17.2. "socket" — Low-level networking interface
***********************************************

This module provides access to the BSD *socket* interface. It is
available on all modern Unix systems, Windows, Mac OS X, BeOS, OS/2,
and probably additional platforms.

Note: Some behavior may be platform dependent, since calls are made
  to the operating system socket APIs.

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
platform-specific reference material for the various socket-related
system calls are also a valuable source of information on the details
of socket semantics.  For Unix, refer to the manual pages; for
Windows, see the WinSock (or Winsock 2) specification. For IPv6-ready
APIs, readers may want to refer to **RFC 3493** titled Basic Socket
Interface Extensions for IPv6.

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 higher-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 follows: A single string is used
for the "AF_UNIX" address family. A pair "(host, port)" is used 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 IPv4 address like "'100.50.200.5'", and *port* is an integer.
For "AF_INET6" address family, a four-tuple "(host, port, flowinfo,
scopeid)" is used, where *flowinfo* and *scopeid* represents
"sin6_flowinfo" and "sin6_scope_id" member in "struct sockaddr_in6" in
C. For "socket" module methods, *flowinfo* and *scopeid* can be
omitted just for backward compatibility. Note, however, omission of
*scopeid* can cause problems in manipulating scoped IPv6 addresses.
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.

For IPv4 addresses, two special forms are accepted instead of a host
address: the empty string represents "INADDR_ANY", and the string
"'<broadcast>'" represents "INADDR_BROADCAST". The behavior is not
available for IPv6 for backward compatibility, therefore, you may want
to avoid these if you intend to support IPv6 with your Python
programs.

If you use a hostname in the *host* portion of IPv4/v6 socket address,
the program may show a nondeterministic behavior, as Python uses the
first address returned from the DNS resolution.  The socket address
will be resolved differently into an actual IPv4/v6 address, depending
on the results from DNS resolution and/or the host configuration.  For
deterministic behavior use a numeric address in *host* portion.

New in version 2.5: AF_NETLINK sockets are represented as  pairs "pid,
groups".

New in version 2.6: Linux-only support for TIPC is also available
using the "AF_TIPC" address family. TIPC is an open, non-IP based
networked protocol designed for use in clustered computer
environments.  Addresses are represented by a tuple, and the fields
depend on the address type. The general tuple form is "(addr_type, v1,
v2, v3 [, scope])", where:

* *addr_type* is one of "TIPC_ADDR_NAMESEQ", "TIPC_ADDR_NAME", or
  "TIPC_ADDR_ID".

* *scope* is one of "TIPC_ZONE_SCOPE", "TIPC_CLUSTER_SCOPE", and
  "TIPC_NODE_SCOPE".

* If *addr_type* is "TIPC_ADDR_NAME", then *v1* is the server type,
  *v2* is the port identifier, and *v3* should be 0.

  If *addr_type* is "TIPC_ADDR_NAMESEQ", then *v1* is the server type,
  *v2* is the lower port number, and *v3* is the upper port number.

  If *addr_type* is "TIPC_ADDR_ID", then *v1* is the node, *v2* is the
  reference, and *v3* should be set to 0.

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 mode is supported through "setblocking()".  A
generalization of this based on timeouts is supported through
"settimeout()".

The module "socket" exports the following constants and functions:

exception socket.error

   This exception is raised for socket-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 "os.error". See the module
   "errno", which contains names for the error codes defined by the
   underlying operating system.

   Changed in version 2.6: "socket.error" is now a child class of
   "IOError".

exception socket.herror

   This exception is raised for address-related errors, i.e. for
   functions that use *h_errno* in the C API, including
   "gethostbyname_ex()" and "gethostbyaddr()".

   The accompanying value is a pair "(h_errno, string)" representing
   an error returned by a library call. *string* represents the
   description of *h_errno*, as returned by the "hstrerror()" C
   function.

exception socket.gaierror

   This exception is raised for address-related errors, for
   "getaddrinfo()" and "getnameinfo()". The accompanying value is a
   pair "(error, string)" representing an error returned by a library
   call. *string* represents the description of *error*, as returned
   by the "gai_strerror()" C function. The *error* value will match
   one of the "EAI_*" constants defined in this module.

exception socket.timeout

   This exception is raised when a timeout occurs on a socket which
   has had timeouts enabled via a prior call to "settimeout()".  The
   accompanying value is a string whose value is currently always
   “timed out”.

