python 高级特性：Generator(生成器)

格式对比：

    List Imprehensions 格式：[ 语句 for .. in ..]

    Generator 格式：（语句 for .. in ..）

区别：List存储的是具体内容，每个元素都占用空间，当需要存储N条数据时，占用空间大

            而Generator存储的是表达式，通过计算而推算出下个元素，因此占空间少。

一、2种输入方式

#!/usr/bin/python

#generator.next()输出：很痛苦的方式
g = (x for x in range(0, 10))
print "g:"
print "1g.next():", g.next();
print "2g.next():", g.next();
print "3g.next():", g.next();
print "4g.next():", g.next();
print "5g.next():", g.next();
print "6g.next():", g.next();
print "7g.next():", g.next();
print "8g.next():", g.next();
print "9g.next():", g.next();
print "10g.next():", g.next();
#print "11g.next():", g.next();  --->超范围后，提示 StopIteration

可见 generator是从中断处继续向下执行。

#for循环输出

k = (x for x in range(0, 10))
for x in k:
    print x

二、斐波拉契数列：list 与 generator对比

##Fibonacci : 1 2 3 5 8 13 21 ..

##in list
def fib(n):
    i, a, b = 0, 0, 1;
    print "in list:"
    while i < n:
        print b;
        a, b = b, a+b
        i += 1;

print fib(6);

##in generator  -----> yield ：generator标志
def fibo(n):
    i, a, b = 0, 0, 1;
    print "in generator:"
    while i < n:
        yield b;
        a, b = b, a+b
        i += 1;
for n in fibo(6):
    print n;