click.Path(resolve_path=True) does not resolve non-existing paths under Windows python<=3.9

[dairiki]

Under Windows, python<=3.9, arguments and parameters of type click.Path(resolve_path=True) are not resolved to an absolute path if they refer to non-existing locations.

This behavior started with PR #2094.
Python’s pathlib.Path.resolve() shares this same broken behavior: on Windows, python <= 3.9, Path("missing-file").resolve() == Path("file"); in other environments or with other python versions, Path("missing-file").resolve() correctly produces an absolute path (by prefixing "missing-file" with the current working directory).

Under Windows, with python<=3.9, this test script will fail (at least for me).

from pathlib import Path

import click
from click.testing import CliRunner


@click.command()
@click.argument("path", type=click.Path(resolve_path=True))
def app(path):
    print(path)


def test_resolve_arg():
    runner = CliRunner()
    with runner.isolated_filesystem():
        result = runner.invoke(app, ["missing-file"])
        assert Path(result.output.strip()).is_absolute()

Environment:

• OS: Windows (only)
• Python version: 3.7, 3.8, 3.9
• Click version: >= 8.0.3

[davidism]

Sounds like the fix is to upgrade Python to fix a bug in Python.

[dairiki]

That sounds like a cop-out to me.

It's not always possible to upgrade Python version, and anyway, Click is used by many applications that themselves purport to support their use on Windows with python >= 3.7. If a Click feature (trivial as it may be) doesn't work for a subset of that "supported" space, it makes the feature worse than useless for such projects — use of such a feature means the application is subtly broken in some environments. ("Works everywhere but Windows" — the best kind of bug!)

AFAICT, Click purports to support python >= 3.7 on Windows. If it can be fixed easily in Click (so that Click behaves in the way its documentation claims it does), why not?

I think the fix might be as simple as replacing this:

rv = os.fsdecode(pathlib.Path(rv).resolve())

with

rv = os.fsdecode(pathlib.Path(rv).absolute().resolve())

---

(If a fix is not possible, the documentation¹² should be updated to note that resolve_path does not always work correctly on Windows, python<=3.9, so that users will know not to rely on it if they are targeting that environment.)

Footnotes

1. https://click.palletsprojects.com/en/8.1.x/parameters/#parameter-types ↩

2. https://click.palletsprojects.com/en/8.1.x/api/#click.Path ↩