Relative paths for install_requires

[klimburg]

From reading PEP-508 it would appear to me that I should be able to use a relative path to specify an install_requires using a path with no scheme e.g. bar @ foo/bar vs bar @ file://localhost/home/klimburg/myproject/foo/bar

The relevant parts of the grammar from PEP-508 are below

urlspec       = '@' wsp* <URI_reference>
URI_reference = <URI | relative_ref>
relative_ref  = relative_part ( '?' query )? ( '#' fragment )?
relative_part = '//' authority path_abempty | path_absolute | path_noscheme | path_empty

The pkg_resources docs state that the requirements syntax is specified by PEP 508

The syntax of a requirement specifier is defined in full in PEP 508.

But the implementation shows that no scheme paths are explicitly not allowed and raise an InvalidRequirement exception.

...
        if req.url:
            parsed_url = urllib.parse.urlparse(req.url)
            if parsed_url.scheme == "file":
                if urllib.parse.urlunparse(parsed_url) != req.url:
                    raise InvalidRequirement("Invalid URL given")
            elif not (parsed_url.scheme and parsed_url.netloc) or (
                not parsed_url.scheme and not parsed_url.netloc
            ):
                raise InvalidRequirement(f"Invalid URL: {req.url}")

Is not supporting relative paths intentional?

Right now the only way to work around this is using a setup.py file instead of setup.cfg
and inject the full path with os.getcwd like this stackoverflow post suggests.

Perhaps the work to move setuptools to use PEP 621 compliant pyproject.toml can also address this or be done as a follow up.
#1688

I would love to see setuptools and the larger PyPA ecosystem support relative path based dependencies similar to how they are supported in Rust's Cargo.

Finally I think pip install -e . should do editable installs of path based dependencies this would make the development experience while working in a multi-project repository much nicer. Maybe this becomes doable with PEP 660 support work?
#2816

[klimburg]

Looks like this has been discussed previously in this thread and the conclusion was to disallow relative paths from install_requires https://discuss.python.org/t/what-is-the-correct-interpretation-of-path-based-pep-508-uri-reference/2815/18