IIRC there was a debate over allowing top level assignment expression without parentheses in original PEP 572 issue and most of the docs and examples follow using parentheses to avoid this confusion. I just noticed while reading the PEP that list comprehension case is not covered in the exceptional cases section that focused mostly on unparenthesized assignment expressions. If this will not be supported maybe this also could be added to PEP there since PEP serves as reference docs in most cases. The restriction can be lifted in future version if there is a simple way to fix this where this can be reopened.

https://www.python.org/dev/peps/pep-0572/#exceptional-cases