   New in version 2.3.

socket.AF_UNIX
socket.AF_INET
socket.AF_INET6

   These constants represent the address (and protocol) families, used
   for the first argument to "socket()".  If the "AF_UNIX" constant is
   not defined then this protocol is unsupported.

socket.SOCK_STREAM
socket.SOCK_DGRAM
socket.SOCK_RAW
socket.SOCK_RDM
socket.SOCK_SEQPACKET

   These constants represent the socket types, used for the second
   argument to "socket()". (Only "SOCK_STREAM" and "SOCK_DGRAM" appear
   to be generally useful.)

SO_*
socket.SOMAXCONN
MSG_*
SOL_*
IPPROTO_*
IPPORT_*
INADDR_*
IP_*
IPV6_*
EAI_*
AI_*
NI_*
TCP_*

   Many constants of these forms, documented in the Unix documentation
   on sockets and/or the IP protocol, are also defined in the socket
   module. They are generally used in arguments to the "setsockopt()"
   and "getsockopt()" methods of socket objects.  In most cases, only
   those symbols that are defined in the Unix header files are
   defined; for a few symbols, default values are provided.

SIO_*
RCVALL_*

   Constants for Windows’ WSAIoctl(). The constants are used as
   arguments to the "ioctl()" method of socket objects.

   New in version 2.6.

TIPC_*

   TIPC related constants, matching the ones exported by the C socket
   API. See the TIPC documentation for more information.

   New in version 2.6.

socket.has_ipv6

   This constant contains a boolean value which indicates if IPv6 is
   supported on this platform.

   New in version 2.3.

socket.create_connection(address[, timeout[, source_address]])

   Connect to a TCP service listening on the Internet *address* (a
   2-tuple "(host, port)"), and return the socket object.  This is a
   higher-level function than "socket.connect()": if *host* is a non-
   numeric hostname, it will try to resolve it for both "AF_INET" and
   "AF_INET6", and then try to connect to all possible addresses in
   turn until a connection succeeds.  This makes it easy to write
   clients that are compatible to both IPv4 and IPv6.

   Passing the optional *timeout* parameter will set the timeout on
   the socket instance before attempting to connect.  If no *timeout*
   is supplied, the global default timeout setting returned by
   "getdefaulttimeout()" is used.

   If supplied, *source_address* must be a 2-tuple "(host, port)" for
   the socket to bind to as its source address before connecting.  If
   host or port are ‘’ or 0 respectively the OS default behavior will
   be used.

   New in version 2.6.

   Changed in version 2.7: *source_address* was added.

socket.getaddrinfo(host, port[, family[, socktype[, proto[, flags]]]])

   Translate the *host*/*port* argument into a sequence of 5-tuples
   that contain all the necessary arguments for creating a socket
   connected to that service. *host* is a domain name, a string
   representation of an IPv4/v6 address or "None". *port* is a string
   service name such as "'http'", a numeric port number or "None".  By
   passing "None" as the value of *host* and *port*, you can pass
   "NULL" to the underlying C API.

   The *family*, *socktype* and *proto* arguments can be optionally
   specified in order to narrow the list of addresses returned.  By
   default, their value is "0", meaning that the full range of results
   is selected. The *flags* argument can be one or several of the
   "AI_*" constants, and will influence how results are computed and
   returned.  Its default value is "0".  For example, "AI_NUMERICHOST"
   will disable domain name resolution and will raise an error if
   *host* is a domain name.

   The function returns a list of 5-tuples with the following
   structure:

   "(family, socktype, proto, canonname, sockaddr)"

   In these tuples, *family*, *socktype*, *proto* are all integers and
   are meant to be passed to the "socket()" function.  *canonname*
   will be a string representing the canonical name of the *host* if
   "AI_CANONNAME" is part of the *flags* argument; else *canonname*
   will be empty.  *sockaddr* is a tuple describing a socket address,
   whose format depends on the returned *family* (a "(address, port)"
   2-tuple for "AF_INET", a "(address, port, flow info, scope id)"
   4-tuple for "AF_INET6"), and is meant to be passed to the
   "socket.connect()" method.

