Okay, so I think this needed to be documented in signal.set_wakeup_fd(). 

IMO this reduces usefulness of set_wakeup_fd(). If I need to install custom handler by my self, I can write to fd in my own custom handler.

And,  installing custom handler omits existing handler, but we can not execute existing signal handler from python script. In my real-world case, SIGWINCH signal does not delivered to curses library. 

pep-475(https://www.python.org/dev/peps/pep-0475/#backward-compatibility) claims that 

    "For applications using event loops, signal.set_wakeup_fd() is the recommanded option to handle signals. "

But I think side effect of signal.set_wakeup_fd() is lager than pep-475 authors expected.