Server Creation Notes¶

There are five classes in an inheritance diagram, four of which representsynchronous servers of four types:

+------------+|BaseServer|+------------+|v+-----------++------------------+|TCPServer|------->|UnixStreamServer|+-----------++------------------+|v+-----------++--------------------+|UDPServer|------->|UnixDatagramServer|+-----------++--------------------+

Note thatUnixDatagramServer derives fromUDPServer, not fromUnixStreamServer — the only difference between an IP and a Unixstream server is the address family, which is simply repeated in both Unixserver classes.

classsocketserver.ForkingMixIn¶

classsocketserver.ThreadingMixIn¶

Forking and threading versions of each type of server can be createdusing these mix-in classes. For instance,ThreadingUDPServeris created as follows:

classThreadingUDPServer(ThreadingMixIn,UDPServer):pass

The mix-in class comes first, since it overrides a method defined inUDPServer. Setting the various attributes also changes thebehavior of the underlying server mechanism.

ForkingMixIn and the Forking classes mentioned below areonly available on POSIX platforms that supportfork().

socketserver.ForkingMixIn.server_close() waits until all childprocesses complete, except ifsocketserver.ForkingMixIn.block_on_close attribute is false.

socketserver.ThreadingMixIn.server_close() waits until all non-daemonthreads complete, except ifsocketserver.ThreadingMixIn.block_on_close attribute is false. Usedaemonic threads by settingThreadingMixIn.daemon_threads toTrue to not wait until threadscomplete.

Changed in version 3.7:socketserver.ForkingMixIn.server_close() andsocketserver.ThreadingMixIn.server_close() now waits until allchild processes and non-daemonic threads complete.Add a newsocketserver.ForkingMixIn.block_on_close classattribute to opt-in for the pre-3.7 behaviour.

classsocketserver.ForkingTCPServer¶

classsocketserver.ForkingUDPServer¶

classsocketserver.ThreadingTCPServer¶

classsocketserver.ThreadingUDPServer¶

These classes are pre-defined using the mix-in classes.

To implement a service, you must derive a class fromBaseRequestHandlerand redefine itshandle() method.You can then run various versions ofthe service by combining one of the server classes with your request handlerclass. The request handler class must be different for datagram or streamservices. This can be hidden by using the handler subclassesStreamRequestHandler orDatagramRequestHandler.

Of course, you still have to use your head! For instance, it makes no sense touse a forking server if the service contains state in memory that can bemodified by different requests, since the modifications in the child processwould never reach the initial state kept in the parent process and passed toeach child. In this case, you can use a threading server, but you will probablyhave to use locks to protect the integrity of the shared data.

On the other hand, if you are building an HTTP server where all data is storedexternally (for instance, in the file system), a synchronous class willessentially render the service “deaf” while one request is being handled –which may be for a very long time if a client is slow to receive all the data ithas requested. Here a threading or forking server is appropriate.

In some cases, it may be appropriate to process part of a request synchronously,but to finish processing in a forked child depending on the request data. Thiscan be implemented by using a synchronous server and doing an explicit fork inthe request handler classhandle() method.

Another approach to handling multiple simultaneous requests in an environmentthat supports neither threads norfork() (or where these are tooexpensive or inappropriate for the service) is to maintain an explicit table ofpartially finished requests and to useselectors to decide whichrequest to work on next (or whether to handle a new incoming request). This isparticularly important for stream services where each client can potentially beconnected for a long time (if threads or subprocesses cannot be used). Seeasyncore for another way to manage this.

Server Objects¶

classsocketserver.BaseServer(server_address,RequestHandlerClass)¶

This is the superclass of all Server objects in the module. It defines theinterface, given below, but does not implement most of the methods, which isdone in subclasses. The two parameters are stored in the respectiveserver_address andRequestHandlerClass attributes.

fileno()¶

Return an integer file descriptor for the socket on which the server islistening. This function is most commonly passed toselectors, toallow monitoring multiple servers in the same process.

handle_request()¶

Process a single request. This function calls the following methods inorder:get_request(),verify_request(), andprocess_request(). If the user-providedhandle() method of thehandler class raises an exception, the server’shandle_error() methodwill be called. If no request is received withintimeoutseconds,handle_timeout() will be called andhandle_request()will return.

serve_forever(poll_interval=0.5)¶

Handle requests until an explicitshutdown() request. Poll forshutdown everypoll_interval seconds.Ignores thetimeout attribute. Italso callsservice_actions(), which may be used by a subclass or mixinto provide actions specific to a given service. For example, theForkingMixIn class usesservice_actions() to clean up zombiechild processes.

Changed in version 3.3:Addedservice_actions call to theserve_forever method.

service_actions()¶

This is called in theserve_forever() loop. This method can beoverridden by subclasses or mixin classes to perform actions specific toa given service, such as cleanup actions.

