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<!--HEVEA command line is: /usr/bin/hevea -O -e latexonly png.hva htmlonly book -->
<!--CUT STYLE book--><!--CUT DEF chapter 1 --><p><span style="font-size:x-large">Python for Informatics: Exploring Information</span></p><p><span style="font-size:large">Charles Severance</span></p><p>Version 0.0.9-d2</p>
<!--TOC chapter id="sec1" Preface-->
<h1 class="chapter" id="sec1">Chapter 0  Preface</h1><!--SEC END --><!--TOC section id="sec2" Python for Informatics: Remixing an Open Book-->
<h2 class="section" id="sec2">Python for Informatics: Remixing an Open Book</h2><!--SEC END --><p>It is quite natural for academics who are continuously told to
“publish or perish” to want to always create something from scratch
that is their own fresh creation. This book is an
experiment in not starting from scratch, but instead “re-mixing”
the book titled
<em>Think Python: How to Think Like
a Computer Scientist</em>
written by Allen B. Downey, Jeff Elkner and others.</p><p>In December of 2009, I was preparing to teach
<span class="c009">SI502 - Networked Programming</span> at the University of Michigan
for the fifth semester in a row and decided it was time
to write a Python textbook that focused on exploring data
instead of understanding algorithms and abstractions.
My goal in SI502 is to teach people life-long data handling
skills using Python. Few of my
students were planning to be professional
computer programmers. Instead, they
planned be librarians, managers, lawyers, biologists, economists, etc.
who happened to want to skillfully use technology in their chosen field.</p><p>I never seemed to find the perfect data-oriented Python
book for my course so I set out
to write just such a book. Luckily at a faculty meeting three weeks
before I was about to start my new book from scratch over
the holiday break,
Dr. Atul Prakash showed me the <em>Think Python</em> book which he had
used to teach his Python course that semester.
It is a well-written Computer Science text with a focus on
short, direct explanations and ease of learning. </p><p>The overall book structure
has been changed to get to doing data analysis problems as quickly as
possible and have a series of running examples and exercises
about data analysis from the very beginning. </p><p>The chapters 2-10 are similar to the <em>Think Python</em> book
but there have been major changes. Number-oriented examples and
exercises have been replaced with data-oriented exercises.
Topics are presented in the order needed to build increasingly
sophisticated data analysis solutions. Some topics like <span class="c001">try</span> and
<span class="c001">except</span> are pulled forward and presented as part of the chapter
on conditionals. Functions are given very light treatment until
they are needed to handle program complexity rather introduced
as an early lesson in abstraction. Nearly all user-defined functions
have been removed from the example code and exercises outside Chapter 4.
The word “recursion”<sup><a id="text1" href="#note1">1</a></sup>
does not appear in the book at all.</p><p>In chapters 1 and 11-16, all of the material is brand new, focusing
on real-world uses and simple examples of Python for data analysis
including regular expressions for searching and parsing,
automating tasks on your computer, retrieving data across
the network, scraping web pages for data,
using web services, parsing XML and JSON data, and creating
and using databases using Structured Query Language.</p><p>The ultimate goal of all of these changes is a shift from a
Computer Science to an Informatics
focus is to only include topics into a first technology
class that can be useful even if one chooses not to
become a professional programmer.</p><p>Students who find this book interesting and want to further explore
should look at Allen B. Downey’s <em>Think Python</em> book. Because there
is a lot of overlap between the two books,
students will quickly pick up skills in the additional
areas of technical programming and algorithmic thinking
that are covered in <em>Think Python</em>.
And given that the books have a similar writing style, you should be
able to move quickly through <em>Think Python</em> with a minimum of effort.</p><p><a id="hevea_default0"></a>
<a id="hevea_default1"></a>
<a id="hevea_default2"></a>
As the copyright holder of <em>Think Python</em>,
Allen has given me permission to change the book’s license
on the material from his book that remains in this book
from the
GNU Free Documentation License
to the more recent
Creative Commons Attribution — Share Alike
license.
This follows a general shift in open documentation licenses moving
from the GFDL to the CC-BY-SA (i.e. Wikipedia).
Using the CC-BY-SA license maintains the book’s
strong copyleft tradition while making it even more straightforward
for new authors to reuse this material as they see fit.</p><p>I feel that this book serves an example of why open
materials are so important to the future of education,
and want to thank Allen B. Downey and Cambridge University
Press for their forward looking decision to make the book available
under an open Copyright. I hope they are pleased with the
results of my efforts and I hope that you the reader are pleased with
<em>our</em> collective efforts.</p><p>I would like to thank Allen B. Downey and Lauren Cowles for their help,
patience, and guidance in dealing with and resolving the copyright
issues around this book.</p><p>Charles Severance<br />
www.dr-chuck.com<br />
Ann Arbor, MI, USA<br />
September 9, 2013</p><p>Charles Severance is a
Clinical Associate Professor
at the University of Michigan School of Information.</p>
<!--BEGIN NOTES chapter-->
<hr class="footnoterule" /><dl class="thefootnotes"><dt class="dt-thefootnotes">
<a id="note1" href="#text1">1</a></dt><dd class="dd-thefootnotes"><div class="footnotetext">Except of course for this line.</div>
</dd></dl>
<!--END NOTES-->
<!--TOC chapter id="sec3" Why should you learn to write programs?-->
<h1 class="chapter" id="sec3">Chapter 1  Why should you learn to write programs?</h1><!--SEC END --><p>Writing programs (or programming) is a very creative
and rewarding activity. You can write programs for
many reasons ranging from making your living to solving
a difficult data analysis problem to having fun to helping
someone else solve a problem. This book assumes that
<em>everyone</em> needs to know how to program and that once
you know how to program, you will figure out what you want
to do with your newfound skills. </p><p>We are surrounded in our daily lives with computers ranging
from laptops to cell phones. We can think of these computers
as our “personal assistants” who can take care of many things
on our behalf. The hardware in our current-day computers
is essentially built to continuously ask us the question,
“What would you like me to do next?”.</p><div class="center"><img src="book001.png" /></div><p>Programmers add an operating system and a set of applications
to the hardware and we end up with a Personal Digital
Assistant that is quite helpful and capable of helping
many different things.</p><p>Our computers are fast and have vast amounts of memory and
could be very helpful to us if we only knew the language to
speak to explain to the computer what we would like it to
“do next”. If we knew this language we could tell the
computer to do tasks on our behalf that were repetitive.
