This is the only case where I think that using whitespace indentation for delimiting blocks of code is not a good idea. It makes this particular problem very difficult to solve elegantly. In languages where blocks are delimited by brackets is easer.
It's an argument to weigh in the balance, yes. Given that Python is more readable than languages without significant whitespace (even when those languages have the advantage of easy multiline lambdas), I don't find it convincing.