New in version 3.3.

shutdown()¶

Tell theserve_forever() loop to stop and wait until it does.shutdown() must be called whileserve_forever() is running in adifferent thread otherwise it will deadlock.

server_close()¶

Clean up the server. May be overridden.

address_family¶

The family of protocols to which the server’s socket belongs.Common examples aresocket.AF_INET andsocket.AF_UNIX.

RequestHandlerClass¶

The user-provided request handler class; an instance of this class is createdfor each request.

server_address¶

The address on which the server is listening. The format of addresses variesdepending on the protocol family;see the documentation for thesocket modulefor details. For Internet protocols, this is a tuple containing a string givingthe address, and an integer port number:('127.0.0.1',80), for example.

socket¶

The socket object on which the server will listen for incoming requests.

The server classes support the following class variables:

allow_reuse_address¶

Whether the server will allow the reuse of an address. This defaults toFalse, and can be set in subclasses to change the policy.

request_queue_size¶

The size of the request queue. If it takes a long time to process a singlerequest, any requests that arrive while the server is busy are placed into aqueue, up torequest_queue_size requests. Once the queue is full,further requests from clients will get a “Connection denied” error. The defaultvalue is usually 5, but this can be overridden by subclasses.

socket_type¶

The type of socket used by the server;socket.SOCK_STREAM andsocket.SOCK_DGRAM are two common values.

timeout¶

Timeout duration, measured in seconds, orNone if no timeout isdesired. Ifhandle_request() receives no incoming requests within thetimeout period, thehandle_timeout() method is called.

There are various server methods that can be overridden by subclasses of baseserver classes likeTCPServer; these methods aren’t useful to externalusers of the server object.

finish_request(request,client_address)¶

Actually processes the request by instantiatingRequestHandlerClass andcalling itshandle() method.

get_request()¶

Must accept a request from the socket, and return a 2-tuple containing thenewsocket object to be used to communicate with the client, and the client’saddress.

handle_error(request,client_address)¶

This function is called if thehandle()method of aRequestHandlerClass instance raisesan exception. The default action is to print the traceback tostandard error and continue handling further requests.

Changed in version 3.6:Now only called for exceptions derived from theExceptionclass.

handle_timeout()¶

This function is called when thetimeout attribute has been set to avalue other thanNone and the timeout period has passed with norequests being received. The default action for forking servers isto collect the status of any child processes that have exited, whilein threading servers this method does nothing.

process_request(request,client_address)¶

Callsfinish_request() to create an instance of theRequestHandlerClass. If desired, this function can create a new processor thread to handle the request; theForkingMixIn andThreadingMixIn classes do this.

server_activate()¶

Called by the server’s constructor to activate the server. The default behaviorfor a TCP server just invokeslisten()on the server’s socket. May be overridden.

server_bind()¶

Called by the server’s constructor to bind the socket to the desired address.May be overridden.

verify_request(request,client_address)¶

Must return a Boolean value; if the value isTrue, the request willbe processed, and if it’sFalse, the request will be denied. Thisfunction can be overridden to implement access controls for a server. Thedefault implementation always returnsTrue.

Changed in version 3.6:Support for thecontext manager protocol was added. Exiting thecontext manager is equivalent to callingserver_close().

Request Handler Objects¶

classsocketserver.BaseRequestHandler¶

This is the superclass of all request handler objects. It definesthe interface, given below. A concrete request handler subclass mustdefine a newhandle() method, and can override any ofthe other methods. A new instance of the subclass is created for eachrequest.

setup()¶

Called before thehandle() method to perform any initialization actionsrequired. The default implementation does nothing.

handle()¶

This function must do all the work required to service a request. Thedefault implementation does nothing. Several instance attributes areavailable to it; the request is available asself.request; the clientaddress asself.client_address; and the server instance asself.server, in case it needs access to per-server information.

The type ofself.request is different for datagram or streamservices. For stream services,self.request is a socket object; fordatagram services,self.request is a pair of string and socket.

finish()¶

Called after thehandle() method to perform any clean-up actionsrequired. The default implementation does nothing. Ifsetup()raises an exception, this function will not be called.

classsocketserver.StreamRequestHandler¶

classsocketserver.DatagramRequestHandler¶

TheseBaseRequestHandler subclasses override thesetup() andfinish()methods, and provideself.rfile andself.wfile attributes.Theself.rfile andself.wfile attributes can beread or written, respectively, to get the request data or return datato the client.

Therfile attributes of both classes support theio.BufferedIOBase readable interface, andDatagramRequestHandler.wfile supports theio.BufferedIOBase writable interface.

