Annotate pylint code with type annotations

[PCManticore]

Since we're on Python 3, we can use annotations for pylint.

[sushobhit27]

@PCManticore since the person opted to work on this issue has backed off, I would like to work on this issue.

[PCManticore]

Hey @sushobhit27 I'm not sure who opted to work on this issue, but feel free to take it! The rough idea of this change would be:

• first add mypy in tox but not on Travis
• try to annotate as much pylint code as possible with type annotations
• if there are any bugs with the types that we pass from one place to another, fix them
• Once we can safely run mypy on pylint without any errors, integrate mypy on Travis

Feel free to send multiple smaller PRs for this, since it might require quite a few changes. I suggest starting just with the public APIs for now, e.g methods and functions that are public and that we also use internally. Probably we can skip the visit methods for now, because we already know the type, but if it helps, we can add annotations for them if we can find more bugs in the visit methods.

[sushobhit27]

@PCManticore Agree that multiple smaller PRs will be required for this and starting just with the public APIs for now will be good starting point.
Will analyze more and start working on this.

[AWhetter]

Before we add too many annotations to pylint, will we get more value out of adding annotations to astroid first? From what I understand of transitioning to a typed codebase, typing modules that are lower in the dependency tree first is a good idea because it makes it easier to correct mistakes later and you have more types available to you in the modules that have the dependency when it comes to typing those.

[PCManticore]

Sounds like a good plan to me, @AWhetter !

[PCManticore]

#2338 was merged but I'm not sure if we should close this issue now or after we have annotations for all the pylint code. Probably the latter as it's easier to track progress with an opened issue.

[AWhetter]

Sounds good to me. Perhaps we close this once we start getting useful information from mypy, not necessarily when pylint is 100% annotated?

[PCManticore]

@AWhetter Definitely. Since that is not measurable enough, I'd say we can close this once we have annotations where it matters. It's important that we now have the setup for running mypy against pylint, the annotations can be its own goal.

[sushobhit27]

@PCManticore @AWhetter I will suggest to do in stages, first annotating functions used across pylint code and then covering checker functions.