Python自动化之select解析
select原理
网络通信被Unix系统抽象为文件的读写,通常是一个设备,由设备驱动程序提供,驱动可以知道自身的数据是否可用。支持阻塞操作的设备驱动通常会实现一组自身的等待队列,如读/写等待队列用于支持上层(用户层)所需的block或non-block操作。设备的文件的资源如果可用(可读或者可写)则会通知进程,反之则会让进程睡眠,等到数据到来可用的时候,再唤醒进程。
这些设备的文件描述符被放在一个数组中,然后select调用的时候遍历这个数组,如果对于的文件描述符可读则会返回改文件描述符。当遍历结束之后,如果仍然没有一个可用设备文件描述符,select让用户进程则会睡眠,直到等待资源可用的时候在唤醒,遍历之前那个监视的数组。每次遍历都是线性的。
select方法
Python的select()方法直接调用操作系统的IO接口,它监控sockets,open files, and pipes(所有带fileno()方法的文件句柄)何时变成readable 和writeable, 或者通信错误,select()使得同时监控多个连接变的简单,并且这比写一个长循环来等待和监控多客户端连接要高效,因为select直接通过操作系统提供的C的网络接口进行操作,而不是通过Python的解释器。
只支持Unix,不支持Windows
示例
import select
import socket
import sys
import Queue
# Create a TCP/IP socket
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.setblocking(0)
# Bind the socket to the port
server_address = ('localhost', 10000)
print >>sys.stderr, 'starting up on %s port %s' % server_address
server.bind(server_address)
# Listen for incoming connections
server.listen(5)
select()方法接收并监控3个通信列表, 第一个是所有的输入的data,就是指外部发过来的数据,第2个是监控和接收所有要发出去的data(outgoing data),第3个监控错误信息,接下来我们需要创建2个列表来包含输入和输出信息来传给select().
readable , writable , exceptional = select.select(inputs, outputs, inputs, timeout)
所有客户端的进来的连接和数据将会被server的主循环程序放在上面的list中处理,我们现在的server端需要等待连接可写(writable)之后才能过来,然后接收数据并返回(因此不是在接收到数据之后就立刻返回),因为每个连接要把输入或输出的数据先缓存到queue里,然后再由select取出来再发出去
当你把inputs,outputs,exceptional(这里跟inputs共用)传给select()后,它返回3个新的list,我们上面将他们分别赋值为readable,writable,exceptional, 所有在readable list中的socket连接代表有数据可接收(recv),所有在writable list中的存放着你可以对其进行发送(send)操作的socket连接,当连接通信出现error时会把error写到exceptional列表中。
Readable list 中的socket 可以有3种可能状态,第一种是如果这个socket是main "server" socket,它负责监听客户端的连接,如果这个main server socket出现在readable里,那代表这是server端已经ready来接收一个新的连接进来了,为了让这个main server能同时处理多个连接,在下面的代码里,我们把这个main server的socket设置为非阻塞模式。
# Handle inputs
for s in readable:
if s is server:
# A "readable" server socket is ready to accept a connection
connection, client_address = s.accept()
print >>sys.stderr, 'new connection from', client_address
connection.setblocking(0)
inputs.append(connection)
# Give the connection a queue for data we want to send
message_queues[connection] = Queue.Queue()
第二种情况是这个socket是已经建立了的连接,它把数据发了过来,这个时候你就可以通过recv()来接收它发过来的数据,然后把接收到的数据放到queue里,这样你就可以把接收到的数据再传回给客户端了。
else:
data = s.recv(1024)
if data:
# A readable client socket has data
print >>sys.stderr, 'received "%s" from %s' % (data, s.getpeername())
message_queues[s].put(data)
# Add output channel for response
if s not in outputs:
outputs.append(s)
第三种情况就是这个客户端已经断开了,所以你再通过recv()接收到的数据就为空了,所以这个时候你就可以把这个跟客户端的连接关闭了。
else:
# Interpret empty result as closed connection
print >>sys.stderr, 'closing', client_address, 'after reading no data'
# Stop listening for input on the connection
if s in outputs:
outputs.remove(s) #既然客户端都断开了,我就不用再给它返回数据了,所以这时候如果这个客户端的连接对象还在outputs列表中,就把它删掉
inputs.remove(s) #inputs中也删除掉
s.close() #把这个连接关闭掉
# Remove message queue
del message_queues[s]
else:
# Interpret empty result as closed connection
print >>sys.stderr, 'closing', client_address, 'after reading no data'
# Stop listening for input on the connection
if s in outputs:
outputs.remove(s) #既然客户端都断开了,我就不用再给它返回数据了,所以这时候如果这个客户端的连接对象还在outputs列表中,就把它删掉
inputs.remove(s) #inputs中也删除掉
s.close() #把这个连接关闭掉
# Remove message queue
del message_queues[s]
对于writable list中的socket,也有几种状态,如果这个客户端连接在跟它对应的queue里有数据,就把这个数据取出来再发回给这个客户端,否则就把这个连接从output list中移除,这样下一次循环select()调用时检测到outputs list中没有这个连接,那就会认为这个连接还处于非活动状态
# Handle outputs
for s in writable:
try:
next_msg = message_queues[s].get_nowait()
except Queue.Empty:
