Avoid function execution for not logged messages

lazylogging.py

import logging

logger = logging.getLogger(__name__)

# Log level is INFO. So anything of lower level (DEBUG) should not execute
logger.setLevel(logging.INFO)


stream_handler = logging.StreamHandler()
logger.addHandler(stream_handler)

urls = ("a", "c", "b", "a")


def sort_urls(urls):
    print("Sorting urls")
    return sorted(urls)


class LazySortedURLs(object):
    def __init__(self, urls):
        print("Instanciating")
        self.urls = urls

    def __str__(self):
        print("Sorting urls")
        return str(sorted(urls))


# The debug message is not logged, but the sorting is still executed. 
# This may be unimportant for small collections, but is can become very costly is the collection is large. 
logger.debug("Debug sorted urls: %s", sort_urls(urls))
logger.info("Info sorted urls: %s", sort_urls(urls))

# When using a custom class to wrap the sorting, it looks like the `__str__`
# method is only evaluated when it is needed ("lazy"). The object is
# instanciated but not printed.
logger.debug("Debug lazy sorted urls: %s", LazySortedURLs(urls))
logger.info("Info lazy sorted urls: %s", LazySortedURLs(urls))