However, this looks better to me:
@(a + b) @(a or (c and d))
Conditions in 'if' and 'while' statements don't need parentheses, so why do decorators?
Those are already closed by the colon so requiring parentheses would be redundant:
if a + b: while a + b:
The minimalist tweak would be to follow Guido's preference and just accept subscripting in addition to calls.
So, in terms of Grammar/Grammar, what about
decorator: '@' atom trailer* NEWLINE
I say "keep it clean", ie no parentheses except where operator priority or clarity require them.
I actually agree with Georg that this is a case where clarity favours enforced parentheses for expressions that are otherwise non-atomic. Things like variable references, function calls and subscripting are already atomic so the parentheses would be optional in those cases.
However, as long as Guido remains -0 extension to arbitrary expressions isn't going to happen, parentheses or no parentheses.