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When I have replaced sys.stdin with my own file-like object and I try to do a multiprocessing.Pool(processes=x) I get errors about sys.stdin not having a fileno or close method.
For at least fileno it is described in the docs (http://docs.python.org/library/stdtypes.html#file-objects) that if your object is not a real file you should not implement it.
This happens to me on Mac OS X, I will add the traceback a bit later as I am currently not on my Mac.

Please add the traceback, I can't seem to find any obvious places where this would happen now.
Also, what version are you currently using?
I agree with the fileno, but I'd say close is a reasonable method to implement, especially for stdin/stdout/stderr

The reported error is only reproducible in 2.6
Python 2.6.1 (r261:67515, Jun 24 2010, 21:47:49) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> import multiprocessing
>>> class A:pass
... 
>>> sys.stdin=A()
>>> p=multiprocessing.Pool(processes=2)
Process PoolWorker-1:
Traceback (most recent call last):
 File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/multiprocessing/process.py", line 223, in _bootstrap
Process PoolWorker-2:
Traceback (most recent call last):
 os.close(sys.stdin.fileno())
AttributeError: A instance has no attribute 'fileno'
>>> File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/multiprocessing/process.py", line 223, in _bootstrap
 os.close(sys.stdin.fileno())
AttributeError: A instance has no attribute 'fileno'

This bug was fixed in #5313
--- a/multiprocessing/process.py
+++ b/multiprocessing/process.py
@@ -225,7 +225,8 @@ class Process(object):
 self._children = set()
 self._counter = itertools.count(1)
 try:
- os.close(sys.stdin.fileno())
+ sys.stdin.close()
+ sys.stdin = open(os.devnull)
 except (OSError, ValueError):
 pass
 _current_process = self

Ok, closing then.