Add type hints to Python function signatures

[ryzokuken]

Add type information to the codebase. Thoughts? @cclauss @targos.

Ref: https://docs.python.org/3/library/typing.html

[targos]

I'm not used to Python and have never worked with typing, so my opinion wouldn't be very informed.
I work with TypeScript though and do like type annotations, so I'm not opposed to that if it makes it easier / safer to develop and if it can be introduced incrementally.

[ryzokuken]

it makes it easier / safer to develop

it can be introduced incrementally

IIUC, these are both true for this.

[cclauss]

I am a huge fan of type hints in Python but they break compatibility with Python 2. If we a green light to drop legacy Python then let's go for it.

[ryzokuken]

Personally, I don't see why we should let Python 2 hold us back in 2020?

[targos]

As long as Node.js and node-gyp support Python 2 in their main development branches, I am against removing support here.

[cclauss]

Unfortunately, this is not a choice that one person can make. How do we get the Node.js Technical Steering Commitee to agree to drop support for legacy Python which died 200 days ago.

[targos]

You don't necessarily need the whole TSC to agree on it. The process is to open a PR dropping Python 2. If we cannot reach consensus (i.e. some collaborator(s) disagree), then the TSC will have to make a decision.

[ryzokuken]

@targos what does the PR need to do? Since the tooling is already Python 3 compatible, I suppose we're looking at a doc-only PR here.

[targos]

Almost doc-only, yes. There's also at least one place where we validate the python version (the configure file)

[cclauss]

Add Python type hints and drop support for Python 2 sounds like a nice PR title to me.

[ryzokuken]

drop support for Python 2 in nodejs/node first!