   The following example fetches address information for a
   hypothetical TCP connection to "example.org" on port 80 (results
   may differ on your system if IPv6 isn’t enabled):

      >>> socket.getaddrinfo("example.org", 80, 0, 0, socket.IPPROTO_TCP)
      [(10, 1, 6, '', ('2606:2800:220:1:248:1893:25c8:1946', 80, 0, 0)),
       (2, 1, 6, '', ('93.184.216.34', 80))]

   New in version 2.2.

socket.getfqdn([name])

   Return a fully qualified domain name for *name*. If *name* is
   omitted or empty, it is interpreted as the local host.  To find the
   fully qualified name, the hostname returned by "gethostbyaddr()" is
   checked, followed by aliases for the host, if available.  The first
   name which includes a period is selected.  In case no fully
   qualified domain name is available, the hostname as returned by
   "gethostname()" is returned.

   New in version 2.0.

socket.gethostbyname(hostname)

   Translate a host name to IPv4 address format.  The IPv4 address is
   returned as a string, such as  "'100.50.200.5'".  If the host name
   is an IPv4 address itself it is returned unchanged.  See
   "gethostbyname_ex()" for a more complete interface.
   "gethostbyname()" does not support IPv6 name resolution, and
   "getaddrinfo()" should be used instead for IPv4/v6 dual stack
   support.

socket.gethostbyname_ex(hostname)

   Translate a host name to IPv4 address format, extended interface.
   Return a triple "(hostname, aliaslist, ipaddrlist)" where
   *hostname* is the primary host name responding to the given
   *ip_address*, *aliaslist* is a (possibly empty) list of alternative
   host names for the same address, and *ipaddrlist* is a list of IPv4
   addresses for the same interface on the same host (often but not
   always a single address). "gethostbyname_ex()" does not support
   IPv6 name resolution, and "getaddrinfo()" should be used instead
   for IPv4/v6 dual stack support.

socket.gethostname()

   Return a string containing the hostname of the machine where  the
   Python interpreter is currently executing.

   If you want to know the current machine’s IP address, you may want
   to use "gethostbyname(gethostname())". This operation assumes that
   there is a valid address-to-host mapping for the host, and the
   assumption does not always hold.

   Note: "gethostname()" doesn’t always return the fully qualified
   domain name; use "getfqdn()" (see above).

socket.gethostbyaddr(ip_address)

   Return a triple "(hostname, aliaslist, ipaddrlist)" where
   *hostname* is the primary host name responding to the given
   *ip_address*, *aliaslist* is a (possibly empty) list of alternative
   host names for the same address, and *ipaddrlist* is a list of
   IPv4/v6 addresses for the same interface on the same host (most
   likely containing only a single address). To find the fully
   qualified domain name, use the function "getfqdn()".
   "gethostbyaddr()" supports both IPv4 and IPv6.

socket.getnameinfo(sockaddr, flags)

   Translate a socket address *sockaddr* into a 2-tuple "(host,
   port)". Depending on the settings of *flags*, the result can
   contain a fully-qualified domain name or numeric address
   representation in *host*.  Similarly, *port* can contain a string
   port name or a numeric port number.

   New in version 2.2.

socket.getprotobyname(protocolname)

   Translate an Internet protocol name (for example, "'icmp'") to a
   constant suitable for passing as the (optional) third argument to
   the "socket()" function.  This is usually only needed for sockets
   opened in “raw” mode ("SOCK_RAW"); for the normal socket modes, the
   correct protocol is chosen automatically if the protocol is omitted
   or zero.

socket.getservbyname(servicename[, protocolname])

   Translate an Internet service name and protocol name to a port
   number for that service.  The optional protocol name, if given,
   should be "'tcp'" or "'udp'", otherwise any protocol will match.

