On Linux:

>>> p = subprocess.Popen("ls", stdout=subprocess.PIPE)
>>> p.stdout.read()
b'...'
>>> p.stdout.seek(0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/heimes/dev/python/py3k/Lib/io.py", line 809, in seek
    pos = self.raw.seek(pos, whence)
IOError: [Errno 29] Illegal seek
>>> p.stdout.read()
b''

On Windows p.stdout.seek(0) does neither raise an error nor works as one
might expect it. The second read() returns an empty byte string, too.