subprocess question

[jyoung79 at kc.rr.com]

Wondering if anyone could shed some light on the subprocess module?  I'll admit I'm not that great at the shell.

If I was wanting to get the size of the trash (on OS X), I could use:

>>> os.system('du -sh ~/.Trash/')
 11M	/Users/jay/.Trash/
0

Which gives me what I want.  However, I've been reading that it's better to use subprocess.  I did a test like so, but is this a good way to do this?

>>> import subprocess
>>> p = subprocess.Popen(['du', '-sh'], cwd='/Users/jay/.Trash/', stdout=subprocess.PIPE)
>>> out, err = p.communicate()
>>> out
' 11M\t.\n'
>>> err
>>> 

And another question - why can't I use the tilde as a shortcut to the home directory?

>>> p = subprocess.Popen(['du', '-sh'], cwd='~/.Trash/', stdout=subprocess.PIPE) 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 672, in __init__
    errread, errwrite)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 1202, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory: '~/.Trash/'

Thanks for looking at my questions.

Jay