Low level file descriptors and high-level Python files

Cameron Simpson cs at zip.com.au
Wed Sep 2 00:01:42 CEST 2015

On 01Sep2015 11:56, random832 at fastmail.us <random832 at fastmail.us> wrote:
>On Tue, Sep 1, 2015, at 10:57, Steven D'Aprano wrote:
>> Q3:  I could probably answer Q2 myself if I knew how to check whether a
>> fd
>> was open or not. With a file object, I can inspect file_obj.closed and it
>> will tell me whether the file is open or not. Is there an equivalent for
>> file descriptors?
>Well, if you try to call os.close, or any other operation for that
>matter, it will raise an OSError with errno=EBADF.

os.fstat might be safer. It won't have side effects.

As additional remarks:

Underwhat circumstances would you imagine probing an fd like this? For what 
purpose? It feels like a code smell for know having enough situational 
awareness, and then you're into guesswork world.

One circumstance where you might use fdopen and _not_ want .close to close the 
underlying service is when you're handed a file descriptor over which you're 
supposed to perform some I/O, and the I/O library functions use high level 
files. In that case you might want code like this:

  fd2 = os.dup(fd)
  fp = open(fd2, 'a+b') # or whatever mode
  ... do stuff, perhaps passing fp to a library function ...

fd2 is not closed, but fd is still open for further use.

Cameron Simpson <cs at zip.com.au>

This is not a bug. It's just the way it works, and makes perfect sense.
        - Tom Christiansen <tchrist at jhereg.perl.com>
I like that line. I hope my boss falls for it.
        - Chaim Frenkel <chaimf at cris.com>

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