[Stackless] why the TaskletExit?

Andrew Dalke dalke at dalkescientific.com
Wed Jan 24 17:50:22 CET 2007

In the main tasklet I create another tasklet, which in turn
creates a 3rd.  The 3rd send some data back to the 2nd then

The 2nd tasklet always does a "receive" on the channel.  Once
the 3rd tasklet finishes I expected either thread 2 to block
or to get an exception derived from Exception.

Instead I get a TaskletExit exception derived from SystemExit,
causing a silent exit of the 2nd tasklet.

  From experimentation it looks like if there's no way for
a tasklet to send anything then all tasklets blocked on a
receive are told to exist.  That's guesswork and I would like
clarification on the behavior.

   Looking through stackless.com and through the source didn't
help me much.

Question: why is TaskletExit called for this case?

   Here's my test code

import stackless

def counter(chan, start, end):
     print "Starting counter"
     while start < end:
         print "Sending value", start
         start = start + 1
     print "Counter finished"

def main():
     chan = stackless.channel()
     # Start a new tasket and put it on the end of the task queue
     stackless.tasklet(counter)(chan, 0, 4)
     print "Created new tasklet"

     while 1:
         # Read from the channel.  If the channel is empty this
         # yields control to another tasklet
         val = chan.receive()
         print "Received", val

if __name__ == "__main__":

Note that if I create a dummy tasklet which only schedules
a few hundred thousand times then I delay the exit (meaning
that the Stackless core knows there's the possibility of the
channel being used).

Note that if I change


then I get

Traceback (most recent call last):
   File "counter.py", line 25, in <module>
   File "counter.py", line 21, in main
     val = chan.receive()
StopIteration: the main tasklet is receiving without a sender available.

I did track that down to  Stackless/module/scheduling.c but couldn't
figure out why StopIteration was better for that case than TaskletExit.

Question 2:  Why is the main tasklet treated differently?

					dalke at dalkescientific.com

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