when does python free memory?

[Robert Eanes]

Hi-
Reading the docs and searching the list archives, it seemed that
python should release memory as soon as the last reference to an
object is deleted.  However, I can't seem to prove this in practice. 
Look at this example:

Python 1.5.2 (#1, May 11 2000, 11:19:54)  [GCC 2.8.1] on sunos5
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> import os
>>> blah = os.popen("ps -ef -o pid -o vsz | grep %s" % os.getpid())
>>> memsize = blah.readlines()
>>> print memsize
['23240 2360\012']
>>> z = ["LSDKFJLSKDFJSDF"]*10000000 #allocate a large list
>>> blah = os.popen("ps -ef -o pid -o vsz | grep %s" % os.getpid())
>>> memsize = blah.readlines()
>>> print memsize
['23240 41424\012']
>>> del z #get rid of the large list
>>> blah = os.popen("ps -ef -o pid -o vsz | grep %s" % os.getpid())
>>> memsize = blah.readlines()
>>> print memsize
['23240 41424\012']

The first number that comes out of the "print memsize" statement is
the pid, the second is the amount of memory used.  It doesn't seem to
be actually freeing the memory that was allocated to variable z.  Now
I played around with it a little more and if I reallocate another list
it does re-use that memory:

>>> x = ["LSDKFJLSKDFJSDF"]*10000000
>>> blah = os.popen("ps -ef -o pid -o vsz | grep %s" % os.getpid())
>>> memsize = blah.readlines()
>>> print memsize
['23240 41424\012']

So apparently python knows that the memory formerly allocated to z is
available, but it doesn't tell the OS.  Is this the case, and if so,
is there any way around it?  I have an application that needs to run
80-150 python processes that can stay around for 15min to an hour, so
I'd like them not to hold onto the maximum amount of memory that they
use throughout the whole life of the process.

Thanks for any help,
-Robert