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./python Lib/test/test_file.py # passes
./python Lib/test/test_file.py </dev/null # fails
nohup ./python Lib/test/test_file.py # fails
======================================================================
FAIL: testStdin (__main__.COtherFileTests)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "Lib/test/test_file.py", line 160, in testStdin
 self.assertRaises((IOError, ValueError), sys.stdin.seek, -1)
AssertionError: (<type 'exceptions.IOError'>, <type 'exceptions.ValueError'>) not raised
======================================================================
FAIL: testStdin (__main__.PyOtherFileTests)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "Lib/test/test_file.py", line 160, in testStdin
 self.assertRaises((IOError, ValueError), sys.stdin.seek, -1)
AssertionError: (<type 'exceptions.IOError'>, <type 'exceptions.ValueError'>) not raised
2.7 only; works fine in 3.2.

testStdin test is useless, it expects that stdin is always unseekable, but since sys.stdin could be any file, i.e. it can be seekable. 
The patch removes the test.

FTR Python 3 raises "io.UnsupportedOperation: underlying stream is not seekable" on unseekable files and "ValueError: negative seek position -1" on seekable ones, whereas Python 2 raises "IOError: [Errno 29] Illegal seek" on unseekable files and doesn't raise anything for seekable ones.

Elena, your patch looks ok to me, thank you.
Could you sign a contributor's agreement? http://www.python.org/psf/contrib/
(or have you already done so?)

New changeset 17542d36effd by Ezio Melotti in branch '2.7':
#14853: remove test that was making too many assumptions about stdin. Patch by Elena Oat.
http://hg.python.org/cpython/rev/17542d36effd
New changeset c5d847ee354a by Ezio Melotti in branch '3.3':
#14853: remove test that was making too many assumptions about stdin. Patch by Elena Oat.
http://hg.python.org/cpython/rev/c5d847ee354a
New changeset 1d0ff57e392e by Ezio Melotti in branch 'default':
#14853: merge with 3.3.
http://hg.python.org/cpython/rev/1d0ff57e392e 

Fixed, thanks for the patch!

The same issue was reported for test_file2k: issue #23168.
I modified test_file2k to skip sys.stdin.seek() test if stdin is not a TTY. I prefer to skip a test in a specific case, instead of removing it completly.
I propose to add again the test in test_file, with the skip. See test_file.patch for Python 2.7 (I will addapt it to Python 3 if the patch is accepted for Python 2).

Long story short: Don’t change Python 2, and just restore the seek(-1) test in 3.
I agree in general it is better to skip a test than completely remove it. However, in Python 2, the test that Victor proposes to restore is redundant with test_file2k. Sys.stdin is always a Python-2-style file object, which for some reason or other can allow seek(-1).
Also, this bug was opened for Python 2, with the comment "works fine in 3.2". I do not understand why the test was removed from Python 3.
For the Python 3 version of the test, the OSF/1 skip should not be necessary. Also, the truncate() test is probably redundant with the test_optional_abilities() method that I added in revision dc9e5f09ac0c (Issue 22854).

New changeset 8ed183391241f0c73e7ba7f42b1d49fc02985f7b by Irit Katriel in branch 'main':
bpo-14853: add back the stdin test, skip if stdin is redirected (GH-27694)
https://github.com/python/cpython/commit/8ed183391241f0c73e7ba7f42b1d49fc02985f7b

New changeset 897c87045c7435254fd330c6ea48204f056e4afc by Miss Islington (bot) in branch '3.9':
bpo-14853: add back the stdin test, skip if stdin is redirected (GH-27694) (GH-27698)
https://github.com/python/cpython/commit/897c87045c7435254fd330c6ea48204f056e4afc

New changeset 4e0147ec50aa62315c5a9aa7c88c181f57aadf42 by Miss Islington (bot) in branch '3.10':
bpo-14853: add back the stdin test, skip if stdin is redirected (GH-27694)
https://github.com/python/cpython/commit/4e0147ec50aa62315c5a9aa7c88c181f57aadf42

Thanks, Irit! ✨ 🍰 ✨