Python 3.7 logging: f-strings vs % [duplicate]

Question

I'm running into a performance problem in a project, and I narrowed it down to some of the log lines. It seems that f-strings are calculated even when my logging facility is above the level of the line that is logging.

Consider this example to demonstrate the issue:

import logging logging.basicConfig(level=logging.INFO) logger = logging.getLogger('MyLogger')  class MyClass:     def __init__(self, name: str) -> None:         self._name = name     def __str__(self) -> str:         print('GENERATING STRING')         return self._name  c = MyClass('foo') logger.debug(f'Created: {c}') 

When this example is run, I get "GENERATING STRING" printed to screen, indicating that the __str__ method is being ran even though my logging level is set to INFO and the log line is for DEBUG.

From what I can tell today, the solution is to use the following vs an f-string.

logger.debug('Created: %s', c) 

There are three things going through my head right now.

• Most of the examples and docs I read seem to be pretty old.
• This project is Python 3.7+ only (not worried about being backwards compatible).
• I have a lot of lines of code to update.

I'm curious to know what others do in this situation. Is the %s the best (most modern) approach? Is there a more modern way that I should be logging as demonstrated above?

I have a lot of code to update (fix), and I'm hoping to align with modern best practices.

Answer 1

The documentation says that the logging lib is optimized to use the %s formatting style. I can't remember where it is mentionned exactly, but I read it a few months ago.

Edit - Found! https://docs.python.org/3/howto/logging-cookbook.html#formatting-styles
Edit2 - (thanks to Robin Nemeth): https://docs.python.org/3/howto/logging.html#optimization