cattrs does not unstructure os.PathLike to str

• cattrs version: 23.1.2
• Python version: 3.11.3
• Operating System: Arch Linux

Description

When unstructuring an attrs class where one of the fields is annotated as os.PathLike (and contains a Path), that field remains a Path (in my case, PosixPath) instead of being unstructured as str.

Curiously, annotating the field as Union[str, os.PathLike] or pathlib.Path (without changing the actual data type) resolves the issue.

What I Did

In [1]: import os
   ...: from pathlib import Path
   ...: from typing import Union
   ...: 
   ...: import attrs
   ...: import cattrs
   ...: import cattrs.preconf.json

In [2]: @attrs.define
   ...: class MyClass:
   ...:     foo: Union[str, os.PathLike] = Path("/foo")
   ...:     bar: os.PathLike = Path("/bar")
   ...: 

In [3]: cattrs.preconf.json.make_converter().unstructure(MyClass())
Out[3]: {'foo': '/foo', 'bar': PosixPath('/bar')}

In [4]: cattrs.unstructure(MyClass())
Out[4]: {'foo': '/foo', 'bar': PosixPath('/bar')}

I'm not saying that this must be a bug (rather than merely my mis-usage of cattrs), but I was not able to find anything in the docs that would suggest that I was doing something incorrectly.

Start by reproducing the example with cattrs 23.1.2 and inspect how unstructuring handles the os.PathLike annotation compared with pathlib.Path and Union[str, os.PathLike]. The issue is resolved when the bar field is unstructured to a string consistently and a regression test covers the reported example.