socket.getservbyport(port[, protocolname])

   Translate an Internet port number and protocol name to a service
   name for that service.  The optional protocol name, if given,
   should be "'tcp'" or "'udp'", otherwise any protocol will match.

socket.socket([family[, type[, proto]]])

   Create a new socket using the given address family, socket type and
   protocol number.  The address family should be "AF_INET" (the
   default), "AF_INET6" or "AF_UNIX".  The socket type should be
   "SOCK_STREAM" (the default), "SOCK_DGRAM" or perhaps one of the
   other "SOCK_" constants.  The protocol number is usually zero and
   may be omitted in that case.

socket.socketpair([family[, type[, proto]]])

   Build a pair of connected socket objects using the given address
   family, socket type, and protocol number.  Address family, socket
   type, and protocol number are as for the "socket()" function above.
   The default family is "AF_UNIX" if defined on the platform;
   otherwise, the default is "AF_INET". Availability: Unix.

   New in version 2.4.

socket.fromfd(fd, family, type[, proto])

   Duplicate the file descriptor *fd* (an integer as returned by a
   file object’s "fileno()" method) and build a socket object from the
   result.  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 (such as a
   server started by the Unix inet daemon).  The socket is assumed to
   be in blocking mode. Availability: Unix.

socket.ntohl(x)

   Convert 32-bit positive integers from network to host byte order.
   On machines where the host byte order is the same as network byte
   order, this is a no-op; otherwise, it performs a 4-byte swap
   operation.

socket.ntohs(x)

   Convert 16-bit positive integers from network to host byte order.
   On machines where the host byte order is the same as network byte
   order, this is a no-op; otherwise, it performs a 2-byte swap
   operation.

socket.htonl(x)

   Convert 32-bit positive integers from host to network byte order.
   On machines where the host byte order is the same as network byte
   order, this is a no-op; otherwise, it performs a 4-byte swap
   operation.

socket.htons(x)

   Convert 16-bit positive integers from host to network byte order.
   On machines where the host byte order is the same as network byte
   order, this is a no-op; otherwise, it performs a 2-byte swap
   operation.

socket.inet_aton(ip_string)

   Convert an IPv4 address from dotted-quad string format (for
   example, ‘123.45.67.89’) to 32-bit packed binary format, as a
   string four characters in length.  This is useful when conversing
   with a program that uses the standard C library and needs objects
   of type "struct in_addr", which is the C type for the 32-bit packed
   binary this function returns.

   "inet_aton()" also accepts strings with less than three dots; see
   the Unix manual page *inet(3)* for details.

   If the IPv4 address string passed to this function is invalid,
   "socket.error" will be raised. Note that exactly what is valid
   depends on the underlying C implementation of "inet_aton()".

   "inet_aton()" does not support IPv6, and "inet_pton()" should be
   used instead for IPv4/v6 dual stack support.

socket.inet_ntoa(packed_ip)

   Convert a 32-bit packed IPv4 address (a string four characters in
   length) to its standard dotted-quad string representation (for
   example, ‘123.45.67.89’).  This is useful when conversing with a
   program that uses the standard C library and needs objects of type
   "struct in_addr", which is the C type for the 32-bit packed binary
   data this function takes as an argument.

   If the string passed to this function is not exactly 4 bytes in
   length, "socket.error" will be raised. "inet_ntoa()" does not
   support IPv6, and "inet_ntop()" should be used instead for IPv4/v6
   dual stack support.

socket.inet_pton(address_family, ip_string)

   Convert an IP address from its family-specific string format to a
   packed, binary format. "inet_pton()" is useful when a library or
   network protocol calls for an object of type "struct in_addr"
   (similar to "inet_aton()") or "struct in6_addr".

   Supported values for *address_family* are currently "AF_INET" and
   "AF_INET6". If the IP address string *ip_string* is invalid,
   "socket.error" will be raised. Note that exactly what is valid
   depends on both the value of *address_family* and the underlying
   implementation of "inet_pton()".

   Availability: Unix (maybe not all platforms).

