Here's a code snippet that shows the problem:

>>> import subprocess
>>> subprocess.Popen(['foo'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/subprocess.py", line 593, in __init__
    errread, errwrite)
  File "/usr/lib/python2.5/subprocess.py", line 1079, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory