# yield-yield from

**1.入门级讲解**

yield本身返回的是一个iter，好处：我想要\[0-无穷]的数组，你难道到返回这么多数吗？返回i=0;;i++就好了

有yield的函数以当前`yield为分界线`，每次到这停止，下次来执行下半部分

```python
def pre():
    yield 1
    yield 2
    yield 3
    yield 4


abc = pre()
print(next(abc)) # 迭代1次
print(list(abc)) #一直迭代直到完
```

```python
# 字符串
astr = 'ABC'
# 列表
alist = [1, 2, 3]
# 字典
adict = {"name": "wangbm", "age": 18}
# 生成器
agen = list((i for i in range(4, 8)))


def gen_yield(*args, **kw):
    '''
    :desc yield
    :param args:
    :param kw:
    :return:
    '''
    for item in args:
        for i in item:
            yield i


def gen_yield_from(*args, **kw):
    '''
    :desc yield from
    :param args:
    :param kw:
    :return:
    '''
    # 细微差别看到了吗？yield from能迭代集合里面的数据
    for item in args:
        yield from item


new_list = gen_yield(astr, alist, adict, agen)
new_list_2 = gen_yield_from(astr, alist, adict, agen)
print(list(new_list))
print(list(new_list_2))

'''
['A', 'B', 'C', 1, 2, 3, 'name', 'age', 4, 5, 6, 7]
['A', 'B', 'C', 1, 2, 3, 'name', 'age', 4, 5, 6, 7]
'''



# ------------例子2，执行顺序详细打印------------
def yield_test(n):
    for i in range(n):
        yield call(i)
        print("range,i=", i)
        # 做一些其它的事情
    print("do something.")
    print("end.")

def call(i):
    print('call,i=', i)
    return i * 2

# 使用for循环
for i in yield_test(5):
    print('print,i=', i)
    print('-' * 20)
'''
call,i= 0
print,i= 0
--------------------
range,i= 0
call,i= 1
print,i= 2
--------------------
range,i= 1
call,i= 2
print,i= 4
--------------------
range,i= 2
call,i= 3
print,i= 6
--------------------
range,i= 3
call,i= 4
print,i= 8
--------------------
range,i= 4
do something.
end.
'''
```

2.生成器

```python
# 子生成器
def average_gen():
    total = 0
    count = 0
    average = 0
    while True:
        new_num = yield average
        count += 1
        total += new_num
        average = total/count

# 委托生成器
def proxy_gen():
    while True:
        yield from average_gen()

# 调用send，向yield average中的average传值
def main():
    calc_average = proxy_gen()
    next(calc_average)            # 预激下生成器
    print(calc_average.send(10))  # 打印：10.0
    print(calc_average.send(20))  # 打印：15.0
    print(calc_average.send(30))  # 打印：20.0

main()
```


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