Foundations of Python Network Programming 978-1-4302-3004-5

CHAPTER 3 ■ TCP
Using TCP Streams like Files
Since TCP supports streams of data, they might have already reminded you of normal files, which also
support reading and writing as fundamental operations. Python does a very good job of keeping these
concepts separate: file objects can read() and write(), sockets can send() and recv(), and no kind of
object can do both. This is actually a substantially cleaner conceptual split than is achieved by the
underlying POSIX interface, which lets a C programmer call read() and write() on a socket
indiscriminately as though it were a normal file descriptor!
But sometimes you will want to treat a socket like a normal Python file object—often because you
want to pass it into code like that of the many Python modules such as pickle, json, and zlib that can
read and write data directly from a file. For this purpose, Python provides a makefile() method on every
socket that returns a Python file object that is really calling recv() and send() behind the scenes:
>>> import socket
>>> s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>>> hasattr(s, 'read')
False
>>> f = s.makefile()
>>> hasattr(f, 'read')
True
Sockets, like normal Python files, also have a fileno() method that lets you discover their file
descriptor number in case you need to supply it to lower-level calls; we will find this very helpful when
we explore select() and poll() in Chapter 7.
Summary
The TCP-powered “stream” socket does whatever is necessary—including re-transmitting lost packets,
reordering the ones that arrive out of sequence, and splitting very large data streams into optimally sized
packets for your network in the first place—to support the transmission and reception of streams of data
over the network between two sockets.
As with UDP, port numbers are used by TCP to distinguish the many stream endpoints that might
exist on a single machine. A program that wants to accept incoming TCP connections needs to bind() to
a port, run listen() on the socket, and then go into a loop that runs accept() over and over to receive a
new socket for each incoming connection with which it can talk to each particular client that connects.
Programs that want to connect to existing server ports need only create a socket and connect() to an
address.
Servers will usually want to set the SO_REUSEADDR option on the sockets they bind(), lest old
connections still closing down on the same port from the last time the server was run prevent the
operating system from allowing the binding.
Data is actually sent and received with send() and recv(). Some protocols will mark up their data so
that clients and servers know automatically when a communication is complete. Other protocols will
treat the TCP socket as a true stream and send and receive until end-of-file is reached. The shutdown()
socket method can be used to produce end-of-file in one direction on a socket (all sockets are
bidirectional by nature) while leaving the other direction open.
Deadlock can occur if two peers are written such that the socket fills with more and more data that
never gets read. Eventually, one direction will no longer be able to send() and might hang forever
waiting for the backlog to clear.
If you want to pass a socket to a Python routine that knows how to read to or write from a normal
file object, the makefile() socket method will give you a Python object that calls recv() and send()
behind the scenes when the caller needs to read and write.
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