[Tutor] Should I be thinking of threads for this ?

Kent Johnson kent37 at tds.net
Sat Aug 27 15:47:44 CEST 2005

Nick Lunt wrote:
> Hello folks,
> I have the following code taken from the Twisted examples -
> [code]
> # filewatcher.py
> from twisted.application import internet
> def watch(fp):
>         fp.seek(fp.tell())
>         for line in fp.readlines():
>                 sys.stdout.write(line)
> import sys
> from twisted.internet import reactor
> s = internet.TimerService(1.0, watch, file(sys.argv[1]))
> s.startService()
> reactor.run()
> s.stopService()
> [/code]
> I find this piece of code amazing and I am keen to put it to use.
> If I run './filewatcher.py myfile' it will print out any changes made to 
> 'myfile', very similar to 'tail -f' .
> Now if I want to have this program monitor several files at once I could 
> run './filewatcher.py file1 file2 filex' or './filewatcher.py file1 & 
> ./filewatcher file2 & etc' both with minor modifications to the code,  
> but I think that could cause performance problems relating to the OS.
> So I'm thinking I will need to learn python threads (no bad thing) 
> instead, but Im hoping that someone could tell me if that seems the best 
> way to go ?

What performance problems you you anticipate? I don't know much about Twisted but my understanding is that tasks are run in a single thread when they are ready. In your case you are scheduling a simple task to run every second. I would think that you could schedule several such tasks and they would each run every second. If your task were time-consuming you might have to worry about doing something different but in this case I think it will be fine. Just try something like

for name in sys.argv[1:]:
  s = internet.TimerService(1.0, watch, file(name))

I suppose if you used threads there would be the possibility of a context switch while watch() is running, if one thread becomes blocked on I/O then another thread can run. I don't know how Twisted handles this - I think you have to wrap the file and stdio in Twisted object that handle the blocking. twisted.protocols.basic.FileSender and twisted.internet.stdio.StandardIO look like they may be starting points.


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