[flake8-type-checking] Allows TC001-004 to quote more expressions

[Daverball]

This idea came up during the discussion of #14676

This is not yet fully baked, but far enough along to clearly see the vision, so I would appreciate some feedback, before I put more work into this @AlexWaygood @MichaReiser

Summary

This adds additional settings to allow TC001-004 to be more liberal about moving imports into type checking blocks.

• Allowing cast type expressions to be quoted is a more conservative approach to TC006.
• Allowing annotated type alias value expressions to be quoted is a more liberal approach to TC007.

Test Plan

cargo nextest run

[Daverball]

I'm guessing this is not the right way to cross reference another rule, do I have to manually link to the rule's page? It seems like all the other references to other settings in here manually link via fragments, rather than relying on the docs to create the correct cross reference.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[AlexWaygood]

Interesting, thanks for working on this! My overall reaction is that this is a good idea. Do we need another configuration knob, though? What's the profile of somebody who wants their annotations to be quoted if it removes the need for a runtime import, but doesn't want the first argument to cast() quoted even if it would remove the need for a runtime import?

I get that the quote-annotations setting isn't particularly well named for the purpose, because the first argument to cast() is not an annotation. But I think it's much less confusing to users if we keep the number of configuration options to a minimum. So I would simply expand the quote-annotations option so that it also quotes the first argument to cast() (where it would remove the need for a runtime import), and expand the docs for TC001-004 to state that this is the new behaviour, and expand the docs for the setting to state that this is the new behaviour. The only disadvantage I can think of, other than the name of the setting not being ideal, is that this would count as an expansion in scope of these rules -- so we might have to make the change as part of the next minor release rather than in a patch release.

It looks like your patch already does the right thing for all of these, but if you haven't already added some, it would be great to add tests for them, because it would obviously be incorrect to quote either of them:

from typing import Annotated, TypeAlias
from expensive import Foo
from expensive2 import Foo2

Y: TypeAlias = Annotated[Foo, "metadata"]
some_object = Y()

class Z(Annotated[Foo2, "metadata"]): ...

[Daverball]

Interesting, thanks for working on this! My overall reaction is that this is a good idea. Do we need another configuration knob, though? What's the profile of somebody who wants their annotations to be quoted if it removes the need for a runtime import, but doesn't want the first argument to cast() quoted even if it would remove the need for a runtime import?

I get that the quote-annotations setting isn't particularly well named for the purpose, because the first argument to cast() is not an annotation. But I think it's much less confusing to users if we keep the number of configuration options to a minimum. So I would simply expand the quote-annotations option so that it also quotes the first argument to cast() (where it would remove the need for a runtime import), and expand the docs for TC001-004 to state that this is the new behaviour, and expand the docs for the setting to state that this is the new behaviour. The only disadvantage I can think of, other than the name of the setting not being ideal, is that this would count as an expansion in scope of these rules -- so we might have to make the change as part of the next minor release rather than in a patch release.

I do think we want some additional knobs, but I do agree that having three separate toggleable switches is awkward, that's the main reason I called this half-baked. I think a sliding scale could make sense, at the low end you would only quote casts, then runtime evaluated annotations too and finally annotated type aliases at the high end. I do agree that there's likely nobody that would have e.g. annotations turned on but casts turned off, but I do think it's likely that there are people that want to have casts turned on, but annotations and type aliases turned off.

Especially type aliases is something some people might not want to ever turn on if they have a bunch of them in a single module that is used by other modules at runtime. They could just turn off TC001-004 in that module, but that seems awkward.

Maybe we add a new setting and deprecate quote-annotations. We can call it quote-type-expressions with settings of none, safe, balanced and eager for none, just casts, casts and annotations and everything. That way we leave ourselves the option to add additional things to the safe category in the future.

It might also make it easier to argue for making safe the default setting, rather than never.

Then we can emit a warning for people that have preview turned on and still use the old setting. People that have preview turned off will return the old default of never.

It looks like your patch already does the right thing for all of these, but if you haven't already added some, it would be great to add tests for them, because it would obviously be incorrect to quote either of them:

from typing import Annotated, TypeAlias
from expensive import Foo
from expensive2 import Foo2

Y: TypeAlias = Annotated[Foo, "metadata"]
some_object = Y()

class Z(Annotated[Foo2, "metadata"]): ...

I do plan to add more tests. I added just enough to make sure that the type alias part works correctly, since that is the most difficult one to get right.

[Daverball]

@AlexWaygood @MichaReiser Any inputs on the idea of using a sliding scale for configuring what kind of type expressions we're allowed to quote? This would be similar in spirit to lint.flake8-tidy-imports.ban-relative-imports where you can choose between all and parents, except you can choose between four options.

