is logging the correct messages; just as important is checking that your application has correctly configured log handers. This can be done using a unit test such as the following:

from testfixtures import Comparison as C, compare
from unittest import TestCase
import logging
import sys

class LoggingConfigurationTests(TestCase):

    # We mock out the handlers list for the logger we're
    # configuring in such a way that we have no handlers
    # configured at the start of the test and the handlers our
    # configuration installs are removed at the end of the test.

    def setUp(self):
        self.logger = logging.getLogger()
        self.orig_handlers = self.logger.handlers
        self.logger.handlers = []
        self.level = self.logger.level

    def tearDown(self):
        self.logger.handlers = self.orig_handlers
        self.logger.level = self.level

    def test_basic_configuration(self):
        # Our logging configuration code, in this case just a
        # call to basicConfig:
        logging.basicConfig(format='%(levelname)s %(message)s',
                            level=logging.INFO)

        # Now we check the configuration is as expected:

        compare(self.logger.level, 20)
        compare([
            C('logging.StreamHandler',
              stream=sys.stderr,
              formatter=C('logging.Formatter',
                          _fmt='%(levelname)s %(message)s',
                          strict=False),
              level=logging.NOTSET,
              strict=False)
            ], self.logger.handlers)