1? yield基本用法

典型的例子：

　　斐波那契（Fibonacci）數(shù)列是一個非常簡單的遞歸數(shù)列，除第一個和第二個數(shù)外，任意一個數(shù)都可由前兩個數(shù)相加得到。1 2 3 5 8……

def fab(max): n, a, b = 0, 0, 1 while n < max: yield b # print b a, b = b, a + b n = n + 1

　　yield 的作用就是把一個函數(shù)變成一個generator，帶有 yield 的函數(shù)不再是一個普通函數(shù)，Python 解釋器會將其視為一個生成器，如調(diào)用fab函數(shù)， 不會執(zhí)行該函數(shù)，而是返回一個iterable迭代對象！

　　在for循環(huán)執(zhí)行時，每次循環(huán)都會相當于執(zhí)行生成器的next函數(shù)，才開始執(zhí)行fab函數(shù)的內(nèi)部代碼，執(zhí)行到y(tǒng)ield b時，fab函數(shù)就返回一個迭代值，然后掛起。

　　下次迭代時，代碼從yield b的下一條語句繼續(xù)執(zhí)行，而函數(shù)的本地變量看起來和上次中斷執(zhí)行前是完全一樣的，于是函數(shù)繼續(xù)執(zhí)行，直到再次遇到y(tǒng)ield。

更多yield例子：

#!/usr/bin/python def a():print ("do a() will not print out")yield 5 a() print ("===============test a()")def b():print ("list generator will in def , print here...")yield 5 g_obj = b() print ("===============g_obj test b: %s" % g_obj) print ("just generator obj, not in b def") print ("list_g: %s" % list(g_obj))def c():print ("next() will here... test generator next(), next attrbute not in python3, python2.6 is exist")yield 5print ("test generator next2") g_obj = c() print ("===============g_obj test c: %s" % g_obj) #g_obj.next() #print ("dir g_obj: %s " % dir(g_obj))def d():global mglobal nprint ("send() will here... test generator send()")m = yield 5print ("send input is m : %s" % m)n = yield 6print ("test generator send2")g_obj = d() print ("===============g_obj test d: %s" % g_obj) s_return1 = g_obj.send(None) s_return2 = g_obj.send("send twice") print ("the next send input will be the result of last yield, just like m is : %s, s_return1 is : %s, s_return2 is : %s" % (m, s_return1, s_return2)) print ("not next send so n is undefind, n is : %s" % n)

運行結(jié)果：

===============test a() ===============g_obj test b: <generator object b at 0x7f740b7fc750> just generator obj, not in b def list generator will in def , print here... list_g: [5] ===============g_obj test c: <generator object c at 0x7f740b7fc7e0> ===============g_obj test d: <generator object d at 0x7f740b7fc750> send() will here... test generator send() send input is m : send twice the next send input will be the result of last yield, just like m is : send twice, s_return1 is : 5, s_return2 is : 6 Traceback (most recent call last):File "./yield0.py", line 40, in <module>print ("not next send so n is undefind, n is : %s" % n) NameError: name 'n' is not defined

send用法說明：

關(guān)于輸入：send的輸入是本次遇到y(tǒng)ield時，先賦值給yield表達式的結(jié)果。有點難懂，詳細說明。

1、如m = yield 5，這個表達式，是分兩次yield完成的，第一次執(zhí)行后一半，即返回5，下次send時，才執(zhí)行前一半，即把后一次send的輸入賦值給m。

2、所以，第一次使用send，輸入必須是None，開啟生成器，因為本次遇到y(tǒng)ield后，yield返回后，就完結(jié)了，并沒有一個執(zhí)行到賦值給m的過程。m的初值是第二個send()輸入?yún)?shù)。

3、最后一個最后一個send，把輸入給了上一次yield表達式，所以最后一個n=yield 6，語句執(zhí)行后，n是未定義的。

關(guān)于輸出：比較簡單，就是yield的結(jié)果。如s_return = send(None)，由于yield 5，所以s_return= 5

2 ?使用yield實現(xiàn)協(xié)程

舉例：生產(chǎn)者生產(chǎn)消息后，直接通過yield跳轉(zhuǎn)到消費者開始執(zhí)行，待消費者執(zhí)行完畢后，切換回生產(chǎn)者繼續(xù)生產(chǎn)。

