stupid thread question

[Michael Vanier]

Python gurus,

Apologies if this has come up before...

I wrote a little python script using the thread module the other day.  I
wanted to be able to set a flag in the parent thread that would be read in
the child thread.  So I set a flag variable (an integer in this case) in the
parent, created the child thread, and then checked the value of the flag
every time through a loop in the child thread to see if it had changed.  It
turns out that the child thread apparently made a copy of the flag, because
when it changed in the parent it didn't change in the child.

To fix this, I changed the flag to be a list with one integer element.  Now,
when I change the list element in the parent, the child sees it too.  I
gather that what this means is that there is a fundamental distinction
between atomic types like integers and reference types like lists or
dictionaries.  However, since python treats everything as a reference this
seems inconsistent to me.  So what I want to know is: what is the cause of
this behavior, and what is the justification for it?  Or could it be a bug?

FWIW this is on a RH Linux 6.1 system running python 1.6a2 with pthreads (I
assume).  

TIA,

Mike

--------------------------------------------------------------
Mike Vanier	mvanier at bbb.caltech.edu
Department of Computation and Neural Systems, Caltech 216-76
GNU/Linux: We can't lose; we're on a mission from God.