>>> sum((yield x for x in S)) >>> >>> but perhaps we can make this work: >>> >>> sum(x for x in S) Paul> I like the look of this. In this context, it looks very natural. How would it look if you used the optional start arg to sum()? Would either of these work? sum(x for x in S, start=5) sum(x for x in S, 5) or would you have to parenthesize the first arg? sum((x for x in S), start=5) sum((x for x in S), 5) Again, why parens? Why not sum(<x for x in S>, start=5) sum(<x for x in S>, 5) or something similar? Also, sum(x for x in S) and sum([x for x in S]) look very similar. I don't think it would be obvious to the casual observer what the difference between them was or why the first form didn't raise a SyntaxError. >> It's a little clearer with parentheses, of course, so perhaps they >> should be required: >> >> for x in (y*2 for y in z if y<20): >> ... Paul> I'd rather not require parentheses in general. Parens are required in certain situations within list comprehensions around tuples (probably for syntactic reasons, but perhaps to aid the reader as well) where tuples can often be defined without enclosing parens. Here's a contrived example: >>> [(a,b) for (a,b) in zip(range(5), range(10))] [(0, 0), (1, 1), (2, 2), (3, 3), (4, 4)] >>> [a,b for (a,b) in zip(range(5), range(10))] File "<stdin>", line 1 [a,b for (a,b) in zip(range(5), range(10))] ^ SyntaxError: invalid syntax Paul> Guido's example of sum(x for x in S) looks too nice for me to want Paul> to give it up without a fight. But I'm happy to have cases where Paul> the syntax is ambiguous, or even out-and-out unparseable, without Paul> the parentheses. Whether it's possible to express this in a way Paul> that Python's grammar can deal with, I don't know. I rather suspect parens would be required for tuples if they were added to the language today. I see no reason to make an exception here. Skip