[Python-ideas] PEP 484 (Type Hints) -- second draft

[David Foster]

* It's a bit vague how local variables should be typed in the presence of
   multiple assignments. Consider:

   ```
   if x > 0:
       root = sqrt(x)  # type: int
   else:
       root = None
   ```

   It seems that the type of `root` should be inferred as `Union[int, None]`
   (AKA `Optional[int]`), but there's no discussion about this type of
   inference in the PEP.

   Cases like this are the only way that the proposed blank `Undefined`
   value makes sense to me, using an example like:

   ```
   root = Undefined    # type: Optional[int]
   if x > 0:
       root = sqrt(x)  # type: int
   else:
       root = None
   ```

* +1 for only allowing .pyi as the extension for stub files (and not 
also .py).
   Makes it very clear that they should only be interpreted as stubs.

* -1 that `Optional[T1]` is unnecessary. It's conceptually more clean and
   directly corresponds conceptually to types like `int?` and `?int` from
   other languages.

* +1 for the argument order cast(x, T). This is consistent with (x: T) 
elsewhere.

--
David Foster
http://dafoster.net/

P.S. Apologies if this doesn't add to the message chain correctly. 
There's no clearly correct way to reply to a digest email from Mailman.