What variable type is returned from Open()?

[Chris Angelico]

On Sun, Apr 19, 2020 at 7:48 AM boB Stepp <robertvstepp at gmail.com> wrote:
> Can you give guidelines or point to a good article how
> to best and most efficiently use type annotation without
> over-annotating?  Bear in mind that I have yet to fully grasp all of
> what mypy (What I am currently using.) can correctly infer from the
> code.
>

Like with most things, the best way is to start by doing as little as
you possibly can, and then work your way up from there. Try using mypy
on your code absolutely as-is, without any type hints at all. If it
seems to be quite useless, add some hints, try again. Continue until
it's able to help you with your goals. Oh, I forgot to mention. Start
with a goal. Why are you using mypy? Is it to help you detect
misspelled variable/parameter names? Is it to save you the hassle of
writing unit tests? Is it to improve IDE features (tab completion
etc)?

The one specific thing I'll say is: function signatures are usually
the lowest-hanging fruit. For equivalent effort, they'll usually give
you the best results, since you can create "knowledge boundaries" of a
sort, where the type inference doesn't need to search the entire
codebase for everything. But other than that, just explore your own
codebase to try to find where it'd be useful.

ChrisA