python的【yield from】【yield 】【with】【@contextlib.contextmanager】混合【使用缺陷】

1. yield from 【Python3.5后被代替】

https://www.jianshu.com/p/87da832730f5
Python3.3之后引入了新语法yield from 
Python3.5开始引入了新的语法 async 和 await ，而await替代的就是yield from（为了不与实现内层for循环的yield from误解）

# 如果生成器函数需要产出另一个生成器生成的值，传统的解决方法是使用嵌套的for循环:
>>> def chain(*iterables):
...     for it in iterables:
...         for i in it:
...             yield i
>>> s = 'ABC'
>>> t = tuple(range(3))
>>> list(chain(s, t))
['A', 'B', 'C', 0, 1, 2]


# 使用yield from 方法
>>> def chain(*iterables):
...     for i in iterables:
...         yield from i
...
>>> list(chain(s, t))
['A', 'B', 'C', 0, 1, 2]



# 经典的例子【处理嵌套，返回没有嵌套的单值序列】
from collections.abc import Iterable

def flatten(items, ignore_types=(str, bytes)):
    for x in items:
        if isinstance(x, Iterable) and not isinstance(x, ignore_types):
            yield from flatten(x)
        else:
            yield x

items = [1, 2, [3, 4, [5, 6], 7], 8]
for x in flatten(items):
    print(x)

items = ['Dave', 'Paula', ['Thomas', 'Lewis']]
for x in flatten(items):
    print(x)

2. 【enter】【exit】【with】

class DatabaseConnection(object):

    def __enter__(self):
        # make a database connection and return it
        ...
        return self.dbconn

    def __exit__(self, exc_type, exc_val, exc_tb):
        # make sure the dbconnection gets closed
        self.dbconn.close()
        ...


with DatabaseConnection() as mydbconn:  # 使用方法，with 对象（class()）
    # do stuff

3. 使用【@contextlib.contextmanager】【with】

import contextlib
import time

@contextlib.contextmanager
def timeit():
    start = time.time()
    print('before yield ')
    yield start                   # 理解 yield start执行完后返回，如果是 res = yield start ，那么赋值操作不会执行，# yield 在等式的右边，执行完就返回了，没有进行赋值 ；类似于 a = return 1【return这样用非法】，a 是NONE
    print('after yield stating')
    end = time.time()
    usedTime = (end - start) * 1000
    print ('Use time %d ms' % usedTime)
    print('after yield ending')

with timeit() as stattime:   # with timeit()  ，注意可以用as的情况
    print('in with staring')
    time.sleep(1)
    print('in with doing something')
    print('in with ending')

'''
结果：
before yield 
in with staring
in with doing something
in with ending
after yield stating
Use time 1000 ms
after yield ending
'''