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The bug is related to http://bugs.python.org/issue1415 and occurs only
with the latest patch from #1415.
Writing to an invalid fd doesn't raise an exception:
>>> f = open(100, 'w')
>>> f.fileno()
100
>>> f.write("test")
4
However reading or opening an invalid fd for reading and writing raises
an exception.
>>> f = open(100, 'r')
>>> f.read()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/heimes/dev/python/py3k/Lib/io.py", line 1253, in read
 res += decoder.decode(self.buffer.read(), True)
 File "/home/heimes/dev/python/py3k/Lib/io.py", line 756, in read
 current = self.raw.read(to_read)
IOError: [Errno 9] Bad file descriptor
>>> f = open(100, 'w+')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/heimes/dev/python/py3k/Lib/io.py", line 195, in __new__
 return open(*args, **kwargs)
 File "/home/heimes/dev/python/py3k/Lib/io.py", line 169, in open
 buffer = BufferedRandom(raw, buffering)
 File "/home/heimes/dev/python/py3k/Lib/io.py", line 948, in __init__
 raw._checkSeekable()
 File "/home/heimes/dev/python/py3k/Lib/io.py", line 301, in _checkSeekable
 if msg is None else msg)
IOError: File or stream is not seekable.
I expected that fileio_write() raises an exception when fd is invalid:
 n = write(self->fd, ptr, n);
 if (n < 0) {
 if (errno == EAGAIN)
 Py_RETURN_NONE;
 PyErr_SetFromErrno(PyExc_IOError);
 return NULL;
 }

Python 2.5 and probably 2.6 suffer from the same problem although it's
harder to trigger it.
Python 2.5.1 (r251:54863, Oct 5 2007, 13:36:32)
[GCC 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> f = open("/tmp/test", 'w')
>>> f.fileno()
3
>>> os.close(f.fileno())
>>> f.write("test")
>>> f.write("test")
>>>
close failed: [Errno 9] Bad file descriptor
$ ls -la /tmp/test
-rw-r----- 1 heimes heimes 0 2007年11月11日 20:46 /tmp/test

I don't think this is worth fixing; you'll get an I/O error as soon as
I/O is attempted, which is upon the first read() for input, or upon the
firsh (implicit or explicit) flush() on output. You can't tell whether
a fd is valid or not without attempting I/O on it, so trying to test on
each write() call would defeat the purpose of buffering. Testing on
open() is insufficient as long as anybody could call os.close() any time.