pathlib __truediv__ returns the "self" type

[dckc]

The pathlib implementation seems to go to some trouble to preserve subtypes in children. This works fine:

import pathlib


class MyPath(pathlib.PosixPath):
    def parity(self):
        return len(str(self)) % 2

root = MyPath('/')
print(root.parity())
child = root / 'there'
print(child.parity())

$ python3 t.py 
1
0

But this declaration isn't parameterized:

    def __truediv__(self, key: Union[str, PurePath]) -> PurePath: ...

so the program above doesn't typecheck:

$ mypy t.py 
t.py:11: error: "PurePath" has no attribute "parity"

[gvanrossum]

Yes, this is currently a problem. We first need to solve python/mypy#1212 and then it should be easy. See also python/peps#89

[dckc]

I found a work-around:

import pathlib as pl
from typing import TypeVar, Generic, Union


Self = TypeVar('Self')


class PosixPathFix(pl.PosixPath, Generic[Self]):
    def __truediv__(self, key: Union [str, pl.PurePath]) -> Self:   # type: ignore
        return super(PosixPathFix, self).__truediv__(key)  # type: ignore


class MyPath(PosixPathFix):
    def parity(self):
        return len(str(self)) % 2


root = MyPath('/')
print(root, root.parity())
child = root / 'there'
print(child, child.parity())

[gvanrossum]

Yeah, until we have SelfType, type ignore is your friend... :-/

…

--Guido (mobile)