Add support for pathlib.Path

[spacemanspiff2007]

• cattrs version: 0.9.0
• Python version: 3.7
• Operating System: Win

Description

I am trying to serialize a class with a pathlib.Path object

What I Did

from pathlib import Path
import attr

@attr.s
class PathTest():
    my_path: Path = attr.ib(default=Path('c:/test'))

cattr.structure({'my_path': 'c:/my_filepath',}, PathTest)

Error:

ValueError: Unsupported type: <class 'pathlib.Path'>. Register a structure hook for it.

The fix is easy, but I really feel like this is a fundamental type and should be supported out of the box

cattr.register_structure_hook(Path, lambda d, t: Path(d))
cattr.register_unstructure_hook(Path, lambda d: str(d))

[henryiii]

This would be very nice for GenConverter, I'd be willing to take a stab at implementing if this is deemed a good idea. I think it could be cattr.register_unstructure_hook(Path, os.fspath).

Edit: actually, most types don't have converters except for the preconfigured ones, maybe it wouldn't fit with the design?

[Tinche]

I'm not sure this is useful enough to be included by default, but a recipe in the Usages section of the docs would be cool!

[rklasen]

@Tinche , I just stumbled over this too. I think support for Pathlib would be very useful, because the way I understand it, pathlib is supposed to be a modern replacement for os.path.

Oh and thx @spacemanspiff2007 for the fix.

[Tinche]

Two things:

• I don't want cattrs to be a kitchen-sink type of library, since it increases complexity and maintenance burden
• I do want cattrs to be a kind of meta-framework, i.e. a tool that you use as a base to create your own mini-framework for de/serialization

More fundamentally, this approach is also how I think good software should be architected.

As the OP rightly points out, adding support for pathlib.Path is literally two lines of code. So the way we handle this is we add a small section to the Usage Examples page so our users can include this support in their converters if they want it, and learn a little cattrs in the process. I'd merge this PR in a heartbeat.

[spacemanspiff2007]

I absolutely agree with you:
Support for every datatype will unnecessarily inflate cattrs.

However pathlib.Path is the recommended way to deal with file paths and it's part of the python standard installation which makes it a bit different. It is like Enum, which you support out of the box, too and is also not a built-in datatype.

[Tinche]

Alright, I consuled some other folks and I've changed my mind. Let's add this.

[rklasen]

Thank you, I really appreciate this!

[brendan-simon-indt]

Awesome. I use Path everywhere ;-)

BTW, mypy produces the following message.

cattr.register_unstructure_hook( Path, lambda d: str(d) )

"Cannot infer function type argument"

Any suggested "fixes"?

[aarnphm]

This is just a lack of lambda supports for type inference right?
a workaround would look something like:

_convert_str: t.Callable[[t.Any], str] = lambda d: str(d)

cattr.register_unstructure_hook(Path, _convert_str)

[brendan-simon-indt]

Yep, that works. I love the way Python is supposed to be pseudocode like and easy to read and understand 😉 🙄

My lazy workaround was:

cattr.register_unstructure_hook( Path, lambda d: str(d) )       # type: ignore

🤣

[Tinche]

Fixed on main!