I suggest that posixpath.c:_fd_converter() should  PyObject_AsFileDescriptor() to convert a Python object to a fd integer. With PyObject_AsFileDescriptor() functions such as os.chmod() can be called with an opened file as first argument:

Now:

  with open("somefile") as f:
     os.chmod(f.fileno(), 0o644)

With PyObject_AsFileDescriptor():

    with open("somefile") as f:
     os.chmod(f, 0o644)

_fd_converter() also has more elaborate overflow checks. These checks should be added to PyObject_AsFileDescriptor(), too.

Do we want the low-level os.write() to work with file objects, or sockets?

This issue looks to be related to #18269.

os.write() already works with file and sockets object but you have to call ob.fileno() first. The select module uses PyObject_AsFileDescriptor() all over the module to get the file descriptor number from file and socket objects.

Let's unify os' and select's behavior!

> Let's unify os' and select's behavior!
OK, let's do that on Windows first :-)

> OK, let's do that on Windows first :-)

Gotcha :( Damn you winsock.dll!

> OK, let's do that on Windows first :-)

For the PEP 433, I did something like that in os.get_cloexec() and os.set_cloexec():

http://hg.python.org/features/pep-433/file/f32c2b09f332/Modules/posixmodule.c#l10198

On Windows, I added two code paths: one for HANDLE (to support sockets), one for file descriptors (classic files). It means that you need a C function supports HANDLE as input type, rather than int (file descriptor).

Can't we raise a nice error message when we get a socket, whereas the function does not support HANDLE? What is the current message on Windows when passing sock.fileno() to a function expecting a file descriptor? Would it be worse if Python implictly call .fileno() on a socket?

Victor, do we want to support "with open("somefile") as f: os.chmod(f, 0o644)"? The feature request has been languishing for 8 years.

I like calling .fileno() explicitly. It seems like it's not really needed to call it implicitly, since this issue doesn't get much activity. I suggest to reject it.

Fine with me! Thanks for your feedback.