[AlexWaygood]

Sorry for the delayed response. It makes me sad to have to deprecate a configuration setting, because it's quite disruptive for our users. However, I think your plan makes sense; I can't think of a better one. I agree that there is probably a group of people who want to quote their cast() type expressions when it can remove the need for imports, but never want to quote their parameter/return/variable annotations. The crucial difference is that type expressions passed to cast() are not introspectable at all, whereas annotations are introspected by many runtime tools.

We can bikeshed over the names of the sliding scale, but your overall plan sounds good to me.

We'll probably have to do the deprecation as part of the next minor release (I think probably in early January, but not sure), but there's no harm in working on a PR now. It'll make the minor release less frantic if it's already ready and has already been reviewed :-)

[Daverball]

One loosely related thing I have been thinking about is the from __future__ import annotations use-case. Currently from a UX perspective it is not as nice as quote-annotations = true since you have to rely on other rules to enforce its use which will lead to redundant violations/fixes and potentially imports getting moved back and forth.

Before I had this idea I still had intended to deprecate this setting and add a new one in order to allow the strategy to be one of move_import, quote_annotations and add_future_import, but this interacts poorly with this change, since this selection of strategies only makes sense for annotations, not for casts or type aliases.

We could have a separate setting for add-future-import, but this somewhat steps on flake8-future-annotations' toes. The same goes for adding from __future__ import annotations to lint.isort.required-imports. But it still seems preferable over adding TC100, since that would once again be potentially bad for people that select ALL. But we could also choose to not support the use-case and instead direct people towards the isort setting as a reasonable alternative.

[AlexWaygood]

Hmm, I kinda think the status quo of telling people to use the isort setting if they want to have the __future__ import everywhere is ~fine. The violations flagged by different rules should all get fixed in a single Ruff invocation. if you have autofixes enabled. There's value in having focussed rules that do one specific thing each. But if we don't document in the flake8-type-checking rules that you might want to also enable that isort option, then I'd absolutely be open to some docs improvements. (We might do already, I haven't checked!)

[Daverball]

@AlexWaygood My concern are imports that will be flagged by TC004 for runtime use at the same time as the missing __future__ import is flagged, fixing the future import gets rid of the TC004 violation, so if the fix moved the imports, you now have TC001-003 violations, so you need another run with fixes to move them back, which might mess with the ordering/formatting of the imports, when there was never a need to mess with the imports to begin with.

Which I assume is also the reason why TC001-004 have been repurposed by ruff to do the quoting of the annotation and moving of the import in a single step, rather than use a separate TC2xx rule to flag annotations like the flake8 plugin does.

That being said, the __future__ import use-case is certainly not as bad as the quoting use-case, since it's less likely to actually end up being a problem, unless you intend to switch to adding the future import, in which case the imports should already be wrong unless you also intend to adopt the TC rules at the same time. So I'm fine with leaving things as they are.

[AlexWaygood]

@AlexWaygood My concern are imports that will be flagged by TC004 for runtime use at the same time as the missing __future__ import is flagged, fixing the future import gets rid of the TC004 violation, so if the fix moved the imports, you now have TC001-003 violations, so you need another run with fixes to move them back, which might mess with the ordering/formatting of the imports, when there was never a need to mess with the imports to begin with.

Running Ruff once with --fix should never lead to new Ruff errors being emitted if the new errors can be autofixed. Ruff runs all rules in a loop until there are no fixes left to apply. If you have a minimal repro of a snippet where running Ruff once with --fix leads to new Ruff violations that can be autofixed, meaning you have to run Ruff a second time with --fix, that's a bug -- please file an issue! :-)

[Daverball]

@AlexWaygood Sorry, I was being a bit unclear. My concern is the redundant work that ruff does for the consecutive fixes and potentially moving the imports to a slightly different location in the code, since the fixes for TC001-004 always move imports to the top of the file/to a type checking block just below all the imports at the top, regardless of where they were originally defined.

So it's more that it might create a bit of mess, that you will then have to revert, not so much that it will result in broken code or end up in an unfixable state.

[AlexWaygood]

I'm still not convinced that the current situation is that bad. (This conversation is getting a bit too hypothetical -- it's hard to say without concrete examples to look at where this actually creates a mess. And the redundant work that Ruff does doesn't concern me much unless we can demonstrate that there's a measurable speedup from doing something differently -- which I'm sceptical of :)

But this all feels like a bit of a tangent anyway. Even if it is desirable to make the way these violations are fixed be configurable, this could easily be done with a separate configuration option.

[Daverball]

It definitely is a tangent yes. The only reason I brought it up, is, that if there ended up being a need for an additional setting, that may inform the design of this one, in order to minimize confusion. But I'm happy to punt on this and only let it be an issue once it actually becomes one.