Not supporting type comments of anything other than a function (is that even in the PEP?) or assignment is a known bug. I'm currently rewriting the parser (to track location information better and give better errors) but haven't gotten to type comments yet.

Ugh, since when are tuples of types directly supported? I know it's given in an example in the PEP, but it doesn't specify this stuff at all.

@gvanrossum What are we supposed to do about an unclear PEP?

I ran into this today and have resorted to using cast on the context manager:

with cast(BinaryIO, open(path, 'rb')) as fp:
    # use fp...

That makes the work-around nicely localized.

Since this is specified in PEP 484, mypy should support it. But it's not high priority.

The WIP fast parser (#1353) supports parsing types on with statements, so this should be easier to implement after that lands.

Moving to 0.5.0 given we aren't very far with the fast parser yet and this feels pretty unimportant.

(This is still TODO; we parse those type comments but don't use them.)

How would type comments work with multiple targets in with stmts. For example:

with X as (x, y), Z as (z, w):  # type: ...?

Would the type comment be of the form # type: int, float, float, int, where the number of elements in the type-tuple must match the number of targets in the with?

It's a bit complicated by the fact that when parsing, this is read as two distinct targets (each of which has a tuple type), so maybe # type: Tuple[int, float], Tuple[float, int] is sensible?

Maybe like this:

with X as (a, b):  # type: Tuple[int, float]
    ...
with X as (a, b), Y as (c, d):  # type Tuple[int, float], Tuple[str, str]
    ...

This may be tricky because IIRC the # type: parser currently only doesn't allow more than one top-level type.

I think this should work okay for the new parser, actually. Tuple[int, float], Tuple[str, str] is a valid Python expression which parses the same as (Tuple[int, float], Tuple[str, str]), so there shouldn't be an issue. (And I tried mypy --fast-parser -c 'x, y = 1, 2 # type: int, str', which works and gives you the error you'd expect.)