Lint rule to ban imports only at top level

[durumu]

Hello,

I use ruff on a library codebase that gets imported a lot, and we value import speed quite a bit. We have a list of expensive imports that we ban from being imported at module level, allowing them only inline. Currently, we enforce this using a custom Pylint checker, but we'd love to switch it to Ruff. Here's an example of what I mean.

I'd like to implement this as a Ruff rule -- first without autofix, and then maybe adding autofix down the line. However, I can think of a couple approaches and I'm not sure what the preferred one is. One way is extending TID251 to support a new config option, e.g.:

            [tool.ruff.flake8-tidy-imports.banned-api]
            "torch".msg = "Pytorch is banned at module level because it is expensive to import."
            "torch".allow_inline = true

Another way would be creating a whole new rule for this. I don't know of any existing flake8 extensions or other rules that do this, so I guess it might be a RUF rule, though TID or something could work too (even if it's not quite the same as the original!)

Would love to hear your thoughts -- thanks in advance!

[zanieb]

Thanks for starting a discussion!

I definitely identify with this — I've gone out of my way to implement deferred import tooling as well but that comes with its own complications. I'm curious to hear from others :)

[charliermarsh]

I agree that this would be useful. We did this in the past at Spring, to ensure that Tensorflow was always a late-import. My instinct is that this should be an entirely new rule in flake8-tidy-imports or flake8-import-conventions that accepts a list of modules as configuration.

[durumu]

Thank you for the suggestion! I decided to implement it as a new rule in flake8-tidy-imports. I've created a PR to add it here: #6378

[mattiasb]

Would it make sense to have an allow-list in addition to the ban list?

That way one could do something like:

[tool.ruff.lint.flake8-tidy-imports]
# Ban certain modules from being imported at module level
banned-module-level-imports = [
  "package1"
  "package2"
  "package3"
]
allowed-module-level-imports = [
  "package1.sub.cheap"
  "package3.other.also_cheap"
]

My particular use case is to warn about expensive imports for Airflow DAGs where we know that we shouldn't import some packages.

Except, there's a few exceptions to this which makes us then either having to list all packages under banned-module-level-imports or put in a bunch of ignores all over the place. 🤪