   New in version 2.3.

socket.inet_ntop(address_family, packed_ip)

   Convert a packed IP address (a string of some number of characters)
   to its standard, family-specific string representation (for
   example, "'7.10.0.5'" or "'5aef:2b::8'") "inet_ntop()" is useful
   when a library or network protocol returns an object of type
   "struct in_addr" (similar to "inet_ntoa()") or "struct in6_addr".

   Supported values for *address_family* are currently "AF_INET" and
   "AF_INET6". If the string *packed_ip* is not the correct length for
   the specified address family, "ValueError" will be raised.  A
   "socket.error" is raised for errors from the call to "inet_ntop()".

   Availability: Unix (maybe not all platforms).

   New in version 2.3.

socket.getdefaulttimeout()

   Return the default timeout in seconds (float) for new socket
   objects. A value of "None" indicates that new socket objects have
   no timeout. When the socket module is first imported, the default
   is "None".

   New in version 2.3.

socket.setdefaulttimeout(timeout)

   Set the default timeout in seconds (float) for new socket objects.
   A value of "None" indicates that new socket objects have no
   timeout. When the socket module is first imported, the default is
   "None".

   New in version 2.3.

socket.SocketType

   This is a Python type object that represents the socket object
   type. It is the same as "type(socket(...))".

See also:

  Module "SocketServer"
     Classes that simplify writing network servers.

  Module "ssl"
     A TLS/SSL wrapper for socket objects.


17.2.1. Socket Objects
======================

Socket objects have the following methods.  Except for "makefile()"
these correspond to Unix system calls applicable to sockets.

socket.accept()

   Accept a connection. The socket must be bound to an address and
   listening for connections. The return value is a pair "(conn,
   address)" where *conn* is a *new* socket object usable to send and
   receive data on the connection, and *address* is the address bound
   to the socket on the other end of the connection.

socket.bind(address)

   Bind the socket to *address*.  The socket must not already be
   bound. (The format of *address* depends on the address family — see
   above.)

   Note: This method has historically accepted a pair of parameters
     for "AF_INET" addresses instead of only a tuple.  This was never
     intentional and is no longer available in Python 2.0 and later.

socket.close()

   Close the socket.  All future operations on the socket object will
   fail. The remote end will receive no more data (after queued data
   is flushed). Sockets are automatically closed when they are
   garbage-collected.

   Note: "close()" releases the resource associated with a
     connection but does not necessarily close the connection
     immediately.  If you want to close the connection in a timely
     fashion, call "shutdown()" before "close()".

socket.connect(address)

   Connect to a remote socket at *address*. (The format of *address*
   depends on the address family — see above.)

   Note: This method has historically accepted a pair of parameters
     for "AF_INET" addresses instead of only a tuple.  This was never
     intentional and is no longer available in Python 2.0 and later.

socket.connect_ex(address)

   Like "connect(address)", but return an error indicator instead of
   raising an exception for errors returned by the C-level "connect()"
   call (other problems, such as “host not found,” can still raise
   exceptions).  The error indicator is "0" if the operation
   succeeded, otherwise the value of the "errno" variable.  This is
   useful to support, for example, asynchronous connects.

   Note: This method has historically accepted a pair of parameters
     for "AF_INET" addresses instead of only a tuple. This was never
     intentional and is no longer available in Python 2.0 and later.

socket.fileno()

   Return the socket’s file descriptor (a small integer).  This is
   useful with "select.select()".

   Under Windows the small integer returned by this method cannot be
   used where a file descriptor can be used (such as "os.fdopen()").
   Unix does not have this limitation.

socket.getpeername()

   Return the remote address to which the socket is connected.  This
   is useful to find out the port number of a remote IPv4/v6 socket,
   for instance. (The format of the address returned depends on the
   address family — see above.)  On some systems this function is not
   supported.

socket.getsockname()

   Return the socket’s own address.  This is useful to find out the
   port number of an IPv4/v6 socket, for instance. (The format of the
   address returned depends on the address family — see above.)

socket.getsockopt(level, optname[, buflen])

   Return the value of the given socket option (see the Unix man page
   *getsockopt(2)*).  The needed symbolic constants ("SO_*" etc.) are
   defined in this module.  If *buflen* is absent, an integer option
   is assumed and its integer value is returned by the function.  If
   *buflen* is present, it specifies the maximum length of the buffer
   used to receive the option in, and this buffer is returned as a
   string.  It is up to the caller to decode the contents of the
   buffer (see the optional built-in module "struct" for a way to
   decode C structures encoded as strings).

socket.ioctl(control, option)

   Platform:
      Windows

   The "ioctl()" method is a limited interface to the WSAIoctl system
   interface.  Please refer to the Win32 documentation for more
   information.

