range()方法返回一个list列表，一次性生成。
xrange()方法返回的是一个xrange()对象。
我所知道的只是xrange()方法更省内存。
Xrange()方法的官方定义：
xrange(start, stop[, step]) This function is very similar to range(), but returns an xrange object instead of a list.
This is an opaque sequence type which yields the same values as the corresponding list, without actually storing them all simultaneously.
The advantage of xrange() over range() is minimal (since xrange() still has to create the values when asked for them) except
when a very large range is used on a memory-starved machine or when all of the range’s elements are never used (such as when the loop is usually terminated with break).
For more information on xrange objects, see XRange Type and Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange.