Replace comments with string literals

[flying-sheep]

As outlined in #35, both assert isinstance(…) and syntactically relevant comments have severe drawbacks.

Parallel to docstrings, using string literals for inline type hinting provides hints that eliminate those disadvantages:

1. they appear in the standard Python AST and are therefore easier to retrieve (no 3rd party AST necessary)
2. no significant runtime cost (like assert isinstance(…) or Undefined(…) have)
3. equally lightweight syntax as comments (triple quouted or single quoted string literals instead of comment)

Also they are already used in e.g. PyCharm’s type hinting.

Open question: Do we prefer

'''type: List[Employee]'''
x = []

or

'type: List[Employee]'
x = []

[gvanrossum]

It's a clever hack. However it takes up an extra line, whereas the # type: ... comment goes on the same line. I think PyCharm would be quite happy to switch to PEP 484 type comments (it's written in Java so I presume they have their own "forgiving" parser anyways). @JukkaL or @ambv, any comments on this proposal?

[gvanrossum]

Please don't submit PRs for ideas still under active discussion.

[flying-sheep]

Please don't submit PRs for ideas still under active discussion.

oh, sorry, i didn’t know about this policy

[JukkaL]

I don't really like this proposal, because a type annotation will take an extra line and it's less obviously related to the variable, so it doesn't read as well in my opinion. Using Undefined(...) is almost as concise, at least for short variable names, though it's less efficient:

'type: List[Employee]'
# vs
x = Undefined(List[Employee])
x = []

Mypy allows Undefined with a string literal argument. This way it's a little more efficient when using complex types and it would still be there in the AST. For example:

x = Undefined('List[Employee]')
x = []