   On other platforms, the generic "fcntl.fcntl()" and "fcntl.ioctl()"
   functions may be used; they accept a socket object as their first
   argument.

   New in version 2.6.

socket.listen(backlog)

   Listen for connections made to the socket.  The *backlog* argument
   specifies the maximum number of queued connections and should be at
   least 0; the maximum value is system-dependent (usually 5), the
   minimum value is forced to 0.

socket.makefile([mode[, bufsize]])

   Return a *file object* associated with the socket.  (File objects
   are described in File Objects.) The file object does not close the
   socket explicitly when its "close()" method is called, but only
   removes its reference to the socket object, so that the socket will
   be closed if it is not referenced from anywhere else.

   The socket must be in blocking mode (it can not have a timeout).
   The optional *mode* and *bufsize* arguments are interpreted the
   same way as by the built-in "file()" function.

   Note: On Windows, the file-like object created by "makefile()"
     cannot be used where a file object with a file descriptor is
     expected, such as the stream arguments of "subprocess.Popen()".

socket.recv(bufsize[, flags])

   Receive data from the socket.  The return value is a string
   representing the data received.  The maximum amount of data to be
   received at once is specified by *bufsize*.  See the Unix manual
   page *recv(2)* for the meaning of the optional argument *flags*; it
   defaults to zero.

   Note: For best match with hardware and network realities, the
     value of *bufsize* should be a relatively small power of 2, for
     example, 4096.

socket.recvfrom(bufsize[, flags])

   Receive data from the socket.  The return value is a pair "(string,
   address)" where *string* is a string representing the data received
   and *address* is the address of the socket sending the data.  See
   the Unix manual page *recv(2)* for the meaning of the optional
   argument *flags*; it defaults to zero. (The format of *address*
   depends on the address family — see above.)

socket.recvfrom_into(buffer[, nbytes[, flags]])

   Receive data from the socket, writing it into *buffer* instead of
   creating a new string.  The return value is a pair "(nbytes,
   address)" where *nbytes* is the number of bytes received and
   *address* is the address of the socket sending the data.  See the
   Unix manual page *recv(2)* for the meaning of the optional argument
   *flags*; it defaults to zero.  (The format of *address* depends on
   the address family — see above.)

   New in version 2.5.

socket.recv_into(buffer[, nbytes[, flags]])

   Receive up to *nbytes* bytes from the socket, storing the data into
   a buffer rather than creating a new string.  If *nbytes* is not
   specified (or 0), receive up to the size available in the given
   buffer.  Returns the number of bytes received.  See the Unix manual
   page *recv(2)* for the meaning of the optional argument *flags*; it
   defaults to zero.

   New in version 2.5.

socket.send(string[, flags])

   Send data to the socket.  The socket must be connected to a remote
   socket.  The optional *flags* argument has the same meaning as for
   "recv()" above. Returns the number of bytes sent. Applications are
   responsible for checking that all data has been sent; if only some
   of the data was transmitted, the application needs to attempt
   delivery of the remaining data. For further information on this
   concept, consult the Socket Programming HOWTO.

socket.sendall(string[, flags])

   Send data to the socket.  The socket must be connected to a remote
   socket.  The optional *flags* argument has the same meaning as for
   "recv()" above. Unlike "send()", this method continues to send data
   from *string* until either all data has been sent or an error
   occurs.  "None" is returned on success.  On error, an exception is
   raised, and there is no way to determine how much data, if any, was
   successfully sent.

socket.sendto(string, address)
socket.sendto(string, flags, address)