# No messages waiting so stop checking for writability.
print >>sys.stderr, 'output queue for', s.getpeername(), 'is empty'
outputs.remove(s)
else:
print >>sys.stderr, 'sending "%s" to %s' % (next_msg, s.getpeername())
s.send(next_msg)
最后,如果在跟某个socket连接通信过程中出了错误,就把这个连接对象在inputs\outputs\message_queue中都删除,再把连接关闭掉
# Handle "exceptional conditions"
for s in exceptional:
print >>sys.stderr, 'handling exceptional condition for', s.getpeername()
# Stop listening for input on the connection
inputs.remove(s)
if s in outputs:
outputs.remove(s)
s.close()
# Remove message queue
del message_queues[s]
客户端
下面的这个是客户端程序展示了如何通过select()对socket进行管理并与多个连接同时进行交互
import socket
import sys
messages = [ 'This is the message. ',
'It will be sent ',
'in parts.',
]
server_address = ('localhost', 10000)
# Create a TCP/IP socket
socks = [ socket.socket(socket.AF_INET, socket.SOCK_STREAM),
socket.socket(socket.AF_INET, socket.SOCK_STREAM),
]
# Connect the socket to the port where the server is listening
print >>sys.stderr, 'connecting to %s port %s' % server_address
for s in socks:
s.connect(server_address)
接下来通过循环通过每个socket连接给server发送和接收数据
for message in messages:
# Send messages on both sockets
for s in socks:
print >>sys.stderr, '%s: sending "%s"' % (s.getsockname(), message)
s.send(message)
# Read responses on both sockets
for s in socks:
data = s.recv(1024)
print >>sys.stderr, '%s: received "%s"' % (s.getsockname(), data)
if not data:
print >>sys.stderr, 'closing socket', s.getsockname()
服务端完整代码
#_*_coding:utf-8_*_
import select
import socket
import sys
import queue
# Create a TCP/IP socket
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.setblocking(False)
# Bind the socket to the port
server_address = ('localhost', 10000)
print(sys.stderr, 'starting up on %s port %s' % server_address)
server.bind(server_address)
# Listen for incoming connections
server.listen(5)
# Sockets from which we expect to read
inputs = [ server ]
# Sockets to which we expect to write
outputs = [ ]
message_queues = {}
while inputs:
# Wait for at least one of the sockets to be ready for processing
print( '\nwaiting for the next event')
readable, writable, exceptional = select.select(inputs, outputs, inputs)
# Handle inputs
for s in readable:
if s is server:
# A "readable" server socket is ready to accept a connection
connection, client_address = s.accept()
print('new connection from', client_address)
connection.setblocking(False)
inputs.append(connection)
# Give the connection a queue for data we want to send
message_queues[connection] = queue.Queue()
else:
data = s.recv(1024)
if data:
# A readable client socket has data
print(sys.stderr, 'received "%s" from %s' % (data, s.getpeername()) )
message_queues[s].put(data)
# Add output channel for response
if s not in outputs:
outputs.append(s)
else:
# Interpret empty result as closed connection
print('closing', client_address, 'after reading no data')
# Stop listening for input on the connection
if s in outputs:
outputs.remove(s) #既然客户端都断开了,我就不用再给它返回数据了,所以这时候如果这个客户端的连接对象还在outputs列表中,就把它删掉
inputs.remove(s) #inputs中也删除掉
s.close() #把这个连接关闭掉
# Remove message queue
del message_queues[s]
# Handle outputs
for s in writable:
try:
next_msg = message_queues[s].get_nowait()
except queue.Empty:
# No messages waiting so stop checking for writability.
print('output queue for', s.getpeername(), 'is empty')
outputs.remove(s)
else:
print( 'sending "%s" to %s' % (next_msg, s.getpeername()))
s.send(next_msg)
# Handle "exceptional conditions"
for s in exceptional:
print('handling exceptional condition for', s.getpeername() )
# Stop listening for input on the connection
inputs.remove(s)
if s in outputs:
outputs.remove(s)
s.close()
# Remove message queue
del message_queues[s]
客户端代码
import socket
import sys
messages = [ 'This is the message. ',
'It will be sent ',
'in parts.',
]
server_address = ('localhost', 10000)
# Create a TCP/IP socket
socks = [ socket.socket(socket.AF_INET, socket.SOCK_STREAM),
socket.socket(socket.AF_INET, socket.SOCK_STREAM),
]
# Connect the socket to the port where the server is listening
print >>sys.stderr, 'connecting to %s port %s' % server_address
for s in socks:
s.connect(server_address)
for message in messages:
# Send messages on both sockets
for s in socks:
print >>sys.stderr, '%s: sending "%s"' % (s.getsockname(), message)
s.send(message)
# Read responses on both sockets
for s in socks:
data = s.recv(1024)
print >>sys.stderr, '%s: received "%s"' % (s.getsockname(), data)
if not data:
print >>sys.stderr, 'closing socket', s.getsockname()
s.close()