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I have a use case where a daemon thread needs to write periodic updates to sys.stdout. I'd prefer it not to print in the middle of a command being typed. In order to achieve this, I have added to the readline module. There is now a function to determine whether we are reading a line, and one to call rl_forced_update_display. These functions enable me to say (in the daemon thread):
def describe (indicator):
 if readline.reading_line ():
 print ()
 print (indicator.description)
 readline.forced_update_display ()
 else:
 print (indicator.description)
I have read PEP7, and have made patchtest on my patch.

Suggest you integrate your rl_forced_update_display() wrapper and documentation with the existing patches in Issue 23067.
Regarding the reading_line() flag, your use case seems a bit racy to me. Can’t the other thread start reading a line after reading_line() returns false and before your print() statement happens?

You're absolutely right. I had elided the details of the locking from my original use case, but now I look at it again, I see that even what I did have wasn't enough.
I've made a new patch that takes a lock properly. A non-blocking acquisition of the lock will serve the same purpose as looking to see if there is a line being read, and will also prevent the interpreter from starting to read a new line while the text is being printed.
My use case now looks like this.
reading_lock = threading.Lock ()
readline.set_lock (reading_lock)
def describe (indicator):
	if reading_lock.acquire (False):
		print (indicator.description)
		reading_lock.release ()
	else:
		print ()
		print (indicator.description)
		readline.forced_update_display ()

I wonder why you can’t add the locking around input() calls at a higher level, rather than needing to change the readline module itself.