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[Python-Dev] pathlib - current status of discussions

Chris Barker - NOAA Federal chris.barker at noaa.gov
Sat Apr 16 14:47:26 EDT 2016 
> On Apr 13, 2016, at 8:31 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>
>>>   class Special(bytes):
>>>       def __fspath__(self):
>>>         return 'str-val'
>>>   obj = Special('bytes-val', 'utf8')
>>>   path_obj = fspath(obj, allow_bytes=True)
>>>
>>> With #2, path_obj == 'bytes-val'. With #3, path_obj == 'str-val'.
>
> In this kind of case, inheritance tends to trump protocol.

Sure, but...

> example, int subclasses can't override operator.index:
...
> The reasons for that behaviour are more pragmatic than philosophical:
> builtins and their subclasses are extensively special-cased for speed
> reasons,

OK, but in this case, purity can beat practicality. If the author
writes an __fspath__ method, presumably it's because it should be
used.

And I can certainly imagine one might want to store a path
representation as bytes, but NOT want the raw bytes passed off to file
handling libs.

(of course you could use composition rather than subclassing if you had to)

-CHB
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