[Python-Dev] pathlib - current status of discussions

Nick Coghlan ncoghlan at gmail.com
Sat Apr 16 21:38:11 EDT 2016

On 17 April 2016 at 04:47, Chris Barker - NOAA Federal
<chris.barker at noaa.gov> wrote:
>> 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)

Exactly - inheritance is a really strong relationship that directly
affects the in-memory layout of instances (at least in CPython), and
also the kinds of assumption other code will make about that type (for
example, subclasses are special cased to allow them to override the
behaviour of numeric binary operators when they appear as the right
operand with an instance of the parent type as the left operand, while
with unrelated types, the left operand always gets the first chance to
handle the operation).

When folks don't want to trigger those "this is an <X>" behaviours,
the appropriate design pattern is composition, not inheritance (and
many of the ABCs were introduced to make it easier to implement
particular interfaces without inheriting from the corresponding
builtin types).


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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