What version of CPython did you compare with? This behavior was added/changed in 2.7.8. CPython has the exact same test and it doesn't seem to be skipped on Windows. Comparing directly with C fdopen does not make sense because os.fdopen isn't just a direct call to fdopen (includes verification of mode, fd, etc).

http://bugs.python.org/issue21191

On Sat, Feb 21, 2015 at 1:58 PM, Matti Picus <matti.picus@gmail.com> wrote:
Looking at this test test_fdopen_keeps_fd_open_on_errors which fails on win32
http://buildbot.pypy.org/summary/longrepr?testname=AppTestPosix.%28%29.test_fdopen_keeps_fd_open_on_errors&builder=own-win-x86-32&build=442&mod=module.posix.test.test_posix2

It also fails when run with python -A, on cpython. It even fails in C when running the code from the MSDN example of fdopen,
https://msdn.microsoft.com/en-us/library/dye30d82.aspx
if you replace the mode flag "r" with "w", windows will happily repopen a read-only file in write mode.

The MSDN documentation is quiet on the issue of mode conflict between fd and a call to fdopen, where posix explicitly will return an error
http://linux.die.net/man/3/fdopen "The mode of the stream (one of the values "r", "r+", "w", "w+", "a", "a+") must be compatible with the mode of the file descriptor." I could find any google results for complaints about win32's reckless behaviour.

Two courses of action are possible:
- fix the test to reflect current cpython and pypy behaviour on win32
- file a bug with cpython and fix our win32 implementation to pass the current test

Any thoughts?
Matti

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