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From: MRAB <python@mrabarnett.plus.com>
Newsgroups: comp.lang.python
Subject: Re: Looking for ideas to improve library API
Date: Thu, 26 Nov 2015 12:36:21 +0000
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On 2015-11-25 20:52, Chris Lalancette wrote:
> Hello,
>       I'm currently developing a library called pyiso (
> https://github.com/clalancette/pyiso), used for manipulating ISO disk
> images.  I'm pretty far along with it, but there is one part of the API
> that I really don't like.
>      Typical usage of the library is something like:
>
> import pyiso
>
> p = pyiso.PyIso() // create the object
> f = open('/path/to/original.iso', 'r')
> p.open(f)  // parse all of the metadata from the input ISO
> fp = open('/path/to/file/to/add/to/iso', 'r')
> p.add_fp(fp)  // add a new file to the ISO
> out = open('/path/to/modified.iso', 'w')
> p.write(out)  // write out the modified ISO to another file
> out.close()
> fp.close()
> f.close()
>
> This currently works OK.  The problem ends up being the file descriptor
> lifetimes.  I want the user to be able to do multiple operations to the
> ISO, and I also don't want to read the entire ISO (and new files) into
> memory.  That means that internal to the library, I take a reference to the
> file object that the user passes in during open() and add_fp().  This is
> fine, unless the user decides to close the file object before calling the
> write method, at which point the write complains of I/O to a closed file.
> This is especially problematic when it comes to using context managers,
> since the user needs to leave the context open until they call write().
>        I've thought of a couple ways to deal with this:
>
> 1.  Make a copy of the file object internal to the library, using os.dup()
> to copy the file descriptor.  This is kind of nasty, especially since I
> want to support other kinds of file objects (think StringIO).
> 2.  Just document the fact that the user needs to leave the file objects
> open until they are done.  This is simple, but not super user-friendly.
>
> I'm looking for any ideas of how to do this better, or something I missed.
> Any input is appreciated!
>
Why pass a file descriptor? Why not a filename?