python yield关键字作用

python yield关键字作用

1，是当前对象变成一个可迭代对象

def frange(start,stop,step):
    x = start
    while x<stop:
        yield  x
        x+=step

for i in frange(10,20,2.5):
    print(i)

10
12.5
15.0
17.5

============下面是没有yield的情况===========
def frange(start,stop,step):
    x = start
    while x<stop:
        # yield  x
        x+=step

for i in frange(10,20,2.5):
    print(i)

TypeError: 'NoneType' object is not iterable

============下面是用list替代yield的情况===========
def frange(start,stop,step):
    x = start
    a =[]
    while x<stop:
        # yield  x
        x+=step
        a.append(x)
    return a

for i in frange(10,20,2.5):
    print(i)

12.5
15.0
17.5
20.0

2，起到暂停作用，是被迭代的元素一个一个的被迭代，否则就会被for/while一次性循环

def frange(start,stop,step):
    x = start
    while x<stop:
        yield  x
        x+=step

it = frange(10,20,2)
print(next(it))    #10
print(next(it))    #12
print(next(it))    #14
print(next(it))    #16
print(next(it))    #18
print(next(it))    #StopIteration