It looks great! Still I have found something that I think could be confusing.

First, I would expect thatIterable[T][int] andIterable[int] is the same type, butIterable[T][int] == Iterable[int] gives meFalse.

Second (maybe related) issue is that now__repr__ shows the whole "history" of a type. For example,Iterable[Tuple[T,T]][int] is printed as__main__.Iterable<+T_co>[__main__.Tuple[~T, ~T]]<~T>[int]. This reads as first there was anIterable parameterized byT_co, then it was indexed byTuple[T, T] and we had something parameterized byT, and then it was indexed byint. The question is whether we need to expose all this information (that is rather internal) to the user? I think it would be more clear if__repr__ in this case will show something likeIterable[Tuple[int,int]].

Ah, yes. I thought of makingIterable[T][int] == Iterable[int], but I ran out of steam on how to implement it. Then again, thinking about it I'm not sure that it's important to do this -- the runtime behavior of these things is of marginal interest and mypy doesn't let you write that anyway (the PEP is silent on whether this is allowed).

Ditto for repr(). The one important thing about repr() is that when you have

class C(Generic[T]): ...

then repr(C) showsC<~T> rather thanC[~T] as it used to. I think the new form is better. And thenC[Tuple[int, int]] will actually be printed asC<~T>[Tuple[int, int]]. The longer form (with alternating<...> and[...]) is only of theoretical importance.

Anyway, thanks for the encouragement. I hope others look over this important change too! I intend to land this in Python 3.5.2 (whenever it is released).

I am now thinking about one additional thing that might be not completely clear: covariance. If I initially haveIterable[T_co] covariant in its type parameter and write

classC(Iterable[Tuple[T,S]]):    ...

then one could be not sure whetherC is covariant in its variables or invariant. I think the natural answer is that covariance is determined only by the final list of type parameters, so that the class above is invariant in both variables. However, I didn't find anywhere in the PEP any explicit explanation of this point. Maybe it is obvious and it is only my problem of understanding, but if not then maybe one should add a short explanation to the PEP?

I just noticed that this PR also fixes#129 . In general, I think this new "revamped" behavior is much better than the old one.