Consider adding typehints into the codebase

[Winterflower]

Python has supported typehints since version 3.5 (see here https://docs.python.org/3/library/typing.html).
Might be good to investigate if adding typehints would make the codebase a bit easier to maintain in the future?

[stevedodson]

++ on doing this.

pandas has implemented this nicely:

https://github.com/pandas-dev/pandas/blob/master/pandas/_typing.py

pandas-dev/pandas#14468

If @viglia or others want to contribute, this could be a good starting point.

[V1NAY8]

@sethmlarson Since, we are incrementally doing this.
Are type hints required for tests?
Test methods would usually have return type None and pytest parameterized arguments are mostly List[int] or List[str] or List[bool]

[sethmlarson]

Not required for tests, only non-tests. Need to add the py.typed marker too.

[V1NAY8]

What does py.typed do, or what does it contain ? :)

[sethmlarson]

See this example project from PEP 561:

https://github.com/ethanhs/sample-typed-package

It's an empty file that signals to mypy, etc that type hints are included in the source/dist :)