Support optional values through a new validator.

[sam-falvo]

Unfortunately, I'm unable to run tests locally b/c of severe dependency
hell. I am unable to get tox, detox, python setup.py test, and other
means of running tests to work at all. So, since I'm blocked on a
project by the lack of optional validator support, I am going to close
my eyes, look away, and pull the trigger, and hope I hit the target.

If someone who has a working Python configuration can please be kind
enough to let me know if my changes fail any tests, I'd be very
appreciative.

Even better, I'd love to know why a stock Python distribution with
dependencies installed is incapable of running the tests for attrs.
However, this isn't the right forum to answer that question.

(This PR has the 17+ debugging commits cleaned up.)

[codecov-io]

Current coverage is 100.0%

Merging #17 into master will change coverage by 0% by b29e8a6

Coverage Diff

@@            master     #17   diff @@
======================================
  Files            6       6       
  Stmts          298     309    +11
  Branches        59      61     +2
  Methods          0       0       
======================================
+ Hit            298     309    +11
  Partial          0       0       
  Missed           0       0       

Powered by Codecov

[hynek]

As for your dependency hell, the answer is to use virtualenvs for everything: https://hynek.me/articles/virtualenv-lives/ For CLI tools I like to use pipsi that will install them into separate virtualenvs (that’s where I have tox in). To run the tests you only should require tox and run it.

But as you see, your changes passed in Travis so there’s that. :)

---

The idea makes sense to me, a few comments:

• You need to add the function to https://raw.githubusercontent.com/hynek/attrs/master/docs/api.rst so it’s included in the documentation.
• The changelog entry should be a fully qualified name that links to the docs. (i.e. Add attr.validators.optional https://attrs..._.)
• Since optional is agnostic of which validator it is used with, you can remove the interface-related tests.

The rest will follow inline, thank you for your contribution!

[sam-falvo]

I was wondering if you'd had a chance to re-review this patch? This is a project blocker for me. Thanks!

[hynek]

I'm sorry I haven't got a notification about your changes so please ping a PR if you address a review.

Right now at a conference but I'll look at it when I'm back.

[hynek]

Merged in b54bbaa – thanks!