Why is an un-inferred dependency a warning, rather than an error, and should it stay that way?

[TalAmuyal]

TL;DR: Why is it a warning, rather than an error, and should it stay that way?

Issue #17324 discusses some inconsistency for a specific warning type - an un-inferred dependency.
That is, when Pants fails to infer a dependency based on an import statement (Python) it produces a warning.

For example, I can make a typo when adding an import statement to a package (and thus refer to a package that doesn't exist) and will still have an in-memory cached result.

I'd expect an un-inferred dependency to produce an error, since (I imagine) an un-inferred dependency can have negative effects on the end result like missing dependencies in an artifact (like a Pex or a Wheel) or missing a key for invalidation (thus hurting the correctness guarantee).

That might be especially true when a package is technically optional and has a side-effect by simply being installed.

[thejcannon]

For context:
It used to be completely silent until I added the ability to warn/error. After a while we bumped it to warning. I wouldn't be opposed to bumping it to error, but that's hard to stomach for people relying on the default.

[TalAmuyal]

Sounds good, what about recommending in the guide to set it as error for new projects?
Do you think it should be done?

[thejcannon]

Id +1 that but I'd want other maintainers to chime in.

[thejcannon]

You can open a PR for our docs, they are in-repo!

[benjyw]

I'm not convinced it is an error in every case - you could, for example, be importing from a package that you know is available at runtime. Making this an error would probably require support for dependency scopes, so we can express the idea of a "provided" dependency.

But allowing repos to opt-in (or out, eventually) of this being an error sounds like a good idea.

All that said, I'm not sure I see the correctness issue? If you had a typo, for example, then when you fix the typo that file is invalidated.

[thejcannon]

For those really special cases, you can mark them as # pants: no-infer-dep

I'd love for error to be the default. It's the safest option, and with pragma ignores, doesn't limit your options.