socketserver— A framework for network servers

Source code:Lib/socketserver.py

---

Thesocketservermodule simplifies the task of writing network servers.

Availability:not WASI.

This module does not work or is not available on WebAssembly. See WebAssembly platformsfor more information.

There are four basic concrete server classes:

classsocketserver.TCPServer(server_address,RequestHandlerClass,bind_and_activate=True)

This uses the internet TCP protocol, which provides for continuous streams of data between the client and server. Ifbind_and_activateis true, the constructor automatically attempts to invokeserver_bind()and server_activate().The other parameters are passed to theBaseServerbase class.

classsocketserver.UDPServer(server_address,RequestHandlerClass,bind_and_activate=True)

This uses datagrams, which are discrete packets of information that may arrive out of order or be lost while in transit. The parameters are the same as forTCPServer.

classsocketserver.UnixStreamServer(server_address,RequestHandlerClass,bind_and_activate=True)

classsocketserver.UnixDatagramServer(server_address,RequestHandlerClass,bind_and_activate=True)

These more infrequently used classes are similar to the TCP and UDP classes, but use Unix domain sockets; they’re not available on non-Unix platforms. The parameters are the same as for TCPServer.

These four classes process requestssynchronously;each request must be completed before the next request can be started. This isn’t suitable if each request takes a long time to complete, because it requires a lot of computation, or because it returns a lot of data which the client is slow to process. The solution is to create a separate process or thread to handle each request; the ForkingMixInandThreadingMixInmix-in classes can be used to support asynchronous behaviour.

Creating a server requires several steps. First, you must create a request handler class by subclassing theBaseRequestHandlerclass and overriding itshandle()method; this method will process incoming requests. Second, you must instantiate one of the server classes, passing it the server’s address and the request handler class. It is recommended to use the server in awithstatement. Then call the handle_request()or serve_forever()method of the server object to process one or many requests. Finally, callserver_close() to close the socket (unless you used awithstatement).

When inheriting fromThreadingMixInfor threaded connection behavior, you should explicitly declare how you want your threads to behave on an abrupt shutdown. TheThreadingMixInclass defines an attribute daemon_threads,which indicates whether or not the server should wait for thread termination. You should set the flag explicitly if you would like threads to behave autonomously; the default isFalse,meaning that Python will not exit until all threads created byThreadingMixInhave exited.

Server classes have the same external methods and attributes, no matter what network protocol they use.

Server Creation Notes

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

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

Note thatUnixDatagramServerderives fromUDPServer,not from UnixStreamServer— the only difference between an IP and a Unix server is the address family.

classsocketserver.ForkingMixIn

classsocketserver.ThreadingMixIn

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

classThreadingUDPServer(ThreadingMixIn,UDPServer):
pass

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

ForkingMixInand the Forking classes mentioned below are only available on POSIX platforms that supportfork().

block_on_close

ForkingMixIn.server_close waits until all child processes complete, except if block_on_closeattribute isFalse.

ThreadingMixIn.server_close waits until all non-daemon threads complete, except if block_on_closeattribute isFalse.

daemon_threads

ForThreadingMixInuse daemonic threads by setting ThreadingMixIn.daemon_threads toTrueto not wait until threads complete.

Changed in version 3.7:ForkingMixIn.server_closeand ThreadingMixIn.server_closenow waits until all child processes and non-daemonic threads complete. Add a newForkingMixIn.block_on_closeclass attribute to opt-in for the pre-3.7 behaviour.

classsocketserver.ForkingTCPServer

classsocketserver.ForkingUDPServer

classsocketserver.ThreadingTCPServer

classsocketserver.ThreadingUDPServer

classsocketserver.ForkingUnixStreamServer

classsocketserver.ForkingUnixDatagramServer

classsocketserver.ThreadingUnixStreamServer

classsocketserver.ThreadingUnixDatagramServer

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

Added in version 3.12:TheForkingUnixStreamServerandForkingUnixDatagramServerclasses were added.

To implement a service, you must derive a class fromBaseRequestHandler and redefine itshandle()method. You can then run various versions of the service by combining one of the server classes with your request handler class. The request handler class must be different for datagram or stream services. This can be hidden by using the handler subclasses StreamRequestHandlerorDatagramRequestHandler.

Of course, you still have to use your head! For instance, it makes no sense to use a forking server if the service contains state in memory that can be modified by different requests, since the modifications in the child process would never reach the initial state kept in the parent process and passed to each child. In this case, you can use a threading server, but you will probably have 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 stored externally (for instance, in the file system), a synchronous class will essentially 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 it has 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. This can be implemented by using a synchronous server and doing an explicit fork in the request handler classhandle()method.

Another approach to handling multiple simultaneous requests in an environment that supports neither threads norfork()(or where these are too expensive or inappropriate for the service) is to maintain an explicit table of partially finished requests and to useselectorsto decide which request to work on next (or whether to handle a new incoming request). This is particularly important for stream services where each client can potentially be connected for a long time (if threads or subprocesses cannot be used).

Server Objects

classsocketserver.BaseServer(server_address,RequestHandlerClass)

This is the superclass of all Server objects in the module. It defines the interface, given below, but does not implement most of the methods, which is done in subclasses. The two parameters are stored in the respective server_addressandRequestHandlerClassattributes.

fileno()

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

handle_request()

Process a single request. This function calls the following methods in order:get_request(),verify_request(),and process_request().If the user-provided handle()method of the handler class raises an exception, the server’shandle_error()method will be called. If no request is received withintimeout seconds,handle_timeout()will be called andhandle_request() will return.

serve_forever(poll_interval=0.5)

Handle requests until an explicitshutdown()request. Poll for shutdown everypoll_intervalseconds. Ignores thetimeoutattribute. It also callsservice_actions(),which may be used by a subclass or mixin to provide actions specific to a given service. For example, the ForkingMixInclass usesservice_actions()to clean up zombie child processes.

