This is actually valid and I believe how logging is supposed to be used: https://docs.python.org/2/howto/logging.html#logging-variable-data

A quick git grep to the sources are showing both approach being used, no idea if we should consolidate on one or not care.

Ah, ok - I wasn't aware, that logging supports it directly. I'll drop PR for now as there is no code-style preferred.

yes, it is preferred to avoid the % operator in logging calls. Using % forces the code to unconditionally construct the formatted string, even if the line will not be logged.