[Python-Dev] PEP 289: Generator Expressions (second draft)
Guido van Rossum
guido at python.org
Thu Oct 23 11:03:35 EDT 2003
> 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/)
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