Re: [Jython-dev] fileno support is not in jython. Reason?

[Alan K. <jyt...@xh...>]

[Mehendran]
> I need to access fileno(file descriptor number) of a file. But that function is
> not available in jython.

Yes it is available; it's just not returning what you (perhaps
mistakenly) expect.

Jython 2.2.1 on java1.4.2_15
Type "copyright", "credits" or "license" for more information.
>>> import socket
>>> s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>>> s.connect( ('localhost', 80) )
>>> s.fileno()
java.nio.channels.SocketChannel[connected local=/127.0.0.1:1622
remote=localhost/127.0.0.1:80]
>>> s.getchannel()
java.nio.channels.SocketChannel[connected local=/127.0.0.1:1622
remote=localhost/127.0.0.1:80]
>>>

Both the socket.fileno() method and the jython-specific
socket.getchannel() method return a java specific
java.nio.channels.SocketChannel object, which is the only thing that
it is possible to return on the java platform.

[Mehendran]
> Is there any specific reason behind that that is omitted currently?

1. It is not omitted
2. Jython cannot return a numeric file-descriptor because java has no
concept of numeric file-descriptors; numeric file descriptors are a
unix-specific thing (but also a paradigm that has been made available
in most C implementations on other OSes); hence cpython, because it's
written in C, supports them on most platforms.

Java instead has its own concept, called Channels, which are used for
several purposes, with the most obvious being multiplexing.

I could have implemented a file-descriptor table which stored these
java.nio.channels.*Channels in an array, which would then be indexed
by an integer. (Which is exactly what C-language file-descriptors are:
an index into an array of struct FILE*). But my opinion of this
approach/hack was swayed by the following conversation with Guido on
python-dev.

http://www.mail-archive.com/pyt...@py.../msg23202.html

What do you need to use the result of fileno() for? If it is for
multiplexing, then simply pass the socket object directly to the
select call; the standard python select API supports passing the
socket directly. In fact, this is the only portable way to do it in
python; any other mechanism is not portable between python
implementations.

If you need to use socket.fileno() for some other purpose, then you'll
have to explain that purpose, so we can recommend the proper way to do
it.

I thought I had documented this on the wiki documentation, but on
checking I see that I did not.

http://wiki.python.org/jython/NewSocketModule

I will update the wiki docs when I get the time; meantime, if anyone
else wishes to update the wiki page, please feel free.

Regards,

Alan.