[feature-request] Require first-party imports to target highest exporting non-ancestor module

[smackesey]

A symbol can be defined in one module and re-exported from several others. When importing a symbol from an external typed library, the convention is to import from as close to the root as possible. This is how popular auto-import resolution tools work (see my very similar post in this pylance discussion:

If the same name is exported from multiple modules, the auto-import logic in pyright and pylance prefer the shortest module path. Only the shortest path is listed in the completion list, and the longer paths are de-duped.

In large projects, sometimes you have big submodules that you want to provide an "internal public" interface for-- e.g. if you have a large submodule project.foo, you might want project-internal code to import only from project.foo itself, rather than project.foo.* submodules.

I'd like to suggest that ruff offer a rule that enforces this by requiring internal imports resolve to the shortest module that exposes a symbol. So:

### project/foo/__init__.py
# redundant alias is necessary to expose `BAR` as public for `project.foo`
from .bar import BAR as BAR

### project/foo/bar.py
BAR = "BAR"

### project/baz.py
from project.foo.bar import BAR   # ERROR: should import from project.foo instead

[charliermarsh]

Yeah this makes sense. It would also force us to support cross-module checks (since we'd need to build a mapping from module to exported symbols), which would be a good thing.

[chbndrhnns]

I sometimes see modules prefixed with an underscore being treated as private. Here is an example how this could work:

# ./client.py
from .myp._private import Member
from .myp._private import OtherMember

assert Member

# ./myp/__init__.py
__all__ = ["Member"]

from ._private import Member

# ./myp/_private.py
class Member: ...

class OtherMember: ...

Ruff could turn this into:

# ./client.py
from .myp import Member
from .myp._private import OtherMember

assert Member

# ./myp/__init__.py
__all__ = ["Member"]

from ._private import Member

# ./myp/_private.py
class Member: ...

class OtherMember: ...

[smackesey]

Just wondering whether this is now easily doable with all the changes in the last year to ruff. Would be a nice QOL improvement for us.