[Python-Dev] file system path protocol PEP

[Chris Angelico]

On Sat, May 14, 2016 at 2:33 AM, Sven R. Kunze <srkunze at mail.de> wrote:
> On 13.05.2016 17:29, Brett Cannon wrote:
>>
>> Purposeful change. It was what I had in my head after I posted my "groups"
>> breakdown email and what Guido suggested as well independently. This helps
>> alleviate any perf worries as type checks in C are pointer checks that are
>> cheap to make compared to attribute lookup. And even if we do get the
>> community to move on to path objects the check is still cheap enough to not
>> worry. And finally, this is simply how we have almost always coded this kind
>> of special-casing at the C level throughout Python and the stdlib, so it now
>> is more inline with standard coding practices than the original proposal in
>> the PEP.
>
>
> Maybe, we have a misunderstanding here. Does "PyUnicode_Check(path)" respect
> subclass hierarchies such as isinstance(path, str) does? Could not find the
> definition of it in the source.
>
> You said it's just a pointer comparison. To me that implies it could not
> respect subclasses as isinstance does. If that is the case, the C
> implementation differs from the Python version in semantics. I'm sorry if I
> misunderstood the whole implementation here.

https://docs.python.org/3/c-api/unicode.html#c.PyUnicode_Check

https://docs.python.org/3/c-api/unicode.html#c.PyUnicode_CheckExact

"Check" accepts subclasses; "CheckExact" doesn't (it's like "type(x)
is str"). The question is, which one SHOULD be being done? What should
this do:

class TmpPath(str):
    def __fspath__(self):
        return "/tmp/"+self
x = TmpPath("foo/bar")
open(x, "w")

Does that respect __fspath__, or respect the fact that it's a string?

ChrisA