Python 2.6.2 (r262:71600, Jun  4 2009, 16:07:26) [C] on aix5
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.popen('cat','w')

<open file 'cat', mode 'w' at 0x1101ab4f8>


Python 3.0.1 (r301:69556, Jun  4 2009, 16:07:22) [C] on aix5
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.popen('cat','w')

<os._wrap_close object at 0x1103e5748> 

Python 3.1rc1 (r31rc1:73054, Jun  1 2009, 10:49:24) [C] on aix5
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.popen('cat','w')

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Python-3.1rc1/Lib/os.py", line 641, in popen
    return _wrap_close(io.TextIOWrapper(proc.stdin), proc)
IOError: [Errno 29] Illegal seek 

This in turn causes help not to work:

Python 3.1rc1 (r31rc1:73054, Jun  1 2009, 10:49:24) [C] on aix5
Type "help", "copyright", "credits" or "license" for more information.
>>> help(open)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Python-3.1rc1/Lib/site.py", line 429, in __call__
    return pydoc.help(*args, **kwds)
  File "/Python-3.1rc1/Lib/pydoc.py", line 1709, in __call__
    self.help(request)
  File "/Python-3.1rc1/Lib/pydoc.py", line 1756, in help
    else: doc(request, 'Help on %s:')
  File "/Python-3.1rc1/Lib/pydoc.py", line 1505, in doc
    pager(render_doc(thing, title, forceload))
  File "/Python-3.1rc1/Lib/pydoc.py", line 1320, in pager
    pager(text)
  File "/Python-3.1rc1/Lib/pydoc.py", line 1340, in <lambda>
    return lambda text: pipepager(text, 'less')
  File "/Python-3.1rc1/Lib/pydoc.py", line 1359, in pipepager
    pipe = os.popen(cmd, 'w')
  File "/Python-3.1rc1/Lib/os.py", line 641, in popen
    return _wrap_close(io.TextIOWrapper(proc.stdin), proc)
IOError: [Errno 29] Illegal seek