Spec: Version and platform checking underspecified about typing HOT 6 OPEN

mypy has actually always supported sys.platform.startswith, since 2016 (in addition to == and !=)
mypy doesn't support os.name
mypy supports comparisons against a potentially indexed or sliced sys.version_info

Details:

• https://github.com/python/mypy/blob/master/mypy/reachability.py#L218
• https://github.com/python/mypy/blob/master/mypy/reachability.py#L291

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I agree we should precisely specify the set of checks that type checkers are required to support. This allows library authors to be confident that the code they write will be understood consistently by type checkers. However, type checkers should be allowed to support additional version or platform checks if they choose to do so, and projects like typeshed may choose to apply more restrictive policies than the typing spec.

As I see it, there are two main motivations for adding support for some pattern:

• Without the pattern, some important pattern cannot be expressed in the type system. For example, if we didn't have platform checks at all, type checkers couldn't use typeshed stubs to warn users if they're using functions that don't exist on their platform. In the terms of PEP 729, this is about making the type system useful.
• The pattern appears commonly in real-world code. For example, supporting if sys.version_info[0] >= 3 isn't strictly necessary (you could write if sys.version_info >= (3,) instead), but this pattern appears in a lot of existing code, so adding support for this pattern eases adoption of typing. In PEP 729's terms, this is about making the type system usable.

Here's some thoughts on the specific checks that should be allowed:

• It makes sense to look at the platforms supported by CPython, as @srittau suggested above. Those are defined in https://peps.python.org/pep-0011/. I think we should aim to make it possible to write typed code on all supported platforms up to Tier 3, though obviously the higher the tier, the higher the priority. This would imply support for various flavors of Linux, MacOS, Windows, WASM, iOS, and FreeBSD.
• sys.platform.startswith probably makes sense to add since it helps FreeBSD (a Tier 3 platform), is already supported by mypy, and doesn't seem difficult to add to other type checkers.
• Mypy has open issues asking for platform.system() (python/mypy#8166) and os.name (python/mypy#13002) but they're not especially popular. Adding support for these would help some real-world code but I'm not sure they're important enough to add to the spec.
• For sys.version_info checks I'd start with the intersection of the checks supported by mypy and pyright (as listed by @erictraut and @hauntsaninja above). If there are other commonly useful checks that aren't supported by both, we can add them.

from typing.

I'm in favor of making the language in the typing spec more precise here, but I'd prefer not to expand the set of supported expression forms here unless there's a really compelling use case identified that cannot be handled with the currently-supported expression forms. A single example doesn't constitute a compelling use case, IMO.

For the record, pyright supports:

• sys.platform <equality_operator> LS
• sys.version_info[0] >= LI
• sys.version_info <comparison_operator> <version_tuple>
• os.name <equality_operator> LS
• Combinations of the above using not, and, and or

Where LI means "literal integerandLS` means "literal string"

<version_tuple> is one of (LI1, ), (LI1, LI2), (LI1, LI2, LI3), (LI1, LI2, LI3, LS4), or (LI1, LI2, LI3, LS4, LI5)

<equality_operator> is one of == or !=

<comparison_operator> is one of <, <=, ==, !=, >, or >=

The above list was established based on real-world use cases that I've run across over the years.

I don't think mypy currently supports os.name or sys.platform, but someone should confirm.

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Mypy supports sys.platform but not os.name. It also supports sys.version_info[0] (and possibly more variants; I haven't checked the mypy code). Example: https://mypy-play.net/?mypy=latest&python=3.10&gist=cbd11992b8631d8bbf8d217de99386e2.

from typing.

I'm in favor of making the language in the typing spec more precise here, but I'd prefer not to expand the set of supported expression forms here unless there's a really compelling use case identified that cannot be handled with the currently-supported expression forms.

If I remember correctly, we have several cases in the stdlib stubs where BSD constants are not correctly guarded. While the BSD platforms are certainly not the most important ones, I think the typing system should support all supported Python platforms properly. Especially, since the recommended form to use sys.platform currently doesn't work with typing, i.e. users following the docs won't have code that's properly type checked.

from typing.

I'm in favor of making the language in the typing spec more precise here, but I'd prefer not to expand the set of supported expression forms here unless there's a really compelling use case identified that cannot be handled with the currently-supported expression forms.

If I remember correctly, we have several cases in the stdlib stubs where BSD constants are not correctly guarded. While the BSD platforms are certainly not the most important ones, I think the typing system should support all supported Python platforms properly. Especially, since the recommended form to use sys.platform currently doesn't work with typing, i.e. users following the docs won't have code that's properly type checked.

Yeah, there seems to be quite a few instances of these platform checks where correctness is lacking due to limited support in the typing spec:

• https://github.com/python/typeshed/blob/main/stdlib/_socket.pyi#L180
• https://github.com/python/typeshed/blob/main/stdlib/_socket.pyi#L414
• https://github.com/python/typeshed/blob/main/stdlib/_socket.pyi#L533
• https://github.com/python/typeshed/blob/main/stdlib/_socket.pyi#L541
• https://github.com/python/typeshed/blob/main/stdlib/mmap.pyi#L103
• https://github.com/python/typeshed/blob/main/stdlib/time.pyi#L15
• https://github.com/python/typeshed/blob/main/stdlib/time.pyi#L26
• https://github.com/python/typeshed/blob/main/stdlib/os/__init__.pyi#L87
• https://github.com/python/typeshed/blob/main/stdlib/os/__init__.pyi#L710

This also isn't a complete list as this was from just a very brief search for "FreeBSD" (plus the Solaris only one I just happened to find along the way). But to @erictraut's point, it's still a somewhat niche use case and I'm not sure if this warrants breaking existing tools such as Pyright.

I'm curious if any actual non-Windows/Linux/Mac users have actually brought up issues related to this as that usually would imply that likely many more have encountered such issues, as well. For the most part, the current platform checks might be good enough, but there were some that basically had to be incorrect due to the limitations in the current spec:

if sys.version_info >= (3, 9) and sys.platform == "linux":
    # Availability: Linux >= 2.6.20, FreeBSD >= 10.1

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