   Send data to the socket.  The socket should not be connected to a
   remote socket, since the destination socket is specified by
   *address*.  The optional *flags* argument has the same meaning as
   for "recv()" above.  Return the number of bytes sent. (The format
   of *address* depends on the address family — see above.)

socket.setblocking(flag)

   Set blocking or non-blocking mode of the socket: if *flag* is 0,
   the socket is set to non-blocking, else to blocking mode.
   Initially all sockets are in blocking mode.  In non-blocking mode,
   if a "recv()" call doesn’t find any data, or if a "send()" call
   can’t immediately dispose of the data, an "error" exception is
   raised; in blocking mode, the calls block until they can proceed.
   "s.setblocking(0)" is equivalent to "s.settimeout(0.0)";
   "s.setblocking(1)" is equivalent to "s.settimeout(None)".

socket.settimeout(value)

   Set a timeout on blocking socket operations.  The *value* argument
   can be a nonnegative float expressing seconds, or "None". If a
   float is given, subsequent socket operations will raise a "timeout"
   exception if the timeout period *value* has elapsed before the
   operation has completed.  Setting a timeout of "None" disables
   timeouts on socket operations. "s.settimeout(0.0)" is equivalent to
   "s.setblocking(0)"; "s.settimeout(None)" is equivalent to
   "s.setblocking(1)".

   New in version 2.3.

socket.gettimeout()

   Return the timeout in seconds (float) associated with socket
   operations, or "None" if no timeout is set.  This reflects the last
   call to "setblocking()" or "settimeout()".

   New in version 2.3.

Some notes on socket blocking and timeouts: A socket object can be in
one of three modes: blocking, non-blocking, or timeout.  Sockets are
always created in blocking mode.  In blocking mode, operations block
until complete or the system returns an error (such as connection
timed out).  In non-blocking mode, operations fail (with an error that
is unfortunately system-dependent) if they cannot be completed
immediately.  In timeout mode, operations fail if they cannot be
completed within the timeout specified for the socket or if the system
returns an error.  The "setblocking()" method is simply a shorthand
for certain "settimeout()" calls.

Timeout mode internally sets the socket in non-blocking mode.  The
blocking and timeout modes are shared between file descriptors and
socket objects that refer to the same network endpoint.  A consequence
of this is that file objects returned by the "makefile()" method must
only be used when the socket is in blocking mode; in timeout or non-
blocking mode file operations that cannot be completed immediately
will fail.

Note that the "connect()" operation is subject to the timeout setting,
and in general it is recommended to call "settimeout()" before calling
"connect()" or pass a timeout parameter to "create_connection()".  The
system network stack may return a connection timeout error of its own
regardless of any Python socket timeout setting.

socket.setsockopt(level, optname, value)

   Set the value of the given socket option (see the Unix manual page
   *setsockopt(2)*).  The needed symbolic constants are defined in the
   "socket" module ("SO_*" etc.).  The value can be an integer or a
   string representing a buffer.  In the latter case it is up to the
   caller to ensure that the string contains the proper bits (see the
   optional built-in module "struct" for a way to encode C structures
   as strings).

socket.shutdown(how)

   Shut down one or both halves of the connection.  If *how* is
   "SHUT_RD", further receives are disallowed.  If *how* is "SHUT_WR",
   further sends are disallowed.  If *how* is "SHUT_RDWR", further
   sends and receives are disallowed.  Depending on the platform,
   shutting down one half of the connection can also close the
   opposite half (e.g. on Mac OS X, "shutdown(SHUT_WR)" does not allow
   further reads on the other end of the connection).

Note that there are no methods "read()" or "write()"; use "recv()" and
"send()" without *flags* argument instead.

Socket objects also have these (read-only) attributes that correspond
to the values given to the "socket" constructor.

socket.family

   The socket family.

   New in version 2.5.

socket.type

   The socket type.

   New in version 2.5.

socket.proto

   The socket protocol.

   New in version 2.5.


