1. Please consider disallowing the use of side effects of any kind in annotations, in that it is not promised when it will happen, if at all. So that a change 3 years from now will be somewhat less likely to break things. Please consider doing this for version 3.6; it is feature-frozen, but this is not (yet) a feature, and I got the feeling it is hardly controversial.

What you are proposing is a very large and controversial feature change. Just repeating that you think there documented 10-year old behavior is an accident doesn't make it so. This is ABSOLUTELY impossible for 3.6. my feeling is is probably oppose it for 3.7, bit that is the soonest timeframe that is even conceivable. Here's a quick, only slightly contrived, example where i want an annotation to have a side effect: def foo(a, b) -> logdef(): ... This might be used as part of a tracing, cod coverage, or timing library. Obviously it would need to do some magic to get more info about the function being logged, bit that's possible. And yes, I can think of other ways to achieve this effect (decorators maybe), but you want to make change to prohibit this use that might be in production for ten years.