[Python-Dev] Pathlib enhancements - acceptable inputs and outputs for __fspath__ and os.fspath()

[Koos Zevenhoven]

On Mon, Apr 11, 2016 at 9:27 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 11 April 2016 at 02:16, Ethan Furman <ethan at stoneleaf.us> wrote:
>>
>> I guess I don't see the point of this.  Either DirEntry's [1] only get
>> partial support (which is only marginally better than the no support pathlib
>> currently has), or stdlib code will need to catch those errors and then do
>> an isinstance check to see if knows what the type is and how to deal with it
>> [1].
>
> What's wrong with only gaining partial support? Standard library code
> that doesn't currently support DirEntry at all will gain the ability
> to support str-based DirEntry objects, while bytes-based DirEntry
> objects will continue to be a low level object that isn't
> interoperable with most other APIs (which is fine - anyone writing low
> level POSIX-specific code can deal with unpacking the values
> explicitly, it just won't happen implicitly anywhere).
>

While I'm also tempted to lean towards 'marginalizing bytes support',
it seems a little bit dangerous to me. Currently, os.path is heavily
based on duck typing of str and bytes, so there may be code out there
that does all kinds of things with paths without knowing whether it
deals with bytes or str objects. If such code gets in contact with
this pathname protocol, it will raise an exception whenever it happens
to be fed a bytes path. That is, if the approach of 'partial support'
is taken.

And still there is the question I just posted in another branch of
this mess: Who should use os.fspathname(...)? If it's os.path.* and
other traditional (low-level?) functions that deal with paths, then
fspathname should, in the name of backwards compatiblity, be able to
deal with bytes and return bytes in those cases.  Otherwise fspathname
would do nothing for you, and all the work of
isinstance/hasattr/whatever would be left to the caller of
os.fspathname (or maybe this is what you want?).

So a somewhat useful fspathname might indeed look something like this:

 def fspathname(pathlike) -> Union[str, bytes]:
     pathname = getattr(pathlike, '__fspathname__', pathlike)
     if not isinstance(pathname, (str, bytes)):
         raise TypeError("your thing is not pathlike")
     return pathname

But maybe it is enough to have the __fspathname__ attribute, and make
fspathname() some internal implementation detail of os.path.* and the
like.

-Koos