Implement sf_open_fd?

[mgeier]

libsndfile has sf_open_fd() to read a file which was already opened and is available as a file descriptor.

This could be handled like in the wave module: http://docs.python.org/3/library/wave.html

The first argument to the SoundFile constructor would have to be renamed from name to file.

[mgeier]

The logic will have to consider all three methods: sf_open(), sf_open_virtual() (as introduced in #1) and sf_open_fd().

[bastibe]

Are Python file descriptors convertible to C file descriptors? Otherwise, this would have to be handled with sf_open_virtual.

I don't quite see the utility of having this. Can you give an example where this might be useful?

[mgeier]

Are Python file descriptors convertible to C file descriptors?

It looks like they are. You can get a file descriptor from a Python file object with the method fileno().
I'm not quite sure what happens if the file object implements some kind of buffering ... this might break things.
You can also get file descriptors directly, e.g. with os.open().

Otherwise, this would have to be handled with sf_open_virtual.

As discussed in #1, this can only be used if seek() is supported. With sf_open_fd(), however, this should work without seek(), so the streaming feature discussed in #1 should be possible.

I don't quite see the utility of having this. Can you give an example where this might be useful?

To be honest, I don't actually have a use for this, I just think it's good to implement as many libsndfile features as possible, it might be useful to someone sometime.
But to give an example anyway, if someone wants to read from a pipe, this should be only possible with sf_open_fd(), at least according to http://stackoverflow.com/questions/8370499/audio-file-format-that-can-be-written-without-seeking (though I didn't actually try it myself).

I implemented this prototypically in #18, commit f0c7735, and it doesn't add a lot of code.
It just needs a new argument closefd and these two lines (and one line in the cdef):

elif isinstance(file, int):
    self._file = _snd.sf_open_fd(file, self.mode, info, closefd)

BTW, I removed the argument virtual_io because it's redundant, we can automatically check if a file-like object was provided (and the check was even already implemented).

And also BTW, the virtual_io feature is broken, because it checks for a read attribute but then uses the readinto() method. The latter seems not to be available for all types of file objects, maybe this should be checked and read() should be used as a fallback?