chore: Simplify host platform logic

[hoodmane]

There is some duplicated code for deciding what the host platform is. This groups it together in util.py.

I was slightly confused in some places whether startswith("win") or == "win32" was desired. I added a new HOST_IS_WIN32 variable since I think it's as explicit as possible that the intended check is specifically for win32 and not for windows in general. Since HOST_IS_WINDOWS is for that.

[hoodmane]

This used to say sys.platform == "win32" but this changes it to startswith("win"). Am I introducing a bug with this behavior change? If so I'd like to add a comment explaining why we use == "win32" here but in architecture we use startswith("win").

[hoodmane]

I made this PR because it was bothering me that the logic here does not exactly match the logic in _compute_platform_ci().

[hoodmane]

This might be a bit excessive but I think it makes the intended distinction between these two checks 100% clear.

[hoodmane]

Well this seems to pass the tests. Maybe there should be more unit test coverage for the difference between sys.platform == win32 and sys.platform.startswith('win')...

[joerick]

This is one of those cases where backward compatibility was preferred over correctness - the 'win32' value applies even on 64-bit windows. See here:

https://github.com/python/cpython/blob/b15a1a6ac6ea0d7792036e639e90f0e51400c2ee/PC/pyconfig.h#L321-L333

(Found via this SO answer)

So there's no functional difference between sys.platform.startswith("win") and sys.platform == 'win32'.

[henryiii]

It's a recommended practice to always use sys.platform.startswith - for some platforms it matters, and it should be possible to "extend" this without breaking checks. MyPy, etc. support this. So my recommendation is never to check via ==. The docs recommend that as an idiom too: https://docs.python.org/3/library/sys.html#sys.platform

I personally don't like pulling this check out - I'd rather it just always be sys.platform.startswith("win")¹. It used to be better to do that statically, though these days you can use Final and it should get assigned a Literal boolean value (though, there's a chance some type checkers will complain about that, some don't seem to like Final deduction!). But I like the explicit way to do it - no need to look up some magic constant when you can specify exactly what you mean in the same number of lines.

Footnotes

1. I prefer avoiding the 32 since it confuses the reader into thinking it has something to do with 32-bit Windows. ↩

[joerick]

Agreed on the literal sys.platform.startswith("win") over a constant - it's clear and means I don't have to look up a constant definition if i'm in doubt. I'm okay with the startswith() convention too - I guess if we had used that everywhere there wouldn't be the 32/64 confusion in the first place!

[hoodmane]

Okay will close this and do another which switches to using startswith("win") throughout since I think that is the consensus here.