Changed in version 3.3:Addedservice_actionscall to theserve_forevermethod.

service_actions()

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

Added 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 a different 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_INETandsocket.AF_UNIX.

RequestHandlerClass

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

server_address

The address on which the server is listening. The format of addresses varies depending on the protocol family; see the documentation for thesocketmodule for details. For internet protocols, this is a tuple containing a string giving the 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 to False,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 single request, any requests that arrive while the server is busy are placed into a queue, up torequest_queue_sizerequests. Once the queue is full, further requests from clients will get a “Connection denied” error. The default value is usually 5, but this can be overridden by subclasses.

socket_type

The type of socket used by the server;socket.SOCK_STREAMand socket.SOCK_DGRAMare two common values.

timeout

Timeout duration, measured in seconds, orNoneif no timeout is desired. Ifhandle_request()receives no incoming requests within the timeout period, thehandle_timeout()method is called.

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

finish_request(request,client_address)

Actually processes the request by instantiatingRequestHandlerClassand calling itshandle()method.

get_request()

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

handle_error(request,client_address)

This function is called if thehandle() method of aRequestHandlerClassinstance raises an exception. The default action is to print the traceback to standard error and continue handling further requests.

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

handle_timeout()

This function is called when thetimeoutattribute has been set to a value other thanNoneand the timeout period has passed with no requests being received. The default action for forking servers is to collect the status of any child processes that have exited, while in threading servers this method does nothing.

process_request(request,client_address)

Callsfinish_request()to create an instance of the RequestHandlerClass.If desired, this function can create a new process or thread to handle the request; theForkingMixInand ThreadingMixInclasses do this.

server_activate()

Called by the server’s constructor to activate the server. The default behavior for 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 will be processed, and if it’sFalse,the request will be denied. This function can be overridden to implement access controls for a server. The default implementation always returnsTrue.

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

Request Handler Objects

classsocketserver.BaseRequestHandler

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

setup()

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

handle()

This function must do all the work required to service a request. The default implementation does nothing. Several instance attributes are available to it; the request is available asrequest;the client address asclient_address;and the server instance as server,in case it needs access to per-server information.

The type ofrequestis different for datagram or stream services. For stream services,requestis a socket object; for datagram services,requestis a pair of string and socket.

finish()

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

request

Thenewsocket.socketobject to be used to communicate with the client.

client_address

Client address returned byBaseServer.get_request().

server

BaseServerobject used for handling the request.

classsocketserver.StreamRequestHandler

classsocketserver.DatagramRequestHandler

TheseBaseRequestHandlersubclasses override the setup()andfinish() methods, and providerfileandwfileattributes.

rfile

A file object from which receives the request is read. Support theio.BufferedIOBasereadable interface.

wfile

A file object to which the reply is written. Support theio.BufferedIOBasewritable interface

Changed in version 3.6:wfilealso supports the io.BufferedIOBasewritable interface.

Examples

socketserver.TCPServerExample

This is the server side:

importsocketserver

classMyTCPHandler(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 client
self.data=self.request.recv(1024).strip()
print("Received from{}:".format(self.client_address[0]))
print(self.data)
# just send back the same data, but upper-cased
self.request.sendall(self.data.upper())

if__name__=="__main__":
HOST,PORT="localhost",9999

# Create the server, binding to localhost on port 9999
withsocketserver.TCPServer((HOST,PORT),MyTCPHandler)asserver:
# Activate the server; this will keep running until you
# interrupt the program with Ctrl-C
server.serve_forever()

An alternative request handler class that makes use of streams (file-like objects that simplify communication by providing the standard file interface):

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() calls
self.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 client
self.wfile.write(self.data.upper())

The difference is that thereadline()call in the second handler will call recv()multiple times until it encounters a newline character, while the singlerecv()call in the first handler will just return what has been received so far from the client’ssendall()call (typically all of it, but this is not guaranteed by the TCP protocol).

This is the client side:

importsocket
importsys

HOST,PORT="localhost",9999
data="".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 data
sock.connect((HOST,PORT))
sock.sendall(bytes(data+"\n","utf-8"))

# Receive data from the server and shut down
received=str(sock.recv(1024),"utf-8")

print("Sent:{}".format(data))
print("Received:{}".format(received))

The output of the example should look something like this:

Server:

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

Client:

$pythonTCPClient.pyhelloworldwithTCP
Sent: hello world with TCP
Received: HELLO WORLD WITH TCP
$pythonTCPClient.pypythonisnice
Sent: python is nice
Received: PYTHON IS NICE

socketserver.UDPServerExample

This is the server side:

importsocketserver

classMyUDPHandler(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",9999
withsocketserver.UDPServer((HOST,PORT),MyUDPHandler)asserver:
server.serve_forever()

This is the client side:

importsocket
importsys

HOST,PORT="localhost",9999
data="".join(sys.argv[1:])

# SOCK_DGRAM is the socket type to use for UDP sockets
sock=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))

The output of the example should look exactly like for the TCP server example.

Asynchronous Mixins

To build asynchronous handlers, use theThreadingMixInand ForkingMixInclasses.

An example for theThreadingMixInclass:

importsocket
importthreading
importsocketserver

classThreadedTCPRequestHandler(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):
pass

defclient(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 port
HOST,PORT="localhost",0

server=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 request
server_thread=threading.Thread(target=server.serve_forever)
# Exit the server thread when the main thread terminates
server_thread.daemon=True
server_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()

The output of the example should look something like this:

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

TheForkingMixInclass is used in the same way, except that the server will spawn a new process for each request. Available only on POSIX platforms that supportfork().