On 6/28/10 5:47 AM, Li Hui wrote: > When write > >>>i for i in range(16) > I get "SyntaxError: invalid syntax" > > but When I use it like this: > >>>def f(x):\ > ... pass > >>>f(i for i in range(16)) > > all is right > I think it maybe f((i for i in range(16)))
The "<expression> for <name> in <iterator>" syntax is actually two very different things. One is a list comprehension, one is a generator comprehension. (Then there's dictionary stuff later, but I shant complicate matters!) One creates a list, the other creates a "generator" which is a kind of iterator, which (eventually) something else can scan over to get and operate on a sequence. A list comprehension must be within brackets, as so: >>> [i for i in range(4)] [0, 1, 2, 3] If you want to enter a generator expression on its own, you must surround it in parens, as: >>> (i for i in range(4)) <generator object at 0x6a2d8> Note though, that you get a generator object and not a list or anything. You can see what that generator would do by: >>> gen = (i for i in range(4)) >>> for x in gen: ... print x ... 0 1 2 3 Now, if you are entering a generator where its not 'on its own' and its not ambiguous-- such as inside a function call-- you don't have to surround it by its own parens. So you don't have to do f((i for i in range(4)). You only have to group it when its own its own. I mean you *can* wrap parens around it all the time: but just as parens don't create tuples, the parens don't -create- the generator so much as set it apart from possibly confusing surrounding elements when needed. -- ... Stephen Hansen ... Also: Ixokai ... Mail: me+list/python (AT) ixokai (DOT) io ... Blog: http://meh.ixokai.io/ -- http://mail.python.org/mailman/listinfo/python-list
