feat: Overrides mechanism

[henryiii]

This is an initial idea for the overrides system.

The overrides array would requires the select key set:

[[tool.scikit-build.overides]]
select = {python-version=">=3.8", implementation-name="cpython"}

The keys would be based on PEP 508:

• python-version: a SpecifierSet that matches the Python version (note: PEP 508 splits this into python_full_version, maybe we should too, and make them mutually exclusive)
• implementation-name
• implementation-version
• platform-system
• platform-machine

The other values (besides python-version) would support regular expressions.

All other values in that array entry are then used over the original values if the select matches. Table/array merging should be considered, but might not make it in the first version. At least one value is required.

[henryiii]

This will still come somewhat soon, but the key reason I was going to prioritize it (and some cleanup in preparation has already gone into 0.4.2) was for conditional limited API / stable ABI, but we were able to support that naturally without overrides by using the value (cp312 only affect 3.12+, for example). So this might be bumped down in priority for a bit.

[LecrisUT]

Is the naming convention of select standardized? I find it a bit unintuitive. I like this syntax more and I think ansible uses something similar. Specifically an if, when verb would be more intuitive.

Also unclear how you would add more than one overrides

[henryiii]

It's not set in stone. It's based on mypy:

[[tool.mypy.overrides]]
module = 'packagename'
...

And cibuildwheel:

[[tool.mypy.overrides]]
select = 'cp*'
...

Though it's not really the same as either one. "module" is what you are matching against, and "select" sort of is (it's matched against "selectors" in cibuildwheel terms).

[[tool.scikit-build.overrides]]
if = {python-version=">=3.10"}
...

You can also write it:

[[tool.scikit-build.overrides]]
if.python-version=">=3.10"
...

I rather like if, since it's a Python keyword, it would be unlikely to collide with future config options. It also feels more Pythonic to me than when. (Though I like when too)

[henryiii]

Also unclear how you would add more than one overrides

Double brackets in toml are arrays. So you can add it as many times as you like.