Interestingly, the kinds of things computers can do best
are often the kinds of things that we humans find boring
and mind-numbing.</p><p>For example, look at the first three paragraphs of this
chapter and tell me the most commonly used word and how
many times the word is used. While you were able to read
and understand the words in a few seconds, counting them
is almost painful because it is not the kind of problem
that human minds are designed to solve. For a computer
the opposite is true, reading and understanding text
from a piece of paper is hard for a computer to do
but counting the words and telling you how many times
the most used word was used is very easy for the
computer:</p><pre class="verbatim"><span class="c004">python words.py
Enter file:words.txt
to 16
</span></pre><p><span class="c006">Our “personal information analysis assistant” quickly
told us that the word “to” was used sixteen times in the
first three paragraphs of this chapter.</span></p><p><span class="c006">This very fact that computers are good at things
that humans are not is why you need to become
skilled at talking “computer language”. Once you learn
this new language, you can delegate mundane tasks
to your partner (the computer), leaving more time
for you to do the
things that you are uniquely suited for. You bring
creativity, intuition, and inventiveness to this
partnership. </span></p><span class="c005">
</span><!--TOC section id="sec4" <span class="c000"><span class="c005">Creativity and motivation</span></span>-->
<h2 class="section" id="sec4"><span class="c006">1.1  Creativity and motivation</span></h2><!--SEC END --><p><span class="c006">While this book is not intended for professional programmers, professional
programming can be a very rewarding job both financially and personally.
Building useful, elegant, and clever programs for others to use is a very
creative activity. Your computer or Personal Digital Assistant (PDA)
usually contains many different programs from many different groups of
programmers, each competing for your attention and interest. They try
their best to meet your needs and give you a great user experience in the
process. In some situations, when you choose a piece of software, the
programmers are directly compensated because of your choice.</span></p><p><span class="c006">If we think of programs as the creative output of groups of programmers,
perhaps the following figure is a more sensible version of our PDA:</span></p><div class="center"><span class="c006"><img src="book002.png" /></span></div><p><span class="c006">For now, our primary motivation is not to make money or please end-users, but
instead for us to be more productive in handling the data and
information that we will encounter in our lives.
When you first start, you will be both the programmer and end-user of
your programs. As you gain skill as a programmer and
programming feels more creative to you, your thoughts may turn
toward developing programs for others.</span></p><span class="c005">
</span><!--TOC section id="sec5" <span class="c000"><span class="c005">Computer hardware architecture</span></span>-->
<h2 class="section" id="sec5"><span class="c006">1.2  Computer hardware architecture</span></h2><!--SEC END --><p><span class="c005">
</span><a id="hevea_default3"></a><span class="c005">
</span><a id="hevea_default4"></a></p><p><span class="c006">Before we start learning the language we
speak to give instructions to computers to
develop software, we need to learn a small amount about
how computers are built. If you were to take
apart your computer or cell phone and look deep
inside, you would find the following parts:</span></p><div class="center"><span class="c006"><img src="book003.png" /></span></div><p><span class="c006">The high-level definitions of these parts are as follows:</span></p><ul class="itemize"><li class="li-itemize"><span class="c006">The <span class="c009">Central Processing Unit</span> (or CPU) is
that part of the computer that is built to be obsessed
with “what is next?”. If your computer is rated
at 3.0 Gigahertz, it means that the CPU will ask “What next?”
three billion times per second. You are going to have to
learn how to talk fast to keep up with the CPU.</span></li><li class="li-itemize"><span class="c006">The <span class="c009">Main Memory</span> is used to store information
that the CPU needs in a hurry. The main memory is nearly as
fast as the CPU. But the information stored in the main
memory vanishes when the computer is turned off.</span></li><li class="li-itemize"><span class="c006">The <span class="c009">Secondary Memory</span> is also used to store
information, but it is much slower than the main memory.
The advantage of the secondary memory is that it can
store information even when there is no power to the
computer. Examples of secondary memory are disk drives
or flash memory (typically found in USB sticks and portable
music players).</span></li><li class="li-itemize"><span class="c006">The <span class="c009">Input and Output Devices</span> are simply our
screen, keyboard, mouse, microphone, speaker, touchpad, etc.
They are all of the ways we interact with the computer.</span></li><li class="li-itemize"><span class="c006">These days, most computers also have a
<span class="c009">Network Connection</span> to retrieve information over a network.
We can think of the network as a very slow place to store and
retrieve data that might not always be “up”. So in a sense,
the network is a slower and at times unreliable form of
<span class="c009">Secondary Memory</span></span></li></ul><p><span class="c006">While most of the detail of how these components work is best left
to computer builders, it helps to have some terminology
so we can talk about these different parts as we write our programs.</span></p><p><span class="c006">As a programmer, your job is to use and orchestrate
each of these resources to solve the problem that you need solving
and analyze the data you need. As a programmer you will
mostly be “talking” to the CPU and telling it what to
do next. Sometimes you will tell the CPU to use the main memory,
secondary memory, network, or the input/output devices.</span></p><div class="center"><span class="c006"><img src="book004.png" /></span></div><p><span class="c006">You need to be the person who answers the CPU’s “What next?”
question. But it would be very uncomfortable to shrink you
down to 5mm tall and insert you into the computer just so you
could issue a command three billion times per second. So instead,
you must write down your instructions in advance.
We call these stored instructions a <span class="c009">program</span> and the act
of writing these instructions down and getting the instructions to
be correct <span class="c009">programming</span>.</span></p><span class="c005">
</span><!--TOC section id="sec6" <span class="c000"><span class="c005">Understanding programming</span></span>-->
<h2 class="section" id="sec6"><span class="c006">1.3  Understanding programming</span></h2><!--SEC END --><p><span class="c006">In the rest of this book, we will try to turn you into a person
who is skilled in the art of programming. In the end you will be a
<span class="c009">programmer</span> — perhaps not a professional programmer but
at least you will have the skills to look at a data/information
analysis problem and develop a program to solve the problem.</span></p><p><a id="hevea_default5"></a></p><p><span class="c006">In a sense, you need two skills to be a programmer:</span></p><ul class="itemize"><li class="li-itemize"><span class="c006">First you need to know the programming language (Python) -
you need to know the vocabulary and the grammar. You need to be able
to spell the words in this new language properly and how to construct
well-formed “sentences” in this new languages.</span></li><li class="li-itemize"><span class="c006">Second you need to “tell a story”. In writing a story,
you combine words and sentences to convey an idea to the reader.
There is a skill and art in constructing the story and skill in
story writing is improved by doing some writing and getting some
feedback. In programming, our program is the “story” and the
problem you are trying to solve is the “idea”.</span></li></ul><p><span class="c006">Once you learn one programming language such as Python, you will
find it much easier to learn a second programming language such
as JavaScript or C++. The new programming language has very different
vocabulary and grammar but once you learn problem solving skills,
they will be the same across all programming languages.</span></p><p><span class="c006">You will learn the “vocabulary” and “sentences” of Python pretty quickly.
