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

C H A P T E R 5
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Network Data and Network Errors
The first four chapters have given us a foundation: we have learned how hosts are named on an IP
network, and we understand how to set up and tear down both TCP streams and UDP datagram
connections between those hosts.
But what data should we then send across those lengths? How should it be encoded and formatted?
For what kinds of errors will our Python programs need to be prepared?
These questions are all relevant regardless of whether we are using streams or datagrams. We will
look at the basic answers in this chapter, and learn how to use sockets responsibly so that our data
arrives intact.
Text and Encodings
If you were watching for it as you read the first few chapters, you may have caught me using two
different terms for the same concept. Those terms were byte and octet, and by both words I always mean
an 8-bit number—an ordered sequence of eight digits, that are each either a one or a zero. They are the
fundamental units of data on modern computing systems, used both to represent raw binary numbers
and to stand for characters or symbols. The binary number 1010000, for example, usually stands for
either the number 80 or the letter P:
>>> 0b1010000
80
>>> chr(0b1010000)
'P'
The reason that the Internet RFCs are so inveterate in their use of the term “octet” instead of “byte”
is that the earliest of RFCs date from a very ancient era in which bytes could be one of several different
lengths—byte sizes from as little as 5 to as many as 16 bits were used on various systems. So the term
“octet,” meaning a “group of eight things,” is always used in the standards so that their meaning is
unambiguous.
Four bits offer a mere sixteen values, which does not come close to even fitting our alphabet. But
eight bits—the next-higher multiple of two—proved more than enough to fit both the upper and lower
cases of our alphabet, all the digits, lots of punctuation, and 32 control codes, and it still left a whole half
of the possible range of values empty. The problem is that many rival systems exist for the specific
mapping used to turn characters into bytes, and the differences can cause problems unless both ends of
your network connection use the same rules.
The use of ASCII for the basic English letters and numbers is nearly universal among network
protocols these days. But when you begin to use more interesting characters, you have to be careful. In
Python you should always represent a meaningful string of text with a “Unicode string” that is denoted
with a leading u, like this:
>>> elvish = u'Namárië!'
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