At Chris Angelico's suggestion, starting another thread on this:
The \ continuation character may be followed by white space and a comment. If a comment is present, there must be at least one whitespace character between the \ and the comment.
x = y + \ # comment allowed herez
with a as x, \ # comment here may be usefulb as y, \ # or herec as z: \ # or herepass
x = y + # syntax errorz