It will take longer for you to be able to write a coherent program
to solve a brand new problem. We teach programming much like we teach
writing. We start reading and explaining programs and then we write
simple programs and then write increasingly complex programs over time.
At some point you “get your muse” and see the patterns on your own
and can see more naturally how to take a problem and
write a program that solves that problem. And once you get
to that point, programming becomes a very pleasant and creative process. </span></p><p><span class="c006">We start with the vocabulary and structure of Python programs. Be patient
as the simple examples remind you of when you started reading for the first
time. </span></p><span class="c005">
</span><!--TOC section id="sec7" <span class="c000"><span class="c005">Words and sentences</span></span>-->
<h2 class="section" id="sec7"><span class="c006">1.4  Words and sentences</span></h2><!--SEC END --><p><span class="c005">
</span><a id="hevea_default6"></a><span class="c005">
</span><a id="hevea_default7"></a></p><p><span class="c006">Unlike human languages, the Python vocabulary is actually pretty small.
We call this “vocabulary” the “reserved words”. These are words that
have very special meaning to Python. When Python sees these words in
a Python program, they have one and only one meaning to Python. Later
as you write programs you will make your own words that have meaning to
you called <span class="c009">variables</span>. You will have great latitude in choosing
your names for your variables, but you cannot use any of Python’s
reserved words as a name for a variable.</span></p><p><span class="c006">In a sense, when we train a dog, we would use special words like,
“sit”, “stay”, and “fetch”. Also when you talk to a dog and
don’t use any of the reserved words, they just look at you with a
quizzical look on their faces until you say a reserved word.
For example, if you say,
“I wish more people would walk to improve their overall health.”,
what most dogs likely hear is,
“blah blah blah <span class="c009">walk</span> blah blah blah blah.”
That is because “walk” is a reserved word in dog language. Many
might suggest that the language between humans and cats has no
reserved words</span><sup><a id="text2" href="#note2"><span class="c006">1</span></a></sup><span class="c006">.</span></p><p><span class="c006">The reserved words in the language where humans talk to
Python incudes the following:</span></p><pre class="verbatim"><span class="c004">and del from not while
as elif global or with
assert else if pass yield
break except import print
class exec in raise
continue finally is return
def for lambda try
</span></pre><p><span class="c006">That is it, and unlike a dog, Python is already completely trained.
When you say “try”, Python will try every time you say it without
fail.</span></p><p><span class="c006">We will learn these reserved words and how they are used in good time,
but for now we will focus on the Python equivalent of “speak” (in
human to dog language). The nice thing about telling Python to speak
is that we can even tell it what to say by giving it a message in quotes:</span></p><pre class="verbatim"><span class="c004">print 'Hello world!'
</span></pre><p><span class="c006">And we have even written our first syntactically correct Python sentence.
Our sentence starts with the reserved word <span class="c009">print</span> followed
by a string of text of our choosing enclosed in single quotes.</span></p><span class="c005">
</span><!--TOC section id="sec8" <span class="c000"><span class="c005">Conversing with Python</span></span>-->
<h2 class="section" id="sec8"><span class="c006">1.5  Conversing with Python</span></h2><!--SEC END --><p><span class="c006">Now that we have a word and a simple sentence that we know in Python,
we need to know how to start a conversation with Python to test
our new language skills.</span></p><p><span class="c006">Before you can converse with Python, you must first install the Python
software on your computer and learn how to start Python on your
computer. That is too much detail for this chapter so I suggest
that you consult <span class="c001">www.pythonlearn.com</span> where I have detailed
instructions and screencasts of setting up and starting Python
on Macintosh and Windows systems. At some point, you will be in
a terminal or command window and you will type <span class="c009">python</span> and
the Python interpreter will start executing in interactive mode
and appear somewhat as follows:
</span><a id="hevea_default8"></a></p><pre class="verbatim"><span class="c004">Python 2.6.1 (r261:67515, Jun 24 2010, 21:47:49)
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
</span></pre><p><span class="c006">The <span class="c001">>>></span> prompt is the Python interpreter’s way of asking you, “What
do you want me to do next?”. Python is ready to have a conversation with
you. All you have to know is how to speak the Python language and you
can have a conversation.</span></p><p><span class="c006">Lets say for example that you did not know even the simplest Python language
words or sentences. You might want to use the standard line that astronauts
use when they land on a far away planet and try to speak with the inhabitants
of the planet:</span></p><pre class="verbatim"><span class="c004">>>> I come in peace, please take me to your leader
File "<stdin>", line 1
I come in peace, please take me to your leader
^
SyntaxError: invalid syntax
>>>
</span></pre><p><span class="c006">This is not going so well. Unless you think of something quickly,
the inhabitants of the planet are likely to stab you with their spears,
put you on a spit, roast you over a fire, and eat you for dinner.</span></p><p><span class="c006">Luckily you brought a copy of this book on your travels and you thumb to
this very page and try again:</span></p><pre class="verbatim"><span class="c004">>>> print 'Hello world!'
Hello world!
</span></pre><p><span class="c006">This is looking much better so you try to communicate some
more:</span></p><pre class="verbatim"><span class="c004">>>> print 'You must be the legendary god that comes from the sky'
You must be the legendary god that comes from the sky
>>> print 'We have been waiting for you for a long time'
We have been waiting for you for a long time
>>> print 'Our legend says you will be very tasty with mustard'
Our legend says you will be very tasty with mustard
>>> print 'We will have a feast tonight unless you say
File "<stdin>", line 1
print 'We will have a feast tonight unless you say
^
SyntaxError: EOL while scanning string literal
>>>
</span></pre><p><span class="c006">The conversation was going so well for a while and then you
made the tiniest mistake using the Python language and Python
brought the spears back out.</span></p><p><span class="c006">At this point, you should also realize that while Python
is amazingly complex and powerful and very picky about
the syntax you use to communicate with it, Python is <em>not</em> intelligent. You are having a conversation with
yourself but using proper syntax.</span></p><p><span class="c006">In a sense when you use a program written by someone else
the conversation is between you and those other
programmers with Python acting as an intermediary. Python
is a way for the creators of programs to express how the
conversation is supposed to proceed. And
in just a few more chapters, you will be one of those
programmers using Python to talk to the users of your program.</span></p><p><span class="c006">Before we leave our first conversation with the Python
interpreter, you should probably know the proper way
to say “good-bye” when interacting with the inhabitants
of Planet Python:</span></p><pre class="verbatim"><span class="c004">>>> good-bye
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'good' is not defined
>>> if you don't mind, I need to leave
File "<stdin>", line 1
if you don't mind, I need to leave
^
SyntaxError: invalid syntax
>>> quit()
</span></pre><p><span class="c006">You will notice that the error is different for the first two
incorrect attempts. The second error is different because
<span class="c009">if</span> is a reserved word and Python saw the reserved word
and thought we were trying to say something but got the syntax
of the sentence wrong.</span></p><p><span class="c006">The proper way to say “good-bye” to Python is to enter
<span class="c009">quit()</span> at the interactive chevron <span class="c001">>>></span> prompt.
