os.PathLike should be a protocol

[catern]

os.PathLike is a special class which overrides __subclasshook__ so that isinstance(x, os.PathLike) returns true if x has an __fspath__ method.

To achieve equivalent behavior when typechecking, os.PathLike should be treated as a protocol.

[srittau]

Interesting idea. Could you provide a PR so that we can explore this approach and see whether it causes any problems?

[bluetech]

I also think it makes sense, os.PathLike is exactly the use case for Protocols IMO.

I tried this patch:

diff --git a/stdlib/2and3/builtins.pyi b/stdlib/2and3/builtins.pyi
index 63b3b04d..6c721d02 100644
--- a/stdlib/2and3/builtins.pyi
+++ b/stdlib/2and3/builtins.pyi
@@ -7,7 +7,7 @@ from typing import (
     Set, AbstractSet, FrozenSet, MutableSet, Sized, Reversible, SupportsInt, SupportsFloat, SupportsAbs,
     SupportsComplex, IO, BinaryIO, Union,
     ItemsView, KeysView, ValuesView, ByteString, Optional, AnyStr, Type, Text,
-    Protocol,
+    Protocol, runtime_checkable
 )
 from abc import abstractmethod, ABCMeta
 from ast import mod, AST
@@ -1153,7 +1153,8 @@ if sys.version_info >= (3, 6):
     # This class is to be exported as PathLike from os,
     # but we define it here as _PathLike to avoid import cycle issues.
     # See https://github.com/python/typeshed/pull/991#issuecomment-288160993
-    class _PathLike(Generic[AnyStr]):
+    @runtime_checkable
+    class _PathLike(Protocol, Generic[AnyStr]):
         def __fspath__(self) -> AnyStr: ...
     def compile(source: Union[str, bytes, mod, AST], filename: Union[str, bytes, _PathLike[Any]], mode: str, flags: int = ..., dont_inherit: int = ..., optimize: int = ...)>
 elif sys.version_info >= (3,):

but this seems not possible as is:

error: Invariant type variable 'AnyStr' used in protocol where covariant one is expected

[hauntsaninja]

I changed this in #4447