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

CHAPTER 5 ■ NETWORK DATA AND NETWORK ERRORS
Again, most protocol designers make compression optional and simply do their own framing. But if
you know ahead of time that you will always want to use zlib, then a convention like this would let you
take advantage of the stream termination built into zlib and always detect the end of a compressed
stream.
Network Exceptions
The example scripts in this book are generally designed to catch only those exceptions that are integral
to the feature being demonstrated. So when we illustrated socket timeouts in Listing 2-2, we were careful
to catch the exception socket.timeout since that is how timeouts are signaled; but we ignored all of the
other exceptions that will occur if the hostname provided on the command line is invalid, or a remote IP
is used with bind(), or the port used with bind() is already busy, or the peer cannot be contacted or
stops responding.
What errors can result from working with sockets? Though the number of errors that can take place
while using a network connection is quite large—involving every possible misstep that can occur at
every stage of the complex TCP/IP protocol, for example—the number of actual exceptions with which
socket operations can hit your programs is fortunately quite few. The exceptions that are specific to
socket operations are:
• socket.gaierror: This exception is raised when getaddrinfo() cannot find a name
or service that you ask about—hence the letters G, A, and I in its name! It can be
raised not only when you make an explicit call to getaddrinfo(), but if you supply
a hostname instead of an IP address to a call like bind() or connect() and the
hostname lookup fails:
>>> import socket
>>> s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>>> s.connect(('nonexistent.hostname.foo.bar', 80))
Traceback (most recent call last):
...
gaierror: [Errno -5] No address associated with hostname
• socket.error: This is the workhorse of the socket module, and will be raised for
nearly every failure that can happen at any stage in a network transmission.
Starting with Python 2.6, this exception became, appropriately enough, a subclass
of the more general IOError. This can occur during nearly any socket call, even
when you least expect it—because a previous send(), for example, might have
elicited a reset (RST) packet from the remote host and the error will then be
delivered whenever you next try to manipulate the socket.
• socket.timeout: This exception is raised only if you, or a library that you are using,
decides to set a timeout on a socket rather than wait forever for a send() or recv()
to complete. It indicates that the timeout was reached before the operation could
complete normally.
You will see that the Standard Library documentation for the socket module also describes an
herror exception; fortunately, it can occur only if you use certain old-fashioned address lookup calls
instead of following the practices we outlined in Chapter 4.
A big question when you are using higher-level socket-based protocols from Python is whether they
allow raw socket errors to hit your own code, or whether they catch them and turn them into their own
kind of error.
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