I understand that it should have been the reason of this.

But in my opinion, it is very bad to possibly have calculations and user space arbitrary operations inside the logging lock.

If you look at my proposition, you can do the format after the filter, but just before acquiring the lock. As most handlers start by calling the self.format() soon in the "emit()", I don't expect a big difference.

I'm not entirely sure that it is a good solution, but I think that the issue is pretty bad.