The SocketServer module simplifies the task of writing network
There are four basic server classes: TCPServer uses the
Internet TCP protocol, which provides for continuous streams of data
between the client and server. UDPServer uses datagrams, which
are discrete packets of information that may arrive out of order or be
lost while in transit. The more infrequently used
UnixStreamServer and UnixDatagramServer classes are
similar, but use Unix domain sockets; they're not available on
non-Unix platforms. For more details on network programming, consult
a book such as W. Richard Steven's UNIX Network Programming
or Ralph Davis's Win32 Network Programming.
These four classes process requests synchronously; 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
ForkingMixIn and ThreadingMixIn mix-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 the BaseRequestHandler
class and overriding its handle() 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. Finally, call the handle_request() or
serve_forever() method of the server object to process one or
Server classes have the same external methods and attributes, no
matter what network protocol they use:
- fileno ()
Return an integer file descriptor for the socket on which the server
is listening. This function is most commonly passed to
select.select(), to allow monitoring multiple servers in the
- 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's
handle_error() method will be called.
- serve_forever ()
Handle an infinite number of requests. This simply calls
handle_request() inside an infinite loop.
The family of protocols to which the server's socket belongs.
socket.AF_INET and socket.AF_UNIX are two
The user-provided request handler class; an instance of this class is
created for each request.
The address on which the server is listening. The format of addresses
varies depending on the protocol family; see the documentation for the
socket module 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.
The socket object on which the server will listen for incoming requests.
The server classes support the following class variables:
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 to request_queue_size requests. 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.
The type of socket used by the server; socket.SOCK_STREAM
and socket.SOCK_DGRAM are two possible values.
There are various server methods that can be overridden by subclasses
of base server classes like TCPServer; these methods aren't
useful to external users of the server object.
- finish_request ()
Actually processes the request by instantiating
RequestHandlerClass and calling its handle() method.
- get_request ()
Must accept a request from the socket, and return a 2-tuple containing
the new 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 the RequestHandlerClass's
handle() method raises an exception. The default action is
to print the traceback to standard output and continue handling
- process_request (request, client_address)
Calls finish_request() to create an instance of the
RequestHandlerClass. If desired, this function can create a
new process or thread to handle the request; the ForkingMixIn
and ThreadingMixIn classes do this.
- server_activate ()
Called by the server's constructor to activate the server.
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 is true, the request will be
processed, and if it's false, the request will be denied.
This function can be overridden to implement access controls for a server.
The default implementation always return true.
The request handler class must define a new handle() method,
and can override any of the following methods. A new instance is
created for each request.
- finish ()
Called after the handle() method to perform any clean-up
actions required. The default implementation does nothing. If
setup() or handle() raise an exception, this
function will not be called.
- handle ()
This function must do all the work required to service a request.
Several instance attributes are available to it; the request is
available as self.request; the client address as
self.client_address; and the server instance as
self.server, in case it needs access to per-server
The type of self.request is different for datagram or stream
services. For stream services, self.request is a socket
object; for datagram services, self.request is a string.
However, this can be hidden by using the mix-in request handler
StreamRequestHandler or DatagramRequestHandler, which
override the setup() and finish() methods, and
provides self.rfile and self.wfile attributes.
self.rfile and self.wfile can be read or written,
respectively, to get the request data or return data to the client.
- setup ()
Called before the handle() method to perform any
initialization actions required. The default implementation does
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