Support for detecting the flavour of pathlib subclasses

[barneygale]

Feature or enhancement

Provide a way of checking whether a user subclass of pathlib.PurePath and Path uses POSIX or Windows semantics

The Problem

Now that #68320 is resolved, users can subclass pathlib.PurePath and Path directly:

import pathlib

class MyPath(pathlib.Path):
    pass

path = MyPath('foo')

However, there's no (public) procedure to detect whether an instance of MyPath uses POSIX or Windows semantics.

A non-public way of doing it:

path._flavour is posixpath  # Check whether the path object uses POSIX semantics
path._flavour is ntpath  # Check whether the path uses Windows semantics
path._flavour is os.path  # Check whether the path uses the current OS's semantics

Note that checking isinstance(path, PurePosixPath) (etc) won't work, as user subclasses of PurePath and Path do not have the POSIX- and Windows-specific subclasses in their class hierarchy.

The Proposal

Make the _flavour attribute public; document it. Possible names:

• flavour (simply remove the underscore)
• pathmod (used internally in older pathlib versions)
• module (used in the longstanding thirdparty path package)

Alternatives

We could make the PurePosixPath and PureWindowsPath classes 'protocols', which support isinstance() checks even when the passed object isn't a conventional subclass. But:

1. The implementation will be more complex
2. It could open a can of worms about whether PurePosixPath and PureWindowsPath should be proper protocols, and not directly instantiable.

A further alternative would be to add an is_posix() method, which gels pretty nicely with the existing is_*() and as_posix() methods.

Linked PRs

• GH-100502: Add pathlib.PurePath.pathmod attribute #106533

[merwok]

pathmod is not a bad name! It makes it clear that the value is (normally) one of the path modules, and Python being Python it doesn’t prevent someone using another kind of object for custom needs.

flavour is a bit more vague but okay; accessor is also available!

[merwok]

About is_posix: I don’t think the name is clear enough. Without reading docs, it could be interpreted as a question about the current platform, or the path module, or a check for some posix spec conformity, or even a question about the form of the paths?!
But maybe my worry is unfounded and it would be fine, we have os.name == 'posix' and os.path is posixpath.

[barneygale]

Thanks for the feedback! pathmod sounds good to me. It occurs to me that the pathlib documentation already refers to PureWindowsPath, PurePosixPath etc as "flavours", and so re-using that term could be confusing.

Agree also on is_posix()

[barneygale]

Implemented in c6c5665 / #106533

[barneygale]

@merwok I'm beginning to have doubts about the solution I implemented. I think the new, internal PurePathBase class ought to support syntax customization beyond a fat "NT or POSIX" switch, and that perhaps the path module should be an internal implementation detail of PurePath, rather than an attribute of PurePathBase. Would you mind if I revert #106533? It's only been available in 3.13 alphas so far, so it's safe to do so.