When writing programs, oftentimes during development/production there are lots of logging. We may see a common debug log such as the following. Expensive function may be some form of large information retrieval or I/O process.

import logging
...
logging.debug(f"using {expensive_func()}  currently")
...

However, in the case we are using the default log level of INFO in which the log message is not actually output, we are wastefully doing two things:

• Rendering a log message, and for some objects calling their __str__/__repr__ is expensive.
• expensive_func is called when not needed.

To address problem 1, we can call logging methods with a string literal instead of an f-string, with the first argument and the pattern-parameters as argument. This prevents f-string rendering and prevents formatting until needed.

import logging
...
logging.debug("using %d currently", expensive_func())
...

To address problem 2, we can use the isEnabledFor() method which takes the level argument and only returns True if the event would be created by the Logger for that specific level of call. This prevents the calling of the expensive function unless absolutely needed.

import logging
...
if logger.isEnabledFor(logging.DEBUG):
    logging.debug("using %d currently", expensive_func())
...

This means that unless the threshold is set above DEBUG level, the call to expensive_func() is never made.

If this is consistently called, you can use LoggerAdapter instead. This allows you to avoid clutter by removing the consistent need for isEnabledFor().

from logging import LoggerAdapter

class LazyLoggerAdapter(LoggerAdapter):
    def lazy_log(self, level, msg_or_func, *args, **kwargs):
        # Only call the function if we're actually going to log
        if self.isEnabledFor(level):
            if callable(msg_or_func):
                message = msg_or_func(*args, **kwargs)  # Function called HERE
            else:
                message = msg_or_func
            self.log(level, message)

...

lazy_logger.lazy_log(logging.DEBUG, expensive_operation)  # Function not called!
lazy_logger.lazy_log(logging.INFO, expensive_operation)   # Function IS called!

Now this will only operate if the DEBUG logging is enabled.