Does syntactic sugar have to be that strictly limited? If we are allowed to manipulate the statement block containing the sugared expression, we could define a trivial helper function which performs the 'nonlocal' side-effect and returns the value. Then, replace the "x := y" with "helper(y)".

I'm not sure exactly, but my point is that a program transformation can do it. However, no transformation can change an import to other operations while perserving semantics.

Just for fun, this seems to work:

  (locals().pop('x', None), locals().setdefault('x', y))[1]

That only works at top-level scope, where locals() == globals(). Inside a function, changing the dict returned by locals() doesn't actually change the variable.