Changed in version 3.6:StreamRequestHandler.wfile also supports theio.BufferedIOBase writable interface.

Examples¶

importsocketserverclassMyTCPHandler(socketserver.BaseRequestHandler):"""    The request handler class for our server.    It is instantiated once per connection to the server, and must    override the handle() method to implement communication to the    client.    """defhandle(self):# self.request is the TCP socket connected to the clientself.data=self.request.recv(1024).strip()print("{} wrote:".format(self.client_address[0]))print(self.data)# just send back the same data, but upper-casedself.request.sendall(self.data.upper())if__name__=="__main__":HOST,PORT="localhost",9999# Create the server, binding to localhost on port 9999withsocketserver.TCPServer((HOST,PORT),MyTCPHandler)asserver:# Activate the server; this will keep running until you# interrupt the program with Ctrl-Cserver.serve_forever()

classMyTCPHandler(socketserver.StreamRequestHandler):defhandle(self):# self.rfile is a file-like object created by the handler;# we can now use e.g. readline() instead of raw recv() callsself.data=self.rfile.readline().strip()print("{} wrote:".format(self.client_address[0]))print(self.data)# Likewise, self.wfile is a file-like object used to write back# to the clientself.wfile.write(self.data.upper())

importsocketimportsysHOST,PORT="localhost",9999data=" ".join(sys.argv[1:])# Create a socket (SOCK_STREAM means a TCP socket)withsocket.socket(socket.AF_INET,socket.SOCK_STREAM)assock:# Connect to server and send datasock.connect((HOST,PORT))sock.sendall(bytes(data+"\n","utf-8"))# Receive data from the server and shut downreceived=str(sock.recv(1024),"utf-8")print("Sent:{}".format(data))print("Received:{}".format(received))

$pythonTCPServer.py127.0.0.1 wrote:b'hello world with TCP'127.0.0.1 wrote:b'python is nice'

$pythonTCPClient.pyhelloworldwithTCPSent:     hello world with TCPReceived: HELLO WORLD WITH TCP$pythonTCPClient.pypythonisniceSent:     python is niceReceived: PYTHON IS NICE

importsocketserverclassMyUDPHandler(socketserver.BaseRequestHandler):"""    This class works similar to the TCP handler class, except that    self.request consists of a pair of data and client socket, and since    there is no connection the client address must be given explicitly    when sending data back via sendto().    """defhandle(self):data=self.request[0].strip()socket=self.request[1]print("{} wrote:".format(self.client_address[0]))print(data)socket.sendto(data.upper(),self.client_address)if__name__=="__main__":HOST,PORT="localhost",9999withsocketserver.UDPServer((HOST,PORT),MyUDPHandler)asserver:server.serve_forever()

importsocketimportsysHOST,PORT="localhost",9999data=" ".join(sys.argv[1:])# SOCK_DGRAM is the socket type to use for UDP socketssock=socket.socket(socket.AF_INET,socket.SOCK_DGRAM)# As you can see, there is no connect() call; UDP has no connections.# Instead, data is directly sent to the recipient via sendto().sock.sendto(bytes(data+"\n","utf-8"),(HOST,PORT))received=str(sock.recv(1024),"utf-8")print("Sent:{}".format(data))print("Received:{}".format(received))

importsocketimportthreadingimportsocketserverclassThreadedTCPRequestHandler(socketserver.BaseRequestHandler):defhandle(self):data=str(self.request.recv(1024),'ascii')cur_thread=threading.current_thread()response=bytes("{}:{}".format(cur_thread.name,data),'ascii')self.request.sendall(response)classThreadedTCPServer(socketserver.ThreadingMixIn,socketserver.TCPServer):passdefclient(ip,port,message):withsocket.socket(socket.AF_INET,socket.SOCK_STREAM)assock:sock.connect((ip,port))sock.sendall(bytes(message,'ascii'))response=str(sock.recv(1024),'ascii')print("Received:{}".format(response))if__name__=="__main__":# Port 0 means to select an arbitrary unused portHOST,PORT="localhost",0server=ThreadedTCPServer((HOST,PORT),ThreadedTCPRequestHandler)withserver:ip,port=server.server_address# Start a thread with the server -- that thread will then start one# more thread for each requestserver_thread=threading.Thread(target=server.serve_forever)# Exit the server thread when the main thread terminatesserver_thread.daemon=Trueserver_thread.start()print("Server loop running in thread:",server_thread.name)client(ip,port,"Hello World 1")client(ip,port,"Hello World 2")client(ip,port,"Hello World 3")server.shutdown()

$pythonThreadedTCPServer.pyServer loop running in thread: Thread-1Received: Thread-2: Hello World 1Received: Thread-3: Hello World 2Received: Thread-4: Hello World 3