Beginning Python - From Novice to Professional

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CHAPTER 24 ■ PROJECT 5: A VIRTUAL TEA PARTY 457 chat server from your own machine, but that’s not much fun in the long run, is it?) To be able to connect, the user has to know the address of your machine (a machine name such as foo.bar.baz.com or an IP number). In addition, the user must know the port number used by your server. You can set this in your program; in the code in this chapter I use the (rather arbitrary) number 5005. ■Note As mentioned in Chapter 14, certain port numbers are restricted and require administrator privileges. In general, numbers greater than 1023 are okay. To test your server you need a client—the program on the user side of the interaction. A simple program for this sort of thing is telnet (which basically lets you connect to any socket server). In UNIX, you probably have this program available on the command line: $ telnet some.host.name 5005 The preceding command connects to the machine some.host.name on port 5005. To connect to the same machine on which you’re running the telnet command, simply use the machine name localhost. (You might want to supply an escape character through the -e switch to make sure you can quit telnet easily. See the man page for more details.) In Windows, you can use either the standard telnet command (in a DOS window) or a terminal emulator with telnet functionality, such as PuTTY (software and more information available at http://www.chiark.greenend.org.uk/~sgtatham/putty). However, if you are installing new software, you might as well get a client program tailored to chatting. MUD (or MUSH or MOO or some other related acronym) clients are quite suitable for this sort of thing. My client of choice is TinyFugue (software and more information available at http://tf.tcp.com). It is mainly designed for use in UNIX. (There are several clients available for Windows as well; just do a Web search for “mud client” or something similar.) First Implementation Let’s break things down a bit. We need to create two main classes: one representing the chat server and one representing each of the chat sessions (the connected users). The ChatServer Class To create the basic ChatServer, you subclass the dispatcher class from asyncore. The dispatcher is basically just a socket object, but with some extra event handling features, which you’ll be using in a minute. See Listing 24-1 for a basic chat server program (that does very little).
458 CHAPTER 24 ■ PROJECT 5: A VIRTUAL TEA PARTY Listing 24-1. A Minimal Server Program from asyncore import dispatcher import asyncore class ChatServer(dispatcher): pass s = ChatServer() asyncore.loop() If you run this program, nothing happens. To make the server do anything interesting, you should call its create_socket method to create a socket, and its bind and listen methods to bind the socket to a specific port number and to tell it to listen for incoming connections. (That is what servers do, after all.) In addition, you’ll override the handle_accept event handling method to actually do something when the server accepts a client connection. The resulting program is shown in Listing 24-2. Listing 24-2. A Server That Accepts Connections from asyncore import dispatcher import socket, asyncore class ChatServer(dispatcher): def handle_accept(self): conn, addr = self.accept() print 'Connection attempt from', addr[0] s = ChatServer() s.create_socket(socket.AF_INET, socket.SOCK_STREAM) s.bind(('', 5005)) s.listen(5) asyncore.loop() The handle_accept method calls self.accept, which lets the client connect. This returns a connection (a socket that is specific for this client) and an address (information about which machine is connecting). Instead of doing anything useful with this connection, the handle_accept method simply prints that a connection attempt was made. addr[0] is the IP address of the client. The server initialization calls create_socket with two arguments that specify the type of socket you want. You could use different types, but those shown here are what you usually want. The call to the bind method simply binds the server to a specific address (host name and port). The host name is empty (an empty string, essentially meaning “localhost,” or, more technically, “all interfaces on this machine”) and the port number is 5005. The call to listen tells the server to listen for connections; it also specifies a backlog of five connections. The final call to asyncore.loop starts the server’s listening loop as before. This server actually works. Try to run it and then connect to it with your client. The client should immediately be disconnected, and the server should print out the following:
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Beginning Python From Novice to Pro
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Introduction A C program is like a
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270 CHAPTER 12 ■ GRAPHICAL USER I
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298 CHAPTER 14 ■ NETWORK PROGRAMM
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572 APPENDIX C ■ ONLINE RESOURCES
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Index ■Symbols - operator 558 !=
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