Wrap rl_forced_update_display() and lock readline module while active #68915

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1. BarneyStratford commented on Jul 26, 2015

   

   BarneyStratfordmannequin

   on Jul 26, 2015

   MannequinAuthor

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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 PEP-7, and have made patchtest on my patch.
2. BarneyStratford

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-featureA feature request or enhancementA feature request or enhancement

   extension-modulesC modules in the Modules dirC modules in the Modules dir

   and removed

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Jul 26, 2015
3. vadmium commented on Aug 2, 2015

   

   vadmium

   on Aug 2, 2015

   Member

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   Suggest you integrate your rl_forced_update_display() wrapper and documentation with the existing patches in bpo-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?
4. BarneyStratford commented on Aug 2, 2015

   

   BarneyStratfordmannequin

   on Aug 2, 2015

   MannequinAuthor

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   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 ()
5. vadmium commented on Jun 21, 2016

   

   vadmium

   on Jun 21, 2016

   Member

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   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.
6. vadmium

   changed the title [-]Expand readline module[/-] [+]Wrap rl_forced_update_display() and lock readline module while active[/+] on Jun 21, 2016
7. ezio-melotti

   transferred this issue fromon Apr 10, 2022