Add a utility to get docstring node for module/class/function about libcst

[mvismonte]

@jack1142 I can definitely see how this would be useful. As far as I know, we don't have any doc string APIs. The recommended way of doing something like this would be to create a Metadata Provider and reference that in your visitor/transforms in order to determine if a module/class/function has a docstring and making the appropriate action based on that.

I'm sure that's something that would be useful for others, so feel free to put up a PR if you're interested in putting that together =)

[jimmylai]

Yeah, build a DocstringProvider will be very useful.

[Jackenmen]

I'll let someone else make this, I won't have enough time to make this, just wanted to put up this feature request :)

[lensvol]

@jack1142 Will you accept a PR for this or it would be done by someone from your team?

[Jackenmen]

@lensvol I'm not in the team :) But from what I know, maintainers of this project are welcome to PRs from outside.

[lensvol]

@jack1142 Sorry, I was actually trying to mention @mvismonte but my thumb slipped :) I am currently working on something similar, will try to see if it fits the purpose of this issue.

[jimmylai]

@lensvol Yes, your PR will be very welcome! We've accepted PRs from a couple external contributors. https://github.com/Instagram/LibCST/graphs/contributors

[jimmylai]

We recently start adding more helper functions to achieve write less and do more.
We recommend add simple and read-only helpers as attributes of CSTNode, e.g. SimpleString.prefix, quote, raw_value.
https://libcst.readthedocs.io/en/latest/nodes.html#libcst.SimpleString

Docstring is the first string literal statement of module, function or class. It falls under the simple and read-only helper category. So make them as Node.docstring with type Optionoal[str] makes sense.

[jimmylai]

get_docstring helper is implemented in #254
It'll be available in the next LibCST release.