#!/usr/bin/pythondef consumer():r = ''while True:n = yield rif not n:print("not n...")returnprint('[CONSUMER] Consuming %s...' % n)r = '200 OK'def produce(c):f = c.send(None)print('[PRODUCER] Consumer first return: %s' % f)n = 0while n < 2:n = n + 1print('[PRODUCER] Producing %s...' % n)r = c.send(n)print('[PRODUCER] Consumer return: %s' % r)c.close()c = consumer() produce(c)

運行結(jié)果：

[PRODUCER] Consumer first return: [PRODUCER] Producing 1... [CONSUMER] Consuming 1... [PRODUCER] Consumer return: 200 OK [PRODUCER] Producing 2... [CONSUMER] Consuming 2... [PRODUCER] Consumer return: 200 OK

協(xié)程的說明：

注意到consumer函數(shù)是一個generator，把一個consumer傳入produce后：

首先調(diào)用c.send(None)啟動生成器；

然后，一旦生產(chǎn)了東西，通過c.send(n)切換到consumer執(zhí)行；

consumer通過yield拿到消息，處理，又通過yield把結(jié)果傳回；

produce拿到consumer處理的結(jié)果，繼續(xù)生產(chǎn)下一條消息；

produce決定不生產(chǎn)了，通過c.close()關(guān)閉consumer，整個過程結(jié)束。

整個流程無鎖，由一個線程執(zhí)行，produce和consumer協(xié)作完成任務(wù)，所以稱為“協(xié)程”，而非線程的搶占式多任務(wù)。

3 ?yield from基本用法

從python3.3新增語法yield from，在python3.4中asyncio的微線程的實現(xiàn)依賴此語法。

先從generator中套generator的需求入手。

舉例：生成器調(diào)用子生成器，父生成器輸入什么，調(diào)用完子生成器后，同樣返回什么。

def i_yield_whatever_input_is():input = 0while True:print("1: before gi yield input=%s" % input)input = yield inputprint("2: after gi yield input=%s" % input)def wrap_generator1():for i in i_yield_whatever_input_is():print("3: before g1 yield i=%s" % i)yield ig = wrap_generator1() print("4: after send None return: %s" % g.send(None)) print("4: after send 1 return: %s" % g.send(1)) print("4: after send 2 return: %s" % g.send(2))

未到達預(yù)期的運行結(jié)果：

1: before gi yield input=0 3: before g1 yield i=0 4: after send None return: 0 2: after gi yield input=None 1: before gi yield input=None 3: before g1 yield i=None 4: after send 1 return: None 2: after gi yield input=None 1: before gi yield input=None 3: before g1 yield i=None 4: after send 2 return: None

　　顯然不是預(yù)期“輸入什么，返回什么”。由于send的輸入到wrap_generator后，無法輸入給子生成器，因此，子生成器i_yield_whatever_input_is的輸入是None，只能yield None。

　　使用yield from，可以將send的輸入，傳遞給子生成器，父生成器代碼修改如下：

def wrap_generator2():yield from i_yield_whatever_input_is()g = wrap_generator2() print("4: after send None return: %s" % g.send(None)) print("4: after send 1 return: %s" % g.send(1)) print("4: after send 2 return: %s" % g.send(2))

達到預(yù)期的運行結(jié)果：

1: before gi yield input=0 4: after send None return: 0 2: after gi yield input=1 1: before gi yield input=1 4: after send 1 return: 1 2: after gi yield input=2 1: before gi yield input=2 4: after send 2 return: 2

4 使用yield from實現(xiàn)asyncio

簡單的例子（兩個函數(shù)并發(fā)執(zhí)行，函數(shù)內(nèi)部的sleep不互相阻塞其它函數(shù)）：

#!/usr/bin/python import asyncio import threading@asyncio.coroutine def hello():print("2.1 befor yield from asyncio sleep")r = yield from asyncio.sleep(2)print("2.1 after yield from asyncio sleep")def hello2():print("2.2 befor yield from asyncio sleep")r = yield from asyncio.sleep(5)print("2.2 after yield from asyncio sleep")loop = asyncio.get_event_loop() print ("1. after get event loop")#loop.run_until_complete(hello()) tasks = [hello2(), hello()] loop.run_until_complete(asyncio.wait(tasks)) print ("2. after run")loop.close() print ("3. after close")

