Add Optional for types on expires/expires_days

[robinro]

The runtime already allows and deal with None values here. Adding Optional improves the type checking and (since this is public API) allows users to run their type checker with strict-optional.

[bdarnell]

Thanks!

Tornado is compatible with strict-optional mode, which has been the default in mypy since version 0.600 (Tornado currently uses mypy 0.700). The issue here is about implicit-optional mode, which is on by default, but that default is intended to change in the indefinite future (python/peps#689, python/typing#275).

I think it's a good idea to switch to no-implicit-optional mode, but we should go all the way: Add no_implicit_optional = True to our setup.cfg, and fix up every instance where we currently use = None without Optional.

[robinro]

Thanks @bdarnell for having a look and the clarification regarding strict/implicit optional.

I'll have a go at enabling no_implicit_optional and fixing the related issues.

[robinro]

@bdarnell I added no_implicit_optional and made the necessary changes to annotations. Please have a look and let me know what you think.

There is currently still one failing test: https://travis-ci.org/tornadoweb/tornado/jobs/545617971
It seems Python 3.5.2 doesn't like Optional[Type["AsyncHTTPClient"]] (also tried Union[Type[".."], None]. Other python version are fine, though. Do you have an idea what's the issue there? I saw some comments about 3.5.2 vs. other 3.5 patchlevels.

[ploxiln]

ubuntu-16.04 comes with python-3.5.2, so that's why tornado is attempting to retain compatibility. Unfortunately, there were some significant bugs fixed in later python 3.5 point releases.
#2605
https://stackoverflow.com/questions/42942867/optionaltypefoo-raises-typeerror-in-python-3-5-2

[ploxiln]

... so I think one solution is to stringify the whole "Optional[Type[AsyncHTTPClient]]" to defer any parsing to mypy (and mypy checks are not run on python-3.5.2)

[robinro]

Thanks @ploxiln for the hint. The CI is green now. Please let me know if there is anything else to change here.

[bdarnell]

Thanks!