Julien reviewed the pushed change and asked me questions on IRC:

* "nonlocal log": this change is unrelated to the fix, I should have done that in a separated change, sorry, I cannot resist to refactoring :-) The change has no effect, it's more cosmetic to help reviewers: "log is not a local variable nor a global variable, it's a "non-local" variable".

* inner function (calling log.append) renamed from "showarnmsg()" to "showarnmsg_logger()": it's to help debugging. It's a pain when two different functions completely different have the same name, especially for the warnings where many functions are overriden at runtime. Moreover, the function is now used for _showwarnmsg_impl instead of _showwarnmsg, so I also renamed the function to avoid confusion.