[Python-Dev] PEP 289: Generator Expressions (second draft)

[Guido van Rossum]

> If we eventually adopt the notation that {a, b, c} is a set, there is a
> potential ambiguity in expressions such as {x**2 for x in range(n)}.  Which
> is it, a set comprehension or a set with one element that is a generator
> expression?
> 
> It would have to be the former, of course, by analogy with
> [x**2 for x in range(n)], which means that if we introduce generator
> expressions, and we later introduce set literals, we will have to introduce
> set comprehensions at the same time.  Either that or prohibit generator
> expressions as set-literal elements unless parenthesized -- i.e.
> {(x**2 for x in range(n))}.

Don't worry.  The current proposal *always* requires parentheses
around generator expressions (but it may be the only argument to a
function), so your example would be illegal.

--Guido van Rossum (home page: http://www.python.org/~guido/)