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 #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 #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.