I'll also note there's a simpler reason the namespace object exposed at the function level can't just be a write-through proxy for the underlying frame: references to frame.f_locals may outlive the frame backing it, at which point we really do want it to be a plain dictionary with no special behaviour, just as it is for regular execution frames. 

(Think "return locals()" as the last line in a helper function, as well as variants like "data = locals(); data.pop('some_key'); return data")

That means that no matter what, we need to snapshot the frame locals the when frame.f_locals is requested. The question then becomes:

- when we do we update the contents of cell references? (this is what's buggy right now when a trace function is installed)
- when do we update ordinary local variables? (this isn't broken, so we want to avoid changing it)

Providing write-through support *just* for cells should thus make it possible to fix the buggy interaction between cells and trace function, while minimising the risk of any unintended consequences affecting regular function locals.