17.2.2. Example
===============

Here are four minimal example programs using the TCP/IP protocol: a
server that echoes all data that it receives back (servicing only one
client), and a client using it.  Note that a server must perform the
sequence "socket()", "bind()", "listen()", "accept()" (possibly
repeating the "accept()" to service more than one client), while a
client only needs the sequence "socket()", "connect()".  Also note
that the server does not "sendall()"/"recv()" on the socket it is
listening on but on the new socket returned by "accept()".

The first two examples support IPv4 only.

   # Echo server program
   import socket

   HOST = ''                 # Symbolic name meaning all available interfaces
   PORT = 50007              # Arbitrary non-privileged port
   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
   s.bind((HOST, PORT))
   s.listen(1)
   conn, addr = s.accept()
   print 'Connected by', addr
   while 1:
       data = conn.recv(1024)
       if not data: break
       conn.sendall(data)
   conn.close()

   # Echo client program
   import socket

   HOST = 'daring.cwi.nl'    # The remote host
   PORT = 50007              # The same port as used by the server
   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
   s.connect((HOST, PORT))
   s.sendall('Hello, world')
   data = s.recv(1024)
   s.close()
   print 'Received', repr(data)

The next two examples are identical to the above two, but support both
IPv4 and IPv6. The server side will listen to the first address family
available (it should listen to both instead). On most of IPv6-ready
systems, IPv6 will take precedence and the server may not accept IPv4
traffic. The client side will try to connect to the all addresses
returned as a result of the name resolution, and sends traffic to the
first one connected successfully.

   # Echo server program
   import socket
   import sys

   HOST = None               # Symbolic name meaning all available interfaces
   PORT = 50007              # Arbitrary non-privileged port
   s = None
   for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC,
                                 socket.SOCK_STREAM, 0, socket.AI_PASSIVE):
       af, socktype, proto, canonname, sa = res
       try:
           s = socket.socket(af, socktype, proto)
       except socket.error as msg:
           s = None
           continue
       try:
           s.bind(sa)
           s.listen(1)
       except socket.error as msg:
           s.close()
           s = None
           continue
       break
   if s is None:
       print 'could not open socket'
       sys.exit(1)
   conn, addr = s.accept()
   print 'Connected by', addr
   while 1:
       data = conn.recv(1024)
       if not data: break
       conn.send(data)
   conn.close()

   # Echo client program
   import socket
   import sys

   HOST = 'daring.cwi.nl'    # The remote host
   PORT = 50007              # The same port as used by the server
   s = None
   for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC, socket.SOCK_STREAM):
       af, socktype, proto, canonname, sa = res
       try:
           s = socket.socket(af, socktype, proto)
       except socket.error as msg:
           s = None
           continue
       try:
           s.connect(sa)
       except socket.error as msg:
           s.close()
           s = None
           continue
       break
   if s is None:
       print 'could not open socket'
       sys.exit(1)
   s.sendall('Hello, world')
   data = s.recv(1024)
   s.close()
   print 'Received', repr(data)

The last example shows how to write a very simple network sniffer with
raw sockets on Windows. The example requires administrator privileges
to modify the interface:

   import socket

   # the public network interface
   HOST = socket.gethostbyname(socket.gethostname())

   # create a raw socket and bind it to the public interface
   s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_IP)
   s.bind((HOST, 0))

   # Include IP headers
   s.setsockopt(socket.IPPROTO_IP, socket.IP_HDRINCL, 1)

   # receive all packages
   s.ioctl(socket.SIO_RCVALL, socket.RCVALL_ON)

   # receive a package
   print s.recvfrom(65565)

   # disabled promiscuous mode
   s.ioctl(socket.SIO_RCVALL, socket.RCVALL_OFF)

Running an example several times with too small delay between
executions, could lead to this error:

   socket.error: [Errno 98] Address already in use

This is because the previous execution has left the socket in a
"TIME_WAIT" state, and can’t be immediately reused.

There is a "socket" flag to set, in order to prevent this,
"socket.SO_REUSEADDR":

   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
   s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
   s.bind((HOST, PORT))

the "SO_REUSEADDR" flag tells the kernel to reuse a local socket in
"TIME_WAIT" state, without waiting for its natural timeout to expire.
