PEP484 type hints / mypy

[The-Compiler]

As mypy seems a lot more usable than when I last looked at it, maybe qutebrowser should start using PEP484 type hints.

PyQt 5.6 also comes with .pyi files for the whole PyQt, so that might already be beneficial before starting to annotate qutebrowser source.

---

• Check mypy/pytype/pyre
• Make mypy run on Travis
• Add first type annotations
• Get check_untyped_defs running
• Probably: Get rid of the typing. prefix and import things from typing instead
• Maybe: Be explicit about optional arguments, see the discussions in Should we change PEP 484 to disable implicit Optional when default = None? python/typing#275 and PEP 484: Do not require type checkers to treat a None default specially python/peps#689
• Investigate # FIXME:typing comments
• Fix various issues with PyQt5-stubs (mainly: make sure signals are typed correctly) - see https://github.com/stlehmann/PyQt5-stubs/blob/master/issues.md
• Use upstream PyQt5-stubs instead of our own fork
• Play with more tools: typeguard, PyAnnotate, MonkeyType
• Annotate ALL THE THINGS™

[The-Compiler]

This is was blocked by #637.

[The-Compiler]

python/typing#310 (comment) looks useful too

[The-Compiler]

PyAnnotate sounds interesting - though someone™ would need to add Python 3 style annotation support to it.

[The-Compiler]

MonkeyType seems awesome!

[The-Compiler]

mypy now supports PEP 571, which means it'd be possible to release type hints for PyQt rewritten so they actually work with MyPy.

[The-Compiler]

mypy is now running on Travis CI! There's still a long way to go as not much code is annotated right now, but it's a good start.

[The-Compiler]

qutebrowser is now using mypy's check_untyped_defs, which means it checks the entire codebase, even where there aren't any type annotations yet.

This requires quite some # type: ignore comments (~140 at the time of writing), mainly due to issues with PyQt5-stubs. I expect those to go down again in the future, as those issues can be fixed there (or even upstream in PyQt5).

I'll update the original post with some more TODO items about things I've learned along the way.

[The-Compiler]

Might be interesting to use diff-cover to enforce any new code to have type annotations.

See pytest-dev/pytest#6486 for an example.

edit: I added tox -e mypy-diff now - not using it on CI by default since there are a lot of WIP PRs still, but I'll probably turn it on at some point.

[The-Compiler]

Also note that mypy has a pretty nice HTML report feature nowadays - if you run tox -e mypy -- --html-report mypy-html and look at mypy-html/index.html, you will get a list of files needing work:

The ones marked in red are places which likely will need more work (and probably don't have type annotations at all yet).