5.7. Built-in Module socket

For an introduction to socket programming (in C), see the following papers: An Introductory 4.3BSD Interprocess Communication Tutorial, by Stuart Sechrest and An Advanced 4.3BSD Interprocess Communication Tutorial, by Samuel J. Leffler et al, both in the UNIX Programmer's Manual, Supplementary Documents 1 (sections PS1:7 and PS1:8). The UNIX manual pages for the various socket-related system calls also a valuable source of information on the details of socket semantics.

The Python interface is a straightforward transliteration of the UNIX system call and library interface for sockets to Python's object-oriented style: the socket() function returns a socket object whose methods implement the various socket system calls. Parameter types are somewhat higer-level than in the C interface: as with read() and write() operations on Python files, buffer allocation on receive operations is automatic, and buffer length is implicit on send operations.

Socket addresses are represented as a single string for the AF_UNIX address family and as a pair (host, port) for the AF_INET address family, where host is a string representing either a hostname in Internet domain notation like 'daring.cwi.nl' or an IP address like '100.50.200.5', and port is an integral port number. Other address families are currently not supported. The address format required by a particular socket object is automatically selected based on the address family specified when the socket object was created.

All errors raise exceptions. The normal exceptions for invalid argument types and out-of-memory conditions can be raised; errors related to socket or address semantics raise the error socket.error.

Non-blocking and asynchronous mode are not supported; see module select for a way to do non-blocking socket I/O.

The module socket exports the following constants and functions:

error -- exception of module socket

This exception is raised for socket- or address-related errors. The accompanying value is either a string telling what went wrong or a pair (errno, string) representing an error returned by a system call, similar to the value accompanying posix.error.

AF_UNIX -- data of module socket

AF_INET -- data of module socket

These constants represent the address (and protocol) families, used for the first argument to socket().

SOCK_STREAM -- data of module socket

SOCK_DGRAM -- data of module socket

These constants represent the socket types, used for the second argument to socket(). (There are other types, but only SOCK_STREAM and SOCK_DGRAM appear to be generally useful.)

gethostbyname (hostname) -- function of module socket

Translate a host name to IP address format. The IP address is returned as a string, e.g., '100.50.200.5'. If the host name is an IP address itself it is returned unchanged.

getservbyname (servicename, protocolname) -- function of module socket

Translate an Internet service name and protocol name to a port number for that service. The protocol name should be 'tcp' or 'udp'.

socket (family, type, proto) -- function of module socket

Create a new socket using the given address family, socket type and protocol number. The address family should be AF_INET or AF_UNIX. The socket type should be SOCK_STREAM, SOCK_DGRAM or perhaps one of the other `SOCK_' constants. The protocol number is usually zero and may be omitted in that case.

fromfd (fd, family, type, proto) -- function of module socket

Build a socket object from an existing file descriptor (an integer as returned by a file object's fileno method). Address family, socket type and protocol number are as for the socket function above. The file descriptor should refer to a socket, but this is not checked --- subsequent operations on the object may fail if the file descriptor is invalid. This function is rarely needed, but can be used to get or set socket options on a socket passed to a program as standard input or output (e.g. a server started by the UNIX inet daemon).

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Socket Object

Socket Example