How to pass a file opened in Python to a C function for writing?

[Joel Daniels]

Hi there, first time posting here.  I'm kind of a noob with Cython, so bear with me.  :)

I've written a function in C to write to a file, but I need to pass it a file I've already opened in Python.  I assume this is possible, but I can't figure out what the syntax should look like, and Googling hasn't turned anything up.

Thanks in advance!

[Jérôme Kieffer]

In python2 that's pretty easy: there is a function for that:
http://docs.python.org/2/c-api/file.html
FILE* PyFile_AsFile(PyObject *p)

Return the file object associated with p as a FILE*.

If the caller will ever use the returned FILE* object while the GIL is released it must also call the PyFile_IncUseCount() and PyFile_DecUseCount() functions described below as appropriate.

This stops working with python3

Cheers,
--
Jérôme Kieffer <goo...@terre-adelie.org>

[Joel Daniels]

Ah, then I should have mentioned I have to use Python 3.3 :(  Dang, that's a shame.

[Stefan Behnel]

Jérôme Kieffer, 28.02.2014 21:36:

It's a bit more complex than that. In both Py2 and Py3, you can get at the
file descriptor (an integer value). However, you most like don't want to
use that because it doesn't take Python level buffering into account.

Even in Py2, you have to double check that reading from the underlying C
FILE* starts at exactly the same byte as reading from the file object
(seeking, buffering, ...). And this really only works for files opened
directly from the file system, not for arbitrary file-like objects
(BytesIO, urlopen, ...).

So, even in Py2, the value of this C-API is fairly limited, which is why it
was so easily dropped in Py3.

Stefan