It would have probably taken you quite a while to guess that
one so having a book handy probably will turn out
to be helpful.</span></p><span class="c005">
</span><!--TOC section id="sec9" <span class="c000"><span class="c005">Terminology: interpreter and compiler</span></span>-->
<h2 class="section" id="sec9"><span class="c006">1.6  Terminology: interpreter and compiler</span></h2><!--SEC END --><p><span class="c006">Python is a <span class="c009">high-level</span> language intended to be relatively
straightforward for humans to read and write and for computers
to read and process. Other high-level languages include: Java, C++,
PHP, Ruby, Basic, Perl, JavaScript, and many more. The actual hardware
inside the Central Processing Unit (CPU) does not understand any
of these high level languages.</span></p><p><span class="c006">The CPU understands a language we call <span class="c009">machine-language</span>. Machine
language is very simple and frankly very tiresome to write because it
is represented all in zeros and ones:</span></p><pre class="verbatim"><span class="c004">01010001110100100101010000001111
11100110000011101010010101101101
...
</span></pre><p><span class="c006">Machine language seems quite simple on the surface given that there
are only zeros and ones, but its syntax is even more complex
and far more intricate than Python. So very few programmers ever write
machine language. Instead we build various translators to allow
programmers to write in high level languages like Python or JavaScript
and these translators convert the programs to machine language for actual
execution by the CPU.</span></p><p><span class="c006">Since machine language is tied to the computer hardware, machine language
is not <span class="c009">portable</span> across different types of hardware. Programs written in
high-level languages can be moved between different computers by using a
different interpreter on the new machine or re-compiling the code to create
a machine language version of the program for the new machine.</span></p><p><span class="c006">These programming language translators fall into two general categories:
(1) interpreters and (2) compilers.</span></p><p><span class="c006">An <span class="c009">interpreter</span> reads the source code of the program as written by the
programmer, parses the source code, and interprets the instructions on-the-fly.
Python is an interpreter and when we are running Python interactively,
we can type a line of Python (a sentence) and Python processes it immediately
and is ready for us to type another line of Python. </span></p><p><span class="c006">Some of the lines of Python tell Python that you want it to remember some
value for later. We need to pick a name for that value to be remembered and
we can use that symbolic name to retrieve the value later. We use the
term <span class="c009">variable</span> to refer to the labels we use to refer to this stored data.</span></p><pre class="verbatim"><span class="c004">>>> x = 6
>>> print x
6
>>> y = x * 7
>>> print y
42
>>>
</span></pre><p><span class="c006">In this example, we ask Python to remember the value six and use the label <span class="c009">x</span>
so we can retrieve the value later. We verify that Python has actually remembered
the value using <span class="c009">print</span>. Then we ask Python to retrieve <span class="c009">x</span> and multiply
it by seven and put the newly-computed value in <span class="c009">y</span>. Then we ask Python to print out
the value currently in <span class="c009">y</span>.</span></p><p><span class="c006">Even though we are typing these commands into Python one line at a time, Python
is treating them as an ordered sequence of statements with later statements able
to retrieve data created in earlier statements. We are writing our first
simple paragraph with four sentences in a logical and meaningful order.</span></p><p><span class="c006">It is the nature of an <span class="c009">interpreter</span> to be able to have an interactive conversation
as shown above. A <span class="c009">compiler</span> needs to be handed the entire program in a file, and then
it runs a process to translate the high level source code into machine language
and then the compiler puts the resulting machine language into a file for later
execution.</span></p><p><span class="c006">If you have a Windows system, often these executable machine language programs have a
suffix of “.exe” or “.dll” which stand for “executable” and “dynamically loadable
library” respectively. In Linux and Macintosh there is no suffix that uniquely marks
a file as executable.</span></p><p><span class="c006">If you were to open an executable file in a text editor, it would look
completely crazy and be unreadable:</span></p><pre class="verbatim"><span class="c004">^?ELF^A^A^A^@^@^@^@^@^@^@^@^@^B^@^C^@^A^@^@^@\xa0\x82
^D^H4^@^@^@\x90^]^@^@^@^@^@^@4^@ ^@^G^@(^@$^@!^@^F^@
^@^@4^@^@^@4\x80^D^H4\x80^D^H\xe0^@^@^@\xe0^@^@^@^E
^@^@^@^D^@^@^@^C^@^@^@^T^A^@^@^T\x81^D^H^T\x81^D^H^S
^@^@^@^S^@^@^@^D^@^@^@^A^@^@^@^A\^D^HQVhT\x83^D^H\xe8
....
</span></pre><p><span class="c006">It is not easy to read or write machine language so it is nice that we have
<span class="c009">interpreters</span> and <span class="c009">compilers</span> that allow us to write in a high-level
language like Python or C.</span></p><p><span class="c006">Now at this point in our discussion of compilers and interpreters, you should
be wondering a bit about the Python interpreter itself. What language is
it written in? Is it written in a compiled language? When we type
“python”, what exactly is happening?</span></p><p><span class="c006">The Python interpreter is written in a high level language called “C”.
You can look at the actual source code for the Python interpreter by
going to <span class="c001">www.python.org</span> and working your way to their source code.
So Python is a program itself and it is compiled into machine code and
when you installed Python on your computer (or the vendor installed it),
you copied a machine-code copy of the translated Python program onto your
system. In Windows the executable machine code for Python itself is likely
in a file with a name like:</span></p><pre class="verbatim"><span class="c004">C:\Python27\python.exe
</span></pre><p><span class="c006">That is more than you really need to know to be a Python programmer, but
sometimes it pays to answer those little nagging questions right at
the beginning.</span></p><span class="c005">
</span><!--TOC section id="sec10" <span class="c000"><span class="c005">Writing a program</span></span>-->
<h2 class="section" id="sec10"><span class="c006">1.7  Writing a program</span></h2><!--SEC END --><p><span class="c006">Typing commands into the Python interpreter is a great way to experiment
with Python’s features, but it is not recommended for solving more complex problems.</span></p><p><span class="c006">When we want to write a program,
we use a text editor to write the Python instructions into a file,
which is called a <span class="c009">script</span>. By
convention, Python scripts have names that end with <span class="c001">.py</span>.</span></p><p><a id="hevea_default9"></a></p><p><span class="c006">To execute the script, you have to tell the Python interpreter
the name of the file. In a Unix or Windows command window,
you would type <span class="c001">python hello.py</span> as follows:</span></p><pre class="verbatim"><span class="c004">csev$ cat hello.py
print 'Hello world!'
