python3中的yield用法

yield跟return用法相似，yield返回迭代器，遍历返回的迭代器时，程序先截止于yield，下次遍历会接着上次程序同级继续运行，并在最后一次遍历不会在yield截止程序，会继续运行；return返回某个值，程序不再往下运行。

首先来看一段不含yield的代码：

class yield_test(object):
    def test1(self):
        for i in range(5):
            i += 1
            print("now i is:",i)
            # yield i
            for test1_i in range(i):
                print("i in test1 is:",test1_i)
        return i

bb = yield_test()
# bb.test1()
print(bb.test1())

执行结果：

now i is: 1
i in test1 is: 0
now i is: 2
i in test1 is: 0
i in test1 is: 1
now i is: 3
i in test1 is: 0
i in test1 is: 1
i in test1 is: 2
now i is: 4
i in test1 is: 0
i in test1 is: 1
i in test1 is: 2
i in test1 is: 3
now i is: 5
i in test1 is: 0
i in test1 is: 1
i in test1 is: 2
i in test1 is: 3
i in test1 is: 4
5

Process finished with exit code 0

上面执行结果可以看到，没有yield时，程序内的for循环全部执行完成了，最后数字5是return返回的值。

再来看一段含yield的代码：

class yield_test(object):
    def test1(self):
        for i in range(5):
            i += 1
            print("now i is:",i)
            yield i
            for test1_i in range(i):
                print("i in test1 is:",test1_i)
        return i

bb = yield_test()
# bb.test1()
print(bb.test1())
print(next(bb.test1()))

执行结果：

<generator object yield_test.test1 at 0x7f376477cdb0>
now i is: 1
1

可以看到，这次的for循环只执行了yield i上面的语句就停住了，最后那个数字1是yield i返回的值，如果要后面的循环继续执行，需要从这个迭代器里面取出一个值，才能继续，就像下面这样：

class yield_test(object):
    def test1(self):
        for i in range(5):
            i += 1
            print("now i is:",i)
            yield i
            for test1_i in range(i):
                print("i in test1 is:",test1_i)
        # return i

bb = yield_test()
# bb.test1()
print(bb.test1())
# print(next(bb.test1()))
for i in bb.test1():
     # print("now i is:",i)
    continue

执行结果：

<generator object yield_test.test1 at 0x7fb29b28cdb0>
now i is: 1
i in test1 is: 0
now i is: 2
i in test1 is: 0
i in test1 is: 1
now i is: 3
i in test1 is: 0
i in test1 is: 1
i in test1 is: 2
now i is: 4
i in test1 is: 0
i in test1 is: 1
i in test1 is: 2
i in test1 is: 3
now i is: 5
i in test1 is: 0
i in test1 is: 1
i in test1 is: 2
i in test1 is: 3
i in test1 is: 4

Process finished with exit code 0