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List:       python-dev
Subject:    Re: [Python-Dev] The path module PEP
From:       Thomas Wouters <thomas () xs4all ! net>
Date:       2006-01-26 0:41:13
Message-ID: 20060126004113.GD18916 () xs4all ! nl
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On Wed, Jan 25, 2006 at 09:37:04PM +0100, BJörn Lindqvist wrote:

> Inheritance from string (Jason)

> This issue has been brought up before when people were discussing the
> path module. I think the consensus is that, while the inheritance
> isn't pure, practicality beats purity. It would require to big changes
> to Python and would break to much existing code to not extend string.

This is my only problem with the PEP. It's all very nice that subclassing
from string makes it easier not to break things, but subclassing implies a
certain relationship. That relationship doesn't exist, in this case. Having
the string subclass behave differently than strings (consider the __iter__
and join methods) is a bad idea. I can dish up a half dozen contrived
problem cases, but the main reason I don't like it is that it feels wrong.

If the reason to subclass string is that it's too hard to make an object
'string-like' at a low enough level for the C API, I suggest fixing that,
instead. If that means Path has to wait until Python 2.6, then that's too
bad. The inability to feed C functions/types open() non-string objects has
troubled me before, and though I haven't invested a lot of time in it, I
don't quite understand why it isn't possible. Fixing it in a
backward-compatible manner may require a new __hook__, although I think
using __str__ or __unicode__ shouldn't be too problematic.

Even if fixing the "%es"/"%et" etc formats to the arg-parsing methods is
infeasible, I would prefer a new format code for 'string-alike as C string'
over the unpythonic inappropriate subclassing. Although, even if the
subclassing was completely appropriate, I would much rather improve builtin
support for ducktyping than showcase subclassing ;)

But perhaps I've missed that single, all-convincing argument that it has use
subclassing... In that case, could it be added to the PEP? :)

-- 
Thomas Wouters <thomas@xs4all.net>

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