運行結(jié)果：

1. after get event loop 2.2 befor yield from asyncio sleep 2.1 befor yield from asyncio sleep ===等待兩個函數(shù)sleep返回=== 2.1 after yield from asyncio sleep 2.2 after yield from asyncio sleep 2. after run 3. after close

復(fù)雜的例子(并發(fā)同時訪問多個WEB服務(wù)器)：

#!/usr/bin/python import asyncio@asyncio.coroutine def wget(host):print("wget %s..." % host)connect = asyncio.open_connection(host, 80)reader, writer = yield from connectheader = 'GET / HTTP/1.0\r\nHost: %s\r\n\r\n' % hostwriter.write(header.encode('utf-8')) #向服務(wù)器發(fā)送請求yield from writer.drain()while True:line = yield from reader.readline() #讀取服務(wù)器返回的數(shù)據(jù)if line == b'\r\n':breakprint('%s header > %s' % (host, line.decode('utf-8').rstrip()))writer.close()loop = asyncio.get_event_loop() tasks = [wget(host) for host in ['www.baidu.com', 'www.sina.com'，'www.taobao.com']] loop.run_until_complete(asyncio.wait(tasks)) loop.close()

運行結(jié)果：并發(fā)訪問，服務(wù)器一旦返回，立即打印到屏幕。

wget www.baidu.com... wget www.taobao.com... wget www.sina.com... www.baidu.com header > HTTP/1.1 200 OK www.baidu.com header > Date: Fri, 12 Jun 2015 03:17:20 GMT www.baidu.com header > Content-Type: text/html www.baidu.com header > Content-Length: 14613 www.baidu.com header > Last-Modified: Wed, 03 Sep 2014 02:48:32 GMT www.baidu.com header > Connection: Close www.baidu.com header > Vary: Accept-Encoding www.baidu.com header > Set-Cookie: BAIDUID=052DF57419E7322485FE496F7CFD60DF:FG=1; expires=Thu, 31-Dec-37 23:55:55 GMT; max-age=2147483647; path=/; domain=.baidu.com www.baidu.com header > Set-Cookie: BIDUPSID=052DF57419E7322485FE496F7CFD60DF; expires=Thu, 31-Dec-37 23:55:55 GMT; max-age=2147483647; path=/; domain=.baidu.com www.baidu.com header > Set-Cookie: PSTM=1434079040; expires=Thu, 31-Dec-37 23:55:55 GMT; max-age=2147483647; path=/; domain=.baidu.com www.baidu.com header > Set-Cookie: BDSVRTM=0; path=/ www.baidu.com header > P3P: CP=" OTI DSP COR IVA OUR IND COM " www.baidu.com header > Server: BWS/1.1 www.baidu.com header > X-UA-Compatible: IE=Edge,chrome=1 www.baidu.com header > Pragma: no-cache www.baidu.com header > Cache-control: no-cache www.baidu.com header > BDPAGETYPE: 1 www.baidu.com header > BDQID: 0x82714a2100005cd6 www.baidu.com header > BDUSERID: 0 www.baidu.com header > Accept-Ranges: bytes www.sina.com header > HTTP/1.1 301 Moved Permanently www.sina.com header > Server: nginx www.sina.com header > Date: Fri, 12 Jun 2015 03:15:36 GMT www.sina.com header > Content-Type: text/html www.sina.com header > Location: http://www.sina.com.cn/ www.sina.com header > Expires: Fri, 12 Jun 2015 03:17:36 GMT www.sina.com header > Cache-Control: max-age=120 www.sina.com header > Age: 104 www.sina.com header > Content-Length: 178 www.sina.com header > X-Cache: HIT from ja180-186.sina.com.cn www.sina.com header > Connection: close www.taobao.com header > HTTP/1.1 200 OK www.taobao.com header > Server: Tengine www.taobao.com header > Date: Fri, 12 Jun 2015 03:17:20 GMT www.taobao.com header > Content-Type: text/html; charset=gbk www.taobao.com header > Connection: close www.taobao.com header > Vary: Accept-Encoding www.taobao.com header > Set-Cookie: CAT=deleted; expires=Thu, 01-Jan-1970 00:00:01 GMT; Max-Age=0 www.taobao.com header > Set-Cookie: thw=cn; Path=/; Domain=.taobao.com; Expires=Sat, 11-Jun-16 03:17:20 GMT;

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轉(zhuǎn)載于:https://www.cnblogs.com/fangyuan1004/p/4571304.html

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