[feature-request] Detect invalid unquoted imports

[smackesey]

The following code crashes at runtime:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from threading import Thread

def foo(t: Thread):
    return t

The issue is that Thread is not defined at runtime, since it's only imported in a TYPE_CHECKING block. But ruff does not detect any error.

If this module included from __future__ import annotations, then it would be OK because "Thread" would be interpreted as a string-- but the module doesn't include that, so it should be possible to detect that this is illegal.

Further, I think in Python 3.13 string annotations, derived from from __future__ import annotations or otherwise, will go away in favor of wrapping annotations in a function for deferred evaluation. See here and PEP 649 for the dirty details.

[charliermarsh]

We can definitely detect that, we already capture this in our semantic model.

Meanwhile this, I believe, is fine:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from threading import Thread

def foo():
    t: Thread = 1
    return t

foo()

[smackesey]

Great!

Yes, ^^ is fine because Python never evaluates local variable annotations-- the annotation for a local variable is always a string, regardless of from __future__ import annotations.

[smackesey]

Btw IMO there should be autofix that converts it to a quoted string-- so my request here is similar to #5559.

[charliermarsh]

Unfortunately I've become intimately familiar with all the rules around annotation semantics 😂

[charliermarsh]

Do you think this should just be caught by F821, with a custom suggestion and autofix to quote it? Or a separate rule?

[smackesey]

Certainly sense to me to catch under an existing undefined anme rule, but I guess ultimately it depends on how much you are trying to mirror exact behavior of the rules you ported from other linters.

[charliermarsh]

May be easier to just implement this as the TC200 rules from flake8-type-checking.

[charliermarsh]

This is a bit different than the request in #5559, which is to automatically quote any annotations that can be quoted (and are only used in typing contexts), right?

[smackesey]

Yeah it's definitely different, they just both involve adding quotes to annotations.