Subprocess confusion: how file-like must stdin be?

[Cameron Laird]

In article <slrneeassv.c4m.nick at irishsea.home.craig-wood.com>,
Nick Craig-Wood  <nick at craig-wood.com> wrote:
>Dennis Lee Bieber <wlfraed at ix.netcom.com> wrote:
>>  On Thu, 17 Aug 2006 17:16:25 +0000, claird at lairds.us (Cameron Laird)
>>  declaimed the following in comp.lang.python:
>> 
>> > Question:
>> >   import subprocess, StringIO
>> > 
>> >   input = StringIO.StringIO("abcdefgh\nabc\n")
>> 
>>  	Here you override the builtin function "input()"
>> >       # I don't know of a compact, evocative, and
>> >       # cross-platform way to exhibit this behavior.
>> >       # For now, depend on cat(1).
>> >   p = subprocess.Popen(["cat"], stdout = subprocess.PIPE, 
>> > 				stdin = response)
>> 
>>  	Here you specify the non-existant "response" 
>
>Assume the OP meant to write this
>
>>>> import subprocess, StringIO
>>>> inp = StringIO.StringIO("abcdefgh\nabc\n")
>>>> p = subprocess.Popen(["cat"], stdout = subprocess.PIPE, stdin = inp)
>Traceback (most recent call last):
>  File "<stdin>", line 1, in ?
>  File "/usr/lib/python2.4/subprocess.py", line 534, in __init__
>    (p2cread, p2cwrite,
>  File "/usr/lib/python2.4/subprocess.py", line 830, in _get_handles
>    p2cread = stdin.fileno()
>AttributeError: StringIO instance has no attribute 'fileno'
>>>> 
			.
			.
			.
Yes; my apologies for the confusion I introduced by "editing
for publication", and doing it badly.

Your interactive session does indeed exhibit the behavior that
puzzles me.  My expectation was that StringIO and the std* 
parameters to Popen() were made for each other; certainly there
are many cases where stdout and stderr can be redirected *to* a
StringIO.  Is it simply the case that stdin demands a more
file-like object?  While that disappoints me, I certainly can
program around it.  My question, then:  does stdin effectively
require something really in the filesystem, or perhaps the
stdout of a previous subprocess?  Is there no built-in way to
feed it an in-memory construct?