csev$ python hello.py
Hello world!
csev$
</span></pre><p><span class="c006">The “csev$” is the operating system prompt, and the “cat hello.py” is
showing us that the file “hello.py” has a one line Python program to print
a string.</span></p><p><span class="c006">We call the Python interpreter and tell it to read its source code from
the file “hello.py” instead of prompting us for lines of Python code
interactively.</span></p><p><span class="c006">You will notice that there was no need to have <span class="c009">quit()</span> at the end of
the Python program in the file. When Python is reading your source code
from a file, it knows to stop when it reaches the end of the file.</span></p><span class="c005">
</span><!--TOC section id="sec11" <span class="c000"><span class="c005">What is a program?</span></span>-->
<h2 class="section" id="sec11"><span class="c006">1.8  What is a program?</span></h2><!--SEC END --><p><span class="c006">The definition of a <span class="c009">program</span> at its most basic is a sequence
of Python statements that have been crafted to do something.
Even our simple <span class="c009">hello.py</span> script is a program. It is a one-line
program and is not particularly useful, but in the strictest definition,
it is a Python program.</span></p><p><span class="c006">It might be easiest to understand what a program is by thinking about a problem
that a program might be built to solve, and then looking at a program
that would solve that problem.</span></p><p><span class="c006">Lets say you are doing Social Computing research on Facebook posts and
you are interested in the most frequently used word in a series of posts.
You could print out the stream of facebook posts and pore over the text
looking for the most common word, but that would take a long time and be very
mistake prone. You would be smart to write a Python program to handle the
task quickly and accurately so you can spend the weekend doing something
fun.</span></p><p><span class="c006">For example look at the following text about a clown and a car. Look at the
text and figure out the most common word and how many times it occurs.</span></p><pre class="verbatim"><span class="c004">the clown ran after the car and the car ran into the tent
and the tent fell down on the clown and the car
</span></pre><p><span class="c006">Then imagine that you are doing this task looking at millions of lines of
text. Frankly it would be quicker for you to learn Python and write a
Python program to count the words than it would be to manually
scan the words.</span></p><p><span class="c006">The even better news is that I already came up with a simple program to
find the most common word in a text file. I wrote it,
tested it, and now I am giving it to you to use so you can save some time.</span></p><pre class="verbatim"><span class="c004">name = raw_input('Enter file:')
handle = open(name, 'r')
text = handle.read()
words = text.split()
counts = dict()
for word in words:
counts[word] = counts.get(word,0) + 1
bigcount = None
bigword = None
for word,count in counts.items():
if bigcount is None or count > bigcount:
bigword = word
bigcount = count
print bigword, bigcount
</span></pre><p><span class="c006">You don’t even need to know Python to use this program. You will need to get through
Chapter 10 of this book to fully understand the awesome Python techniques that were
used to make the program. You are the end user, you simply use the program and marvel
at its cleverness and how it saved you so much manual effort.
You simply type the code
into a file called <span class="c009">words.py</span> and run it or you download the source
code from <span class="c001">http://www.pythonlearn.com/code/</span> and run it.</span></p><p><a id="hevea_default10"></a><span class="c006">
This is a good example of how Python and the Python language are acting as an intermediary
between you (the end-user) and me (the programmer). Python is a way for us to exchange useful
instruction sequences (i.e. programs) in a common language that can be used by anyone who
installs Python on their computer. So neither of us are talking <em>to Python</em>,
instead we are communicating with each other <em>through</em> Python.</span></p><span class="c005">
</span><!--TOC section id="sec12" <span class="c000"><span class="c005">The building blocks of programs</span></span>-->
<h2 class="section" id="sec12"><span class="c006">1.9  The building blocks of programs</span></h2><!--SEC END --><p><span class="c006">In the next few chapters, we will learn more about the vocabulary, sentence structure,
paragraph structure, and story structure of Python. We will learn about the powerful
capabilities of Python and how to compose those capabilities together to create useful
programs.</span></p><p><span class="c006">There are some low-level conceptual patterns that we use to construct programs. These
constructs are not just for Python programs, they are part of every programming language
from machine language up to the high-level languages.</span></p><dl class="description"><dt class="dt-description"><span class="c010">input:</span></dt><dd class="dd-description"><span class="c006"> Get data from the “outside world”. This might be
reading data from a file, or even some kind of sensor like
a microphone or GPS. In our initial programs, our input will come from the user
typing data on the keyboard.</span></dd><dt class="dt-description"><span class="c010">output:</span></dt><dd class="dd-description"><span class="c006"> Display the results of the program on a screen
or store them in a file or perhaps write them to a device like a
speaker to play music or speak text.</span></dd><dt class="dt-description"><span class="c010">sequential execution:</span></dt><dd class="dd-description"><span class="c006"> Perform statements one after
another in the order they are encountered in the script.</span></dd><dt class="dt-description"><span class="c010">conditional execution:</span></dt><dd class="dd-description"><span class="c006"> Check for certain conditions and
execute or skip a sequence of statements.</span></dd><dt class="dt-description"><span class="c010">repeated execution:</span></dt><dd class="dd-description"><span class="c006"> Perform some set of statements
repeatedly, usually with
some variation.</span></dd><dt class="dt-description"><span class="c010">reuse:</span></dt><dd class="dd-description"><span class="c006"> Write a set of instructions once and give them a name
and then reuse those instructions as needed throughout your program.</span></dd></dl><p><span class="c006">It sounds almost too simple to be true and of course it is never
so simple. It is like saying that walking is simply
“putting one foot in front of the other”. The “art”
of writing a program is composing and weaving these
basic elements together many times over to produce something
that is useful to its users.</span></p><p><span class="c006">The word counting program above directly uses all of
these patterns except for one.</span></p><span class="c005">
</span><!--TOC section id="sec13" <span class="c000"><span class="c005">What could possibly go wrong?</span></span>-->
<h2 class="section" id="sec13"><span class="c006">1.10  What could possibly go wrong?</span></h2><!--SEC END --><p><span class="c006">As we saw in our earliest conversations with Python, we must
communicate very precisely when we write Python code. The smallest
deviation or mistake will cause Python to give up looking at your
program.</span></p><p><span class="c006">Beginning programmers often take the fact that Python leaves no
room for errors as evidence that Python is mean, hateful and cruel.
