Python – Lazy logger message string evaluation

loggingpython

I'm using standard python logging module in my python application:

import logging
logging.basicConfig(level=logging.INFO)
logger = logging.getLogger("log")
while True:
  logger.debug('Stupid log message " + ' '.join([str(i) for i in range(20)]) )
  # Do something

The issue is that although debug level is not enable, that stupid log message is evaluated on each loop iteration, which harms performance badly.

Is there any solution for this?

In C++ we have log4cxx package that provides macros like this:
LOG4CXX_DEBUG(logger, messasage)
That effectively evaluates to 

if (log4cxx::debugEnabled(logger)) {
    log4cxx.log(logger,log4cxx::LOG4CXX_DEBUG, message)
}

But since there are no macros in Python (AFAIK), if there a efficient way to do logging?

Best Answer

The logging module already has partial support for what you want to do. Do this:

log.debug("Some message: a=%s b=%s", a, b)

... instead of this:

log.debug("Some message: a=%s b=%s" % (a, b))

The logging module is smart enough to not produce the complete log message unless the message actually gets logged somewhere.

To apply this feature to your specific request, you could create a lazyjoin class.

class lazyjoin:
    def __init__(self, s, items):
        self.s = s
        self.items = items
    def __str__(self):
        return self.s.join(self.items)

Use it like this (note the use of a generator expression, adding to the laziness):

logger.info('Stupid log message %s', lazyjoin(' ', (str(i) for i in range(20))))

Here is a demo that shows this works.

>>> import logging
>>> logging.basicConfig(level=logging.INFO)
>>> logger = logging.getLogger("log")
>>> class DoNotStr:
...     def __str__(self):
...         raise AssertionError("the code should not have called this")
... 
>>> logger.info('Message %s', DoNotStr())
Traceback (most recent call last):
...
AssertionError: the code should not have called this
>>> logger.debug('Message %s', DoNotStr())
>>>

In the demo, The logger.info() call hit the assertion error, while logger.debug() did not get that far.

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