with open('/etc/passwd') as p1, \
open('/etc/passwd') as p2: # source, destination
A reasonable restriction (to my mind) would be to require at least two spaces or a tab after a \ before a comment (although requiring just one space would also be ok with me although I personally would always use more). This change couldn't break existing code since \ is currently a syntax error if followed by whitespace or a comment.
I would ignore whitespace after a final \ in a string, but would not allow comments.
(Yes, I realize that better variable names would obviate the need for these particular comments but comments are still useful sometimes :-)
--- Bruce