While Python seems to like everyone else, Python knows them
personally and holds a grudge against them. Because of this grudge,
Python takes our perfectly written programs and rejects them as
“unfit” just to torment us.</span></p><pre class="verbatim"><span class="c004">>>> primt 'Hello world!'
File "<stdin>", line 1
primt 'Hello world!'
^
SyntaxError: invalid syntax
>>> primt 'Hello world'
File "<stdin>", line 1
primt 'Hello world'
^
SyntaxError: invalid syntax
>>> I hate you Python!
File "<stdin>", line 1
I hate you Python!
^
SyntaxError: invalid syntax
>>> if you come out of there, I would teach you a lesson
File "<stdin>", line 1
if you come out of there, I would teach you a lesson
^
SyntaxError: invalid syntax
>>>
</span></pre><p><span class="c006">There is little to be gained by arguing with Python. It is a tool,
it has no emotion and it is happy and ready to serve you whenever you
need it. Its error messages sound harsh, but they are just Python’s
call for help. It has looked at what you typed, and it simply cannot
understand what you have entered.</span></p><p><span class="c006">Python is much more like a dog, loving you unconditionally, having a few
key words that it understands, looking you with a sweet look on its
face (<span class="c001">>>></span>) and waiting for you to say something it understands.
When Python says “SyntaxError: invalid syntax”, it is simply wagging
its tail and saying, “You seemed to say something but I just don’t
understand what you meant, but please keep talking to me (<span class="c001">>>></span>).”</span></p><p><span class="c006">As your programs become increasingly sophisticated, you will encounter three
general types of errors:</span></p><dl class="description"><dt class="dt-description"><span class="c010">Syntax errors:</span></dt><dd class="dd-description"><span class="c006"> These are the first errors you will make and the easiest
to fix. A syntax error means that you have violated the “grammar” rules of Python.
Python does its best to point right at the line and character where
it noticed it was confused. The only tricky bit of syntax errors is that sometimes
the mistake that needs fixing is actually earlier in the program than where Python
<em>noticed</em> it was confused. So the line and character that Python indicates in
a syntax error may just be a starting point for your investigation.</span></dd><dt class="dt-description"><span class="c010">Logic errors:</span></dt><dd class="dd-description"><span class="c006"> A logic error is when your program has good syntax but there is a mistake
in the order of the statements or perhaps a mistake in how the statements relate to one another.
A good example of a logic error might be, “take a drink from your water bottle, put it
in your backpack, walk to the library, and then put the top back on the bottle.”</span></dd><dt class="dt-description"><span class="c010">Semantic errors:</span></dt><dd class="dd-description"><span class="c006"> A semantic error is when your description of the steps to take
is syntactically perfect and in the right order, but there is simply a mistake in
the program. The program is perfectly correct but it does not do what
you <em>intended</em> for it to do. A simple example would
be if you were giving a person directions to a restaurant and said, “... when you reach
the intersection with the gas station, turn left and go one mile and the restaurant
is a red building on your left.”. Your friend is very late and calls you to tell you that
they are on a farm and walking around behind a barn, with no sign of a restaurant.
Then you say “did you turn left or right at the gas station?” and
they say, “I followed your directions perfectly, I have
them written down, it says turn left and go one mile at the gas station.”. Then you say,
“I am very sorry, because while my instructions were syntactically correct, they
sadly contained a small but undetected semantic error.”. </span></dd></dl><p><span class="c006">Again in all three types of errors, Python is merely trying its hardest to
do exactly what you have asked.</span></p><span class="c005">
</span><!--TOC section id="sec14" <span class="c000"><span class="c005">The learning journey</span></span>-->
<h2 class="section" id="sec14"><span class="c006">1.11  The learning journey</span></h2><!--SEC END --><p><span class="c006">As you progress through the rest of the book, don’t be afraid if the concepts
don’t seem to fit together well the first time. When you were learning to speak,
it was not a problem for your first few years that you just made cute gurgling noises.
And it was OK if it took six months for you to move from simple vocabulary to
simple sentences and took 5-6 more years to move from sentences to paragraphs, and a
few more years to be able to write an interesting complete short story on your own.</span></p><p><span class="c006">We want you to learn Python much more rapidly, so we teach it all at the same time
over the next few chapters.
But it is like learning a new language that takes time to absorb and understand
before it feels natural.
That leads to some confusion as we visit and revisit
topics to try to get you to see the big picture while we are defining the tiny
fragments that make up the big picture. While the book is written linearly and
if you are taking a course, it will progress in a linear fashion, don’t hesitate
to be very non-linear in how you approach the material. Look forwards and backwards
and read with a light touch. By skimming more advanced material without
fully understanding the details, you can get a better understanding of the “why?”
of programming. By reviewing previous material and even re-doing earlier
exercises, you will realize that you actually learned a lot of material even
if the material you are currently staring at seems a bit impenetrable.</span></p><p><span class="c006">Usually when you are learning your first programming language, there are a few
wonderful “Ah-Hah!” moments where you can look up from pounding away at some rock
with a hammer and chisel and step away and see that you are indeed building
a beautiful sculpture.</span></p><p><span class="c006">If something seems particularly hard, there is usually no value in staying up all
night and staring at it. Take a break, take a nap, have a snack, explain what you
are having a problem with to someone (or perhaps your dog), and then come back to it with
fresh eyes. I assure you that once you learn the programming concepts in the book
you will look back and see that it was all really easy and elegant and it simply
took you a bit of time to absorb it.</span></p><span class="c005">
</span><!--TOC section id="sec15" <span class="c000"><span class="c005">Glossary</span></span>-->
<h2 class="section" id="sec15"><span class="c006">1.12  Glossary</span></h2><!--SEC END --><dl class="description"><dt class="dt-description"><span class="c010">bug:</span></dt><dd class="dd-description"><span class="c006"> An error in a program.
</span><a id="hevea_default11"></a></dd><dt class="dt-description"><span class="c010">central processing unit:</span></dt><dd class="dd-description"><span class="c006"> The heart of any computer. It is what
runs the software that we write; also called “CPU” or “the processor”.
</span><a id="hevea_default12"></a><span class="c005">
</span><a id="hevea_default13"></a></dd><dt class="dt-description"><span class="c010">compile:</span></dt><dd class="dd-description"><span class="c006"> To translate a program written in a high-level language
into a low-level language all at once, in preparation for later
execution.
</span><a id="hevea_default14"></a></dd><dt class="dt-description"><span class="c010">high-level language:</span></dt><dd class="dd-description"><span class="c006"> A programming language like Python that
is designed to be easy for humans to read and write.
</span><a id="hevea_default15"></a></dd><dt class="dt-description"><span class="c010">interactive mode:</span></dt><dd class="dd-description"><span class="c006"> A way of using the Python interpreter by
typing commands and expressions at the prompt.
