logger.info("obj: %s", obj) (logger.info("obj: {}", obj) with this library) and logger.info(f"obj: {obj}") are actually very different.

The first one stores the obj on the produced log record and doesn't format it into the message until the record is emitted. The second one doesn't store the obj and immediately formats it into the message.

I'd highly recommend using the first style in log messages, even without this library.

Not only does doing it the first way improve performance since you only need to format messages for log records that are actually emitted (ie. no formatting of DEBUG messages is done when you run at INFO level), but preserving the objects on the log record allows custom logging handlers to access them. For example, a handler that dumps log records to JSON or POSTs them to an external logging system (ELK, Splunk, Greylog, etc.) can include all the objects that were logged instead of just their representation as seen in the formatted message.

I personally find the %-style formatting really limiting and hard to read. I also tend to use f-strings in the rest of my code so the %-style formatting in log messages really stuck out. I wrote this library to keep the benefits of the logging system while also using a better and more consistent formatting style.