Testing logging¶
Python includes a logging
package, and while it is widely used, many
people assume that logging calls do not need to be tested or 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(capture):
logger = logging.getLogger()
logger.info('a message')
logger.error('an error')
capture.check(
('root', 'INFO', 'a message'),
('root', 'ERROR', 'an error'),
)
Note
This method is not compatible with pytest’s fixture discovery stuff. Instead, put a fixture such as the following in your conftest.py:
import pytest
@pytest.fixture(autouse=True)
def capture():
with LogCapture() as capture:
yield capture
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 log:
logger.info('start of block number %i', 1)
try:
logger.debug('inside try block')
raise RuntimeError('No code to run!')
except:
logger.error('error occurred', exc_info=True)
The check methods¶
LogCapture
instances have check()
and check_present()
methods to make assertions about
entries that have been logged.
check()
will compare the
log messages captured with those you expect. Expected messages are
expressed, by default, 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:
>>> log.check(
... ('root', 'INFO', 'start of block number 1'),
... ('root', 'DEBUG', 'inside try block'),
... ('root', 'ERROR', 'error occurred'),
... )
However, if the actual messages logged were different, you’ll get an
AssertionError
explaining what happened:
>>> log.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', 'DEBUG', 'inside try block'), ('root', 'ERROR', 'error occurred'))
In contrast, check_present()
will only check that the messages you
specify are present, and that their order is as specified. Other messages will be ignored:
>>> log.check_present(
... ('root', 'INFO', 'start of block number 1'),
... ('root', 'ERROR', 'error occurred'),
... )
If the order of messages is non-deterministic, then you can be explict that the order doesn’t matter:
>>> log.check_present(
... ('root', 'ERROR', 'error occurred'),
... ('root', 'INFO', 'start of block number 1'),
... order_matters=False
... )
Printing¶
The LogCapture
has a string representation that
shows what messages it has captured. This can be useful in doc tests:
>>> print(log)
root INFO
start of block number 1
root DEBUG
inside try block
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:
from testfixtures import compare, Comparison as C
compare(C(RuntimeError('No code to run!')), log.records[-1].exc_info[1])
If you wish the extraction specified in the attributes
parameter to the
LogCapture
constructor to be taken into account, you can examine the list
of recorded entries returned by the actual()
method:
assert log.actual()[-1][-1] == 'error occurred'
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:
>>> 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
In other cases this problem can be alleviated by only capturing a specific logger:
>>> 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
However, it may be that while you don’t want to capture all logging, you do want to capture logging from multiple specific loggers:
>>> 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
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 un-installation 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
Once you have the filtered to the entries you would like to make assertions about, you may also
want to look at a different set of attributes that the defaults for
LogCapture
:
>>> with LogCapture(attributes=('levelname', 'getMessage')) as log:
... logger = getLogger()
... logger.debug('a debug message')
... logger.info('something %s', 'info')
... logger.error('an error')
>>> log.check(('DEBUG', 'a debug message'), ('INFO', 'something info'), ('ERROR', 'an error'))
As you can see, if a specified attribute is callable, it will be called and the result used to
form part of the entry. If you need even more control, you can pass a callable to the
attributes
parameter, which can extract any required information from the records and return
it in the most appropriate form:
def extract(record):
return {'level': record.levelname, 'message': record.getMessage()}
>>> with LogCapture(attributes=extract) as log:
... logger = getLogger()
... logger.debug('a debug message')
... logger.error('an error')
>>> log.check(
... {'level': 'DEBUG', 'message': 'a debug message'},
... {'level': 'ERROR', 'message': 'an error'},
... )
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)