</span><a id="hevea_default16"></a></dd><dt class="dt-description"><span class="c010">interpret:</span></dt><dd class="dd-description"><span class="c006"> To execute a program in a high-level language
by translating it one line at a time.
</span><a id="hevea_default17"></a></dd><dt class="dt-description"><span class="c010">low-level language:</span></dt><dd class="dd-description"><span class="c006"> A programming language that is designed
to be easy for a computer to execute; also called “machine code” or
“assembly language.”
</span><a id="hevea_default18"></a></dd><dt class="dt-description"><span class="c010">machine code:</span></dt><dd class="dd-description"><span class="c006"> The lowest level language for software which
is the language that is directly executed by the central processing unit
(CPU).
</span><a id="hevea_default19"></a></dd><dt class="dt-description"><span class="c010">main memory:</span></dt><dd class="dd-description"><span class="c006"> Stores programs and data. Main memory loses
its information when the power is turned off.
</span><a id="hevea_default20"></a></dd><dt class="dt-description"><span class="c010">parse:</span></dt><dd class="dd-description"><span class="c006"> To examine a program and analyze the syntactic structure.
</span><a id="hevea_default21"></a></dd><dt class="dt-description"><span class="c010">portability:</span></dt><dd class="dd-description"><span class="c006"> A property of a program that can run on more
than one kind of computer.
</span><a id="hevea_default22"></a></dd><dt class="dt-description"><span class="c010">print statement:</span></dt><dd class="dd-description"><span class="c006"> An instruction that causes the Python
interpreter to display a value on the screen.
</span><a id="hevea_default23"></a><span class="c005">
</span><a id="hevea_default24"></a></dd><dt class="dt-description"><span class="c010">problem solving:</span></dt><dd class="dd-description"><span class="c006"> The process of formulating a problem, finding
a solution, and expressing the solution.
</span><a id="hevea_default25"></a></dd><dt class="dt-description"><span class="c010">program:</span></dt><dd class="dd-description"><span class="c006"> A set of instructions that specifies a computation.
</span><a id="hevea_default26"></a></dd><dt class="dt-description"><span class="c010">prompt:</span></dt><dd class="dd-description"><span class="c006"> When a program displays a message and pauses for the
user to type some input to the program.
</span><a id="hevea_default27"></a></dd><dt class="dt-description"><span class="c010">secondary memory:</span></dt><dd class="dd-description"><span class="c006"> Stores programs and data and retains its
information even when the power is turned off. Generally slower
than main memory. Examples of secondary memory include disk
drives and flash memory in USB sticks.
</span><a id="hevea_default28"></a></dd><dt class="dt-description"><span class="c010">semantics:</span></dt><dd class="dd-description"><span class="c006"> The meaning of a program.
</span><a id="hevea_default29"></a></dd><dt class="dt-description"><span class="c010">semantic error:</span></dt><dd class="dd-description"><span class="c006"> An error in a program that makes it do something
other than what the programmer intended.
</span><a id="hevea_default30"></a></dd><dt class="dt-description"><span class="c010">source code:</span></dt><dd class="dd-description"><span class="c006"> A program in a high-level language.
</span><a id="hevea_default31"></a></dd></dl><span class="c005">
</span><!--TOC section id="sec16" <span class="c000"><span class="c005">Exercises</span></span>-->
<h2 class="section" id="sec16"><span class="c006">1.13  Exercises</span></h2><!--SEC END --><div class="theorem"><span class="c006"><span class="c009">Exercise 1</span>  <em>
What is the function of the secondary memory in a computer?</em></span><p><span class="c006"><em>a) Execute all of the computation and logic of the program<br />
b) Retrieve web pages over the Internet<br />
c) Store information for the long term - even beyond a power cycle<br />
d) Take input from the user
</em></span></p></div><div class="theorem"><span class="c006"><span class="c009">Exercise 2</span>  <em>
What is a program?
</em></span></div><div class="theorem"><span class="c006"><span class="c009">Exercise 3</span>  <em>
What is the difference between a compiler and an interpreter?
</em></span></div><div class="theorem"><span class="c006"><span class="c009">Exercise 4</span>  <em>
Which of the following contains "machine code"?</em></span><p><span class="c006"><em>a) The Python interpreter<br />
b) The keyboard<br />
c) Python source file<br />
d) A word processing document
</em></span></p></div><div class="theorem"><span class="c006"><span class="c009">Exercise 5</span>  <em>
What is wrong with the following code:</em></span><pre class="verbatim"><span class="c004"><em>>>> primt 'Hello world!'
File "<stdin>", line 1
primt 'Hello world!'
^
SyntaxError: invalid syntax
>>>
</em></span></pre></div><div class="theorem"><span class="c006"><span class="c009">Exercise 6</span>  <em>
Where in the computer is a variable such as "X" stored
after the following Python line finishes?</em></span><pre class="verbatim"><span class="c004"><em>x = 123
</em></span></pre><p><span class="c006"><em>a) Central processing unit<br />
b) Main Memory<br />
c) Secondary Memory<br />
d) Input Devices<br />
e) Output Devices
</em></span></p></div><div class="theorem"><span class="c006"><span class="c009">Exercise 7</span>  <em>
What will the following program print out:</em></span><pre class="verbatim"><span class="c004"><em>x = 43
x = x + 1
print x
</em></span></pre><p><span class="c006"><em>a) 43<br />
b) 44<br />
c) x + 1<br />
d) Error because x = x + 1 is not possible mathematically
</em></span></p></div><div class="theorem"><span class="c006"><span class="c009">Exercise 8</span>  <em>
Explain each of the following using an example of a human capability:
(1) Central processing unit, (2) Main Memory, (3) Secondary Memory,
(4) Input Device, and
(5) Output Device.
For example, "What is the human equivalent to a Central Processing Unit"?
</em></span></div><div class="theorem"><span class="c006"><span class="c009">Exercise 9</span>  <em>
How do you fix a "Syntax Error"?
