the _pyio module at line 16 tries to check whether it is running on Windows platform, by doing:

if os.name == 'win32':
    from msvcrt import setmode as _setmode
else:
    _setmode = None

However, the string returned by os.name is 'nt' and not 'win32' (the latter is returned by sys.platform). Therefore, the value is always False and the setmode function from mscvrt module is never imported.

Thank you.
Cheers,

Cosimo

To make _pyio correspond to the C version I've added

sys.platform in {'win32', 'cygwin'}

condition. See the attached pyio_setmode.diff

It is not clear why the absence of _setmode(fd, os.O_BINARY) is not detected by tests.

(a) a corresponding test should be added
(b) OR _setmode() call should be removed from both Python and C io versions

It is not clear why the absence of _setmode(fd, os.O_BINARY)
is not detected by tests.

It's only a problem when an existing text-mode file descriptor is passed in. For example, in text mode the CRT handles b'\x1a' as an EOF marker:

    Python 3.5.0b4 (v3.5.0b4:c0d641054635, Jul 26 2015, 07:11:12)
    [MSC v.1900 64 bit (AMD64)] on win32
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import os, _pyio
    >>> _ = open('testfile', 'wb').write(b'abc\x1adef')

    >>> fd = os.open('testfile', os.O_RDONLY|os.O_TEXT)
    >>> f = _pyio.open(fd, 'rb')
    >>> f.read()
    b'abc'
    >>> f.close()

    >>> f = _pyio.open('testfile', 'rb')
    >>> f.read()
    b'abc\x1adef'

The setmode call ensures a file descriptor is in the expected binary mode:

    >>> import msvcrt
    >>> fd = os.open('testfile', os.O_RDONLY|os.O_TEXT)
    >>> old_mode = msvcrt.setmode(fd, os.O_BINARY)
    >>> old_mode == os.O_TEXT
    True
    >>> f = _pyio.open(fd, 'rb')
    >>> f.read()
    b'abc\x1adef'

New changeset 687da8760a58 by Serhiy Storchaka in branch '3.5':
Issue bpo-24881: Fixed setting binary mode in Python implementation of FileIO
https://hg.python.org/cpython/rev/687da8760a58

New changeset 2dd9294f679d by Serhiy Storchaka in branch 'default':
Issue bpo-24881: Fixed setting binary mode in Python implementation of FileIO
https://hg.python.org/cpython/rev/2dd9294f679d

Good catch. Thank you for your report Cosimo. Thank you for your patch Akira.

FWIW this patch broke the _pyio module on Cygwin, as the msvcrt module is not built on Cygwin. AFAICT this is only a problem for Python built with MSVCRT, which Cygwin does not use. When test case works as expected on Cygwin without this.

"FWIW this patch broke the _pyio module on Cygwin,"

Please open a new issue, this one is closed.