If you tag objects as being either "owned" by a thread or "shared" then you could (in the python implementation) do something like:

  if (obj.owner == current_thread):
    # just get in and mess with it
  elsif (obj.owner == shared):
    # get in and mess with it under a lock
  else
    throw "not your obj - make this obj shared if you need that"

From a language perspective, you'd just need a way to declare that an object was 'shared'. All other objs would be owned by their creating thread.

No, that doesn't work. Java has something like this (synchronized methods) but they are cumbersome (I find programming nicer with plain old locks and conditions, the Java synchronized model feels just a layer of cruft but its all lock&cond inside).

But Python is different than Java. The Python interpreter has to guard the internal data structures of the interpreter itself, and that is a lot more tricky than it seems on the surface.