Add explanation of singletons in unions to PEP484

[ilevkivskyi]

I tried to keep discussion to a minimum, only including simple examples. I think it is better to add this section right after discussion of Optional, that is actually also a Union with a singleton type (NoneType).

[gvanrossum]

Cannot

[gvanrossum]

Thanks! Some language nits.

I wonder if we should add another example showing an enum with multiple values?

[ilevkivskyi]

I have made corrections and added another example. I think it clarifies that it is not necessary to make many Enum's if one wants more than one special value.

[JukkaL]

Style nit: add spaces around =.

[ilevkivskyi]

@JukkaL Done.

[gvanrossum]

Pushed to the peps repo and filed python/mypy#1803.

Maybe now that the peps repo has moved to GitHub we should switch to doing PRs there?

[ilevkivskyi]

Pushed to the peps repo and filed python/mypy#1803.

Thanks!

Maybe now that the peps repo has moved to GitHub we should switch to doing PRs there?

Yes, I think we could switch to new peps repo. By the way, what are the plans for PEP484, will it be provisional in 3.6? Or will it "graduate" soon?

[gvanrossum]

I would like PEP 484 to drop the provisional status for 3.6, but we have to discuss this on python-dev. For asyncio we did two provisional release (3.4 and 3.5) but IMO that was too long (and yet not long enough -- Yury recently asked me to extend that to 3.6 as well).