Python: how to do lazy debug logging

Question

I have some python like this:

def foo():
    logger = logging.getLogger()
    # do something here
    logger.debug(\'blah blah {}\'.format(expensive_func()))         


        

Answer 1

You can use stringlike library to add laziness to your messages

E.g.:

logger.debug(
    'blah blah {value}'
    .format(
        value=LazyString(expensive_func)
    )
)

Lib link: https://github.com/CovenantEyes/py_stringlike

Answer 2

Look at this part of the documentation.

Update: Logging already supports lazy evaluation, but slightly differently to the way described in your comnment. For example, see the following script:

import logging

def expensive_func(*args):
    print('Expensive func called: %s' % (args,))
    return sum(args)

class DeferredMessage(object):
    def __init__(self, func, *args):
        self.func = func
        self.args = args

    def __str__(self):
        return 'Message {0}'.format(self.func(*self.args))

if __name__ == '__main__':
    logging.basicConfig()
    logging.info(DeferredMessage(expensive_func, 1, 2))
    logging.warning(DeferredMessage(expensive_func, 3, 4))
    logging.error(DeferredMessage(expensive_func, 5, 6))

When the above script is run, it should print

Expensive func called: (3, 4)
WARNING:root:Message 7
Expensive func called: (5, 6)
ERROR:root:Message 11

which shows that a potentially expensive function is only called when necessary. The example above can, of course, be generalised to allow the format string to be passed to the DeferredMessage, and to use kwargs, and so on.

Answer 3

As Vinay Sajip suggests, you can do the following:

def foo():
    logger = logging.getLogger()
    if logger.isEnabledFor(logging.DEBUG):
        logger.debug('blah blah {}'.format(expensive_func()))
        logger.debug('Message: {}'.format(expf_1(expf_2(some_arg))))
        logger.debug('Message: {}', Lazy(expf_1, Lazy(expf_2, some_arg)))
foo()

Which is already lazy!

That's because the then-expressions

        logger.debug('blah blah {}'.format(expensive_func()))
        logger.debug('Message: {}'.format(expf_1(expf_2(some_arg))))
        logger.debug('Message: {}', Lazy(expf_1, Lazy(expf_2, some_arg)))

are only evaluated if and only if logger.isEnabledFor(logging.DEBUG) returns True, i.e. if and only if their evaluation is needed.

---

Even more

logging.info(DeferredMessage(expensive_func, 1, 2))

is not as lazy as one may think: DeferredMessage(expensive_func, 1, 2) have to be evaluated in an eager fashion. Which is in addition slower than evaluating:

    if logger.isEnabledFor(logging.DEBUG):