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

This quick and dirty fix in pydoc.py makes so it no longer aborts help.

(less behaves somewhat strange for some commands but that is better than
no help at all)

def pipepager(text, cmd):
    """Page through text by feeding it to another program."""
    import subprocess
    pipe=subprocess.Popen(cmd,stdin=subprocess.PIPE).stdin
    #pipe = os.popen(cmd, 'w')
    try:
        pipe.write(bytes(text,sys.getdefaultencoding()))
        #pipe.write(text)
        pipe.close()
    except IOError:
        pass # Ignore broken pipes caused by quitting the pager program.

Ouch, this is quite annoying. I will try to fix this before the final
release.

Here is my current interpretation:

subprocess uses os.pipe() to create the file handles used for
communication. These handles normally always raise an error ([Errno 29]
Illegal seek) when trying to seek() on them, which the IO lib interprets
as meaning the stream is not seekable, which it then handles fine.

However, if the first seek() succeeds, the IO lib thinks the stream is
seekable and treats any subsequent seek() failure as an error which it
reports to the user. It may be what you are witnessing.

Can you try applying the following patch?

Just before, could you try to type the following commands:

>>> r, w = os.pipe()
>>> os.lseek(r, 0, 1)

That fails consistently:
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
>>> r,w=os.pipe()
>>> os.lseek(r,0,1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 29] Illegal seek
>>>
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
>>> r,w=os.pipe()
>>> os.lseek(r,0,1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 29] Illegal seek
>>>
Python 3.1rc2 (r31rc2:73411, Jun 15 2009, 10:56:49) [C] on aix5
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> r,w=os.pipe()
>>> os.lseek(r,0,1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 29] Illegal seek
>>>

Ah, then I don't know what happens...

Maybe this has something to do with it?

Python 3.1 (r31:73572, Jul  9 2009, 16:28:28) [C] on aix5
Type "help", "copyright", "credits" or "license" for more information.
>>> open("/dev/tty","a").seekable()
True
>>>

Perhaps, although I don't know where /dev/tty would be involved in
os.popen().

For reference, under Linux I get:
>>> open("/dev/tty","a")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IOError: [Errno 29] Illegal seek

/dev/tty is probably similar to stdout, at least when python is started
without redirection.

What is the output of
>>> sys.stdout.seekable()
when run from an interactive interpreter?
Does it depend on the active console? (xterm, ssh...)

Python 3.1 (r31:73572, Jul  9 2009, 16:28:28) [C] on aix5
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> import sys
>>> sys.stdout.seekable()
True
>>>

I can access the machine only through ssh.

The problem is that the fileio struct in Modules/_io/fileio.c defines
the 2-bit seekable field as int.

From the C99 standard, §6.7.2: for bit-fields, it is
implementation-defined whether the specifier int designates the same
type as signed int or the same type as unsigned int.

Contrary to gcc, both xlc and suncc default to unsigned.

Adding 'signed' solves the problem (and also issue #6348).

Patch attached.

Thanks for the patch! Committed in r75007, r75008, r75009, r75010.
If there's any problem, please reopen.

Fantastic. Applied the patch and it solved the problem with xlc 8.0 on
AIX 5.3.