Fluent Python读书笔记(十一)

协程

协程和generator很像. 基本格式为:

var = yield [expression]

coroutine是generator的进一步发展:同调用者协同工作,从调用者接受数据并产生结果给调用者

协程和.send(),.throw(),.close()方法配合使用.

>>> def simple_coroutine(): #
... print('-> coroutine started')
... x = yield #
... print('-> coroutine received:', x)
...
>>> my_coro = simple_coroutine()
>>> my_coro #
<generator object simple_coroutine at 0x100c2be10>
>>> next(my_coro) #
-> coroutine started
>>> my_coro.send(42) #
-> coroutine received: 42
Traceback (most recent call last): #
...
StopIteration

协程状态

协程有四个状态, 可通过inspect.getgeneratorstate()方法查看:

• GEN_CREATED: 等待执行. 对应上述例子的my_coro = simple_coroutine()

• GEN_RUNNING: 运行中. 对应上述例子的next(my_coro)

• GEN_SUSPENDED: 被yield表达式挂起. 对应上述例子中的x = yield

• GEN_CLOSED: 执行完成

如果一个协程创建后, 立刻通过.send()传输一个不为None的值, 会抛出异常

>>> my_coro = simple_coroutine()
>>> my_coro.send(1729)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can't send non-None value to a just-started generator

但如果通过.send(None)调用则不会.

>>> def simple_coro2(a):
... print('-> Started: a =', a)
... b = yield a
... print('-> Received: b =', b)
... c = yield a + b
... print('-> Received: c =', c)
...
>>> my_coro2 = simple_coro2(14)
>>> from inspect import getgeneratorstate
>>> getgeneratorstate(my_coro2)
'GEN_CREATED'
>>> next(my_coro2)
-> Started: a = 14
14
>>> getgeneratorstate(my_coro2)
'GEN_SUSPENDED'
>>> my_coro2.send(28)
-> Received: b = 28
42
>>> my_coro2.send(99)
-> Received: c = 99
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration
>>> getgeneratorstate(my_coro2)
'GEN_CLOSED'

使用装饰器来装饰协程

使用协程的一般步骤为:

• 创建协程
• 调用next()方法
• 调用.send()方法
• 如果要停止, 调用.close()方法; 如果要抛出异常, 调用.throw()方法

上述的步骤有点繁琐, 如果不记得先要调用next()而直接调用.send()方法, 则会抛出异常. 我们可以使用装饰器来简化此问题.

from functools import wraps
def coroutine(func):
	"""Decorator: primes `func` by advancing to first `yield`"""
	@wraps(func)
	def primer(*args,**kwargs):
		gen = func(*args,**kwargs)
		next(gen)
		return gen
	return primer

generator.throw和generator.close

"""
Coroutine closing demonstration::

# BEGIN DEMO_CORO_EXC_1
    >>> exc_coro = demo_exc_handling()
    >>> next(exc_coro)
    -> coroutine started
    >>> exc_coro.send(11)
    -> coroutine received: 11
    >>> exc_coro.send(22)
    -> coroutine received: 22
    >>> exc_coro.close()
    >>> from inspect import getgeneratorstate
    >>> getgeneratorstate(exc_coro)
    'GEN_CLOSED'

# END DEMO_CORO_EXC_1

Coroutine handling exception::

# BEGIN DEMO_CORO_EXC_2
    >>> exc_coro = demo_exc_handling()
    >>> next(exc_coro)
    -> coroutine started
    >>> exc_coro.send(11)
    -> coroutine received: 11
    >>> exc_coro.throw(DemoException)
    *** DemoException handled. Continuing...
    >>> getgeneratorstate(exc_coro)
    'GEN_SUSPENDED'

# END DEMO_CORO_EXC_2

Coroutine not handling exception::

# BEGIN DEMO_CORO_EXC_3
    >>> exc_coro = demo_exc_handling()
    >>> next(exc_coro)
    -> coroutine started
    >>> exc_coro.send(11)
    -> coroutine received: 11
    >>> exc_coro.throw(ZeroDivisionError)
    Traceback (most recent call last):
      ...
    ZeroDivisionError
    >>> getgeneratorstate(exc_coro)
    'GEN_CLOSED'

# END DEMO_CORO_EXC_3
"""

# BEGIN EX_CORO_EXC
class DemoException(Exception):
    """An exception type for the demonstration."""

def demo_exc_handling():
    print('-> coroutine started')
    while True:
        try:
            x = yield
        except DemoException:  # <1>
            print('*** DemoException handled. Continuing...')
        else:  # <2>
            print('-> coroutine received: {!r}'.format(x))
    raise RuntimeError('This line should never run.')  # <3>
# END EX_CORO_EXC

从协程中返回值

"""
A coroutine to compute a running average.

Testing ``averager`` by itself::

# BEGIN RETURNING_AVERAGER_DEMO1

    >>> coro_avg = averager()
    >>> next(coro_avg)
    >>> coro_avg.send(10)  # <1>
    >>> coro_avg.send(30)
    >>> coro_avg.send(6.5)
    >>> coro_avg.send(None)  # <2>
    Traceback (most recent call last):
       ...
    StopIteration: Result(count=3, average=15.5)

# END RETURNING_AVERAGER_DEMO1

Catching `StopIteration` to extract the value returned by
the coroutine::

# BEGIN RETURNING_AVERAGER_DEMO2

    >>> coro_avg = averager()
    >>> next(coro_avg)
    >>> coro_avg.send(10)
    >>> coro_avg.send(30)
    >>> coro_avg.send(6.5)
    >>> try:
    ...     coro_avg.send(None)
    ... except StopIteration as exc:
    ...     result = exc.value
    ...
    >>> result
    Result(count=3, average=15.5)

# END RETURNING_AVERAGER_DEMO2


"""

# BEGIN RETURNING_AVERAGER
from collections import namedtuple

Result = namedtuple('Result', 'count average')


def averager():
    total = 0.0
    count = 0
    average = None
    while True:
        term = yield
        if term is None:
            break  # <1>
        total += term
        count += 1
        average = total/count
    return Result(count, average)  # <2>
# END RETURNING_AVERAGER

使用yield from

前面我们已经了解了yield from针对generator的用法 yield from <iterable>. yield from 还可以针对subgenerator和caller使用.