Classes and functions

The module defines the following functions:

Listener(address=None, family=None, backlog=1, authenticate=False, authkey=None)

Returns a wrapper for a bound socket or Windows named pipe which is 'listening' for connections.

Client(address, family=None, authenticate=False, authkey=None)

Attempts to set up a connection to the listener which is using address address.

The type of the connection is determined by family argument, but this can generally be omitted since it can usually be inferred from the format of address.

If authentication or authkey is a string then then digest authentication is used. The key used for authentication will be either authkey or currentProcess.getAuthKey() if authkey is None. If authentication fails then AuthenticationError is raised. See Authentication keys.

A Connection object is returned. (See Connection objects.)

Pipe()

Returns a pair of two connection objects (see Connection objects) representing the ends of a duplex connection. It is also available directly from processing.

For example:

from processing import Process, Pipe

def foo(conn):
    conn.send(42)

if __name__ == '__main__':
    c, d = Pipe()
    p = Process(target=foo, args=[d])
    p.start()
    print c.recv()   # prints 42
    p.join()

Note that at most one thread/process should be sending or receiving from a connection object at a given time.

The module exports an exception type:

exception AuthenticationError

Exception raised when there is an authentication error.

Listener objects

Instances of Listener have the following methods:

__init__(address=None, family=None, backlog=1, authenticate=False, authkey=None)

address

The address to be used by the bound socket or named pipe of the listener object.

family

The type of the socket (or named pipe) to use.

This can be one of the strings 'AF_INET' (for a TCP socket), 'AF_UNIX' (for a Unix domain socket) or 'AF_PIPE' (for a Windows named pipe). Of these only the first is guaranteed to be available.

If family is None than the family is inferred from the format of address. If address is also None then a default is chosen. This default is the family which is assumed to be the fastest available. See Address formats.

backlog

If the listener object uses a socket then backlog is passed to the listen() method of the socket once it has been bound.

authenticate

If authenticate is true or authkey is not None then digest authentication is used.

authkey

If authkey is a string then it will be used as the authentication key; otherwise it must be None.

If authkey is None and authenticate is true then currentProcess.getAuthKey() is used as the authentication key.

If authkey is None and authentication is false then no authentication is done.

If authentication fails then AuthenticationError is raised. See Authentication keys.

accept()

Accept a connection on the bound socket or named pipe of the listener object. If authentication is attempted and fails then AuthenticationError is raised.

Returns a Connection object. See Connection objects.

close()

Close the bound socket or named pipe of the listener object.

This is called automatically when the listener is garbage collected.

Listener objects have the following read-only properties:

address

The address which is being used by the listener object.

last_accepted

The address from which the last accepted connection came.

If this is unavailable then None is returned.

Connection objects

A connection object represents one end of a message oriented socket or pipe connection. Connection objects have the following methods:

recv()

Return an object sent from the other end of the connection using send().

send(obj)

Send an object to the other end of the connection which should be read using recv().

The object must be picklable.

poll(timeout)

Return whether there is any data available to be read.

fileno()

Returns the file descriptor or handle used by the connection.

close()

Close the connection.

This is called automatically when the connection is garbage collected.

_recv_string()

Return a string sent from the other end of the connection using send_string().

_send_string(obj)

Send a string to the other end of the connection which should be read using recv_string().

Transferring connection objects between processes

If the C extension processing._process is available and contains support then socket objects, connection objects and file objects can be successfully pickled in one process and unpickled in another.

Note however that on Windows there is no socket.fromfd() function. As a result on Windows an unpickled socket object is not a true socket object: only the recv(), send(), sendall(), close() and fileno() methods will work.

Address formats

• An 'AF_INET' address is a tuple of the form (hostname, port) where hostname is a string and port is an integer

• An 'AF_UNIX' address is a string representing a filename on the filesystem.

• An 'AF_PIPE' address is a string of the form r'\\.\pipe\PipeName'.

  To use Client to connect to a named pipe on a remote computer called ServerName one should use an address of the form r'\\ServerName\pipe\PipeName' instead.

Note that any string beginning with two backslashes is assumed by default to be an 'AF_PIPE' address rather than an 'AF_UNIX' address.

Authentication keys

When one uses the recv() method of a connection object, the data received is automatically unpickled. Unfortunately unpickling data from an untrusted source is a security risk. Therefore Listener and Client use the hmac module to provide digest authentication.

An authentication key is a string which can be thought of as a password: once a connection is established both ends will demand proof that the other knows the authentication key. (Demonstrating that both ends are using the same key does not involve sending the key over the connection.)

If authentication is requested but do authentication key is specified then the return value of currentProcess().getAuthKey() is used (see Process objects). This value will automatically inherited by any Process object that the current process creates. This means that (by default) all processes of a multi-process program will share a single authentication key which can be used when setting up connections between the themselves.

Suitable authentication keys can also be generated by using os.urandom().

Example

The following server code creates a listener which uses 'secret password' as an authentication key. It then waits for a connection, receives an object and then sends it back boxed inside a list:

from processing.connection import Listener

address = ('localhost', 6000)     # family is deduced to be 'AF_INET'
listener = Listener(address, authkey='secret password')

conn = listener.accept()
print 'connection accepted from', listener.last_accepted

obj = conn.recv()
boxed_obj = [obj]
conn.send(boxed_obj)

The following code connects to the server, sends it the number 42, and then prints the reply from the server:

from processing.connection import Client

address = ('localhost', 6000)
conn = Client(address, authkey='secret password')

obj = 42
print 'sending:', obj
conn.send(obj)
print 'received:', conn.recv()    # prints 'received: [42]'