select是全平台通用的IO多路复用模块。最大连接数：1024。

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poll和epoll没有最大连接数限制，但只能用在linux平台。

selectors是再封装模块，推荐使用。下篇会讨论。

select.select(rlist, wlist, xlist[, timeout])¶

• This is a straightforward interface to the Unix select() system call. The first three arguments are sequences of ‘waitable objects’: either integers representing file descriptors or objects with a parameterless method named fileno() returning such an integer:

  Empty sequences are allowed, but acceptance of three empty sequences is platform-dependent. (It is known to work on Unix but not on Windows.)  The optional timeout argument specifies a time-out as a floating point number in seconds.  When the timeout argument is omitted the function blocks until at least one file descriptor is ready.  A time-out value of zero specifies a poll and never blocks.

  The return value is a triple of lists of objects that are ready: subsets of the first three arguments.  When the time-out is reached without a file descriptor becoming ready, three empty lists are returned.

  Among the acceptable object types in the sequences are Python file objects (e.g. sys.stdin, or objects returned by open() or os.popen()), socket objects returned by socket.socket().  You may also define a wrapper class yourself, as long as it

• rlist: wait until ready for reading

• wlist: wait until ready for writing

• xlist: wait for an “exceptional condition” (see the manual page for what your system considers such a condition)

import socket
import os
import select
import queue
import json


class SelectFTP(object):
    def __init__(self, ip, port):
        self.server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.server.setblocking(0)
        self.server.bind((ip, port))
        self.server.listen(20)
        

        self.inputs = [self.server]
        self.outputs = []
        self.file_attr = {}
        # file_attr format:file_attr[socket]:{func:'', filename:'', len:999, recv_len:0}      
        self.socket_queue = {}

    
    def upload(self, sock, write_data):
        # if os.path.isfile(self.file_attr[sock]['filename']):
        with open(self.file_attr[sock]['filename'], 'a+') as file:
            file.write(data)
            self.file_attr[sock][recv_len] += len(write_data)
            if self.file_attr[sock][recv_len] == self.file_attr[sock][len]:
                del self.file_attr[sock]
                file.close()
        
   
    def download(self, sock, *args):
        pass
        
    def run(self):
        while self.inputs:
            read_active, read_output, exception = select.select(self.inputs, self.outputs, self.inputs)
            
            for fd in read_active:
                if fd is server:
                    conn, addr = fd.accept(1024)
                    conn.setblocking(0)
                    self.inputs = self.inputs.append(conn)
                    self.socket_queue[fd] = queue.Queue()
                    
                else:
                    recv_data = fd.recv(1024)

                    if recv_data:
                        data = json.loads(recv_data.decode())
                        
                        if fd not in self.file_attr.keys:
                            self.file_attr[fd] = data
                        
                        else:
                            try:
                                self.socket_queue.put_nowait(data)
                                if fd not in self.outputs:
                                    self.outputs.append(fd)
                            except Exception as e:
                                print(e)
                            
                    else:
                        self.inputs.remove(fd)
                        if fd in self.outputs:
                            self.outputs.remove(fd)
                        del self.socket_queue[fd]
                        
                    send_data = 
                        
            for fd in read_output:
                try:
                    message = self.socket_queue.get_nowait()
                except queue.Empty:
                    self.outputs.remove(fd)
                    print('wait...')
                else:
                    getattr(self.file_attr[fd]['func'])(fd, message)