gevent文檔
Python通過
yield
提供了對協程的基本支援,但是不完全。而第三方的gevent為Python提供了比較完善的協程支援。
gevent是第三方庫,通過greenlet實作協程,其基本思想是:
當一個greenlet遇到IO操作時,比如通路網絡,就自動切換到其他的greenlet,等到IO操作完成,再在适當的時候切換回來繼續執行。由于IO操作非常耗時,經常使程式處于等待狀态,有了gevent為我們自動切換協程,就保證總有greenlet在運作,而不是等待IO。
由于切換是在IO操作時自動完成,是以gevent需要修改Python自帶的一些标準庫,這一過程在啟動時通過monkey patch完成:
from gevent import monkey; monkey.patch_socket()
import gevent
def f(n):
for i in range(n):
print gevent.getcurrent(), i
g1 = gevent.spawn(f, 5)
g2 = gevent.spawn(f, 5)
g3 = gevent.spawn(f, 5)
g1.join()
g2.join()
g3.join()
運作結果:
<Greenlet at 0x10e49f550: f(5)> 0
<Greenlet at 0x10e49f550: f(5)> 1
<Greenlet at 0x10e49f550: f(5)> 2
<Greenlet at 0x10e49f550: f(5)> 3
<Greenlet at 0x10e49f550: f(5)> 4
<Greenlet at 0x10e49f910: f(5)> 0
<Greenlet at 0x10e49f910: f(5)> 1
<Greenlet at 0x10e49f910: f(5)> 2
<Greenlet at 0x10e49f910: f(5)> 3
<Greenlet at 0x10e49f910: f(5)> 4
<Greenlet at 0x10e49f4b0: f(5)> 0
<Greenlet at 0x10e49f4b0: f(5)> 1
<Greenlet at 0x10e49f4b0: f(5)> 2
<Greenlet at 0x10e49f4b0: f(5)> 3
<Greenlet at 0x10e49f4b0: f(5)> 4
可以看到,3個greenlet是依次運作而不是交替運作。
要讓greenlet交替運作,可以通過
gevent.sleep()
交出控制權:
def f(n):
for i in range(n):
print gevent.getcurrent(), i
gevent.sleep(0)
執行結果:
<Greenlet at 0x10cd58550: f(5)> 0
<Greenlet at 0x10cd58910: f(5)> 0
<Greenlet at 0x10cd584b0: f(5)> 0
<Greenlet at 0x10cd58550: f(5)> 1
<Greenlet at 0x10cd584b0: f(5)> 1
<Greenlet at 0x10cd58910: f(5)> 1
<Greenlet at 0x10cd58550: f(5)> 2
<Greenlet at 0x10cd58910: f(5)> 2
<Greenlet at 0x10cd584b0: f(5)> 2
<Greenlet at 0x10cd58550: f(5)> 3
<Greenlet at 0x10cd584b0: f(5)> 3
<Greenlet at 0x10cd58910: f(5)> 3
<Greenlet at 0x10cd58550: f(5)> 4
<Greenlet at 0x10cd58910: f(5)> 4
<Greenlet at 0x10cd584b0: f(5)> 4
3個greenlet交替運作,
把循環次數改為500000,讓它們的運作時間長一點,然後在作業系統的程序管理器中看,線程數隻有1個。
當然,實際代碼裡,我們不會用
gevent.sleep()
去切換協程,而是在執行到IO操作時,gevent自動切換,代碼如下:
from gevent import monkey; monkey.patch_all()
import gevent
import urllib2
def f(url):
print('GET: %s' % url)
resp = urllib2.urlopen(url)
data = resp.read()
print('%d bytes received from %s.' % (len(data), url))
gevent.joinall([
gevent.spawn(f, 'https://www.python.org/'),
gevent.spawn(f, 'https://www.yahoo.com/'),
gevent.spawn(f, 'https://github.com/'),
])
運作結果:
GET: https://www.python.org/
GET: https://www.yahoo.com/
GET: https://github.com/
45661 bytes received from https://www.python.org/.
14823 bytes received from https://github.com/.
304034 bytes received from https://www.yahoo.com/.