Nick, in msg304388 you wrote "[allow] immediate write-through from trace functions". The latest iteration of PEP 558 says instead:

"tracing mode: the way the interpreter behaves when a trace hook has been registered..."

and also says:

"As long as the process remains in tracing mode, then __setitem__ and __delitem__ operations on the proxy will affect not only the dynamic snapshot, but also the corresponding fast local or cell reference on the underlying frame."

So write-through is allowed by the current PEP outside a trace function. Does not this change in some cases the behavior of code that updates the mapping returned by locals() when tracing mode is active from its standard behavior when there is no tracing ?