</em></span></div><span class="c005">
</span><!--BEGIN NOTES chapter-->
<hr class="footnoterule" /><dl class="thefootnotes"><dt class="dt-thefootnotes"><span class="c005">
</span><a id="note2" href="#text2"><span class="c006">1</span></a></dt><dd class="dd-thefootnotes"><span class="c006"><div class="footnotetext"><span class="c001">http://xkcd.com/231/</span></div>
</span></dd></dl>
<!--END NOTES-->
<!--TOC chapter id="sec17" <span class="c000"><span class="c005">Variables, expressions and statements</span></span>-->
<h1 class="chapter" id="sec17"><span class="c006">Chapter 2  Variables, expressions and statements</span></h1><!--SEC END --><span class="c005">
</span><!--TOC section id="sec18" <span class="c000"><span class="c005">Values and types</span></span>-->
<h2 class="section" id="sec18"><span class="c006">2.1  Values and types</span></h2><!--SEC END --><p><span class="c005">
</span><a id="hevea_default32"></a><span class="c005">
</span><a id="hevea_default33"></a><span class="c005">
</span><a id="hevea_default34"></a></p><p><span class="c006">A <span class="c009">value</span> is one of the basic things a program works with,
like a letter or a
number. The values we have seen so far
are <span class="c001">1</span>, <span class="c001">2</span>, and
<code>'Hello, World!'</code>.</span></p><p><span class="c006">These values belong to different <span class="c009">types</span>:
<span class="c001">2</span> is an integer, and <code>'Hello, World!'</code> is a <span class="c009">string</span>,
so-called because it contains a “string” of letters.
You (and the interpreter) can identify
strings because they are enclosed in quotation marks.</span></p><p><a id="hevea_default35"></a></p><p><span class="c006">The <span class="c001">print</span> statement also works for integers. We use the
<span class="c001">python</span> command to start the interpreter.</span></p><pre class="verbatim"><span class="c004">python
>>> print 4
4
</span></pre><p><span class="c006">If you are not sure what type a value has, the interpreter can tell you.</span></p><pre class="verbatim"><span class="c004">>>> type('Hello, World!')
<type 'str'>
>>> type(17)
<type 'int'>
</span></pre><p><span class="c006">Not surprisingly, strings belong to the type <span class="c001">str</span> and
integers belong to the type <span class="c001">int</span>. Less obvious, numbers
with a decimal point belong to a type called <span class="c001">float</span>,
because these numbers are represented in a
format called <span class="c009">floating-point</span>.</span></p><p><a id="hevea_default36"></a><span class="c005">
</span><a id="hevea_default37"></a><span class="c005">
</span><a id="hevea_default38"></a><span class="c005">
</span><a id="hevea_default39"></a><span class="c005">
</span><a id="hevea_default40"></a><span class="c005">
</span><a id="hevea_default41"></a><span class="c005">
</span><a id="hevea_default42"></a></p><pre class="verbatim"><span class="c004">>>> type(3.2)
<type 'float'>
</span></pre><p><span class="c006">What about values like <code>'17'</code> and <code>'3.2'</code>?
They look like numbers, but they are in quotation marks like
strings.</span></p><p><a id="hevea_default43"></a></p><pre class="verbatim"><span class="c004">>>> type('17')
<type 'str'>
>>> type('3.2')
<type 'str'>
</span></pre><p><span class="c006">They’re strings.</span></p><p><span class="c006">When you type a large integer, you might be tempted to use commas
between groups of three digits, as in <span class="c001">1,000,000</span>. This is not a
legal integer in Python, but it is legal:</span></p><pre class="verbatim"><span class="c004">>>> print 1,000,000
1 0 0
</span></pre><p><span class="c006">Well, that’s not what we expected at all! Python interprets <span class="c001">1,000,000</span> as a comma-separated sequence of integers, which it
prints with spaces between.</span></p><p><a id="hevea_default44"></a><span class="c005">
</span><a id="hevea_default45"></a><span class="c005">
</span><a id="hevea_default46"></a></p><p><span class="c006">This is the first example we have seen of a semantic error: the code
runs without producing an error message, but it doesn’t do the
“right” thing.</span></p><span class="c005">
</span><!--TOC section id="sec19" <span class="c000"><span class="c005">Variables</span></span>-->
<h2 class="section" id="sec19"><span class="c006">2.2  Variables</span></h2><!--SEC END --><p><span class="c005">
</span><a id="hevea_default47"></a><span class="c005">
</span><a id="hevea_default48"></a><span class="c005">
</span><a id="hevea_default49"></a></p><p><span class="c006">One of the most powerful features of a programming language is the
ability to manipulate <span class="c009">variables</span>. A variable is a name that
refers to a value.</span></p><p><span class="c006">An <span class="c009">assignment statement</span> creates new variables and gives
them values:</span></p><pre class="verbatim"><span class="c004">>>> message = 'And now for something completely different'
>>> n = 17
>>> pi = 3.1415926535897931
</span></pre><p><span class="c006">This example makes three assignments. The first assigns a string
to a new variable named <span class="c001">message</span>;
the second assigns the integer <span class="c001">17</span> to <span class="c001">n</span>; the third
assigns the (approximate) value of π to <span class="c001">pi</span>.</span></p><p><span class="c006">To display the value of a variable, you can use a print statement:</span></p><pre class="verbatim"><span class="c004">>>> print n
17
>>> print pi
3.14159265359
</span></pre><p><span class="c006">The type of a variable is the type of the value it refers to.</span></p><pre class="verbatim"><span class="c004">>>> type(message)
<type 'str'>
>>> type(n)
<type 'int'>
>>> type(pi)
<type 'float'>
</span></pre><span class="c005">
</span><!--TOC section id="sec20" <span class="c000"><span class="c005">Variable names and keywords</span></span>-->
<h2 class="section" id="sec20"><span class="c006">2.3  Variable names and keywords</span></h2><!--SEC END --><p><span class="c005">
</span><a id="hevea_default50"></a></p><p><span class="c006">Programmers generally choose names for their variables that
are meaningful—they document what the variable is used for.</span></p><p><span class="c006">Variable names can be arbitrarily long. They can contain
both letters and numbers, but they have to begin with a letter.
It is legal to use uppercase letters, but it is a good idea
to begin variable names with a lowercase letter (you’ll
see why later).</span></p><p><span class="c006">The underscore character (<code>_</code>) can appear in a name.
It is often used in names with multiple words, such as
<code>my_name</code> or <code>airspeed_of_unladen_swallow</code>.
Variable names can start with an underscore character, but
we generally avoid using it unles we are writing library
code for others to use.</span></p><p><a id="hevea_default51"></a></p><p><span class="c006">If you give a variable an illegal name, you get a syntax error:</span></p><pre class="verbatim"><span class="c004">>>> 76trombones = 'big parade'
SyntaxError: invalid syntax
>>> more@ = 1000000
SyntaxError: invalid syntax
>>> class = 'Advanced Theoretical Zymurgy'
SyntaxError: invalid syntax
</span></pre><p><span class="c006"><span class="c001">76trombones</span> is illegal because it does not begin with a letter.
<span class="c001">more@</span> is illegal because it contains an illegal character, <span class="c001">@</span>. But what’s wrong with <span class="c001">class</span>?</span></p><p><span class="c006">It turns out that <span class="c001">class</span> is one of Python’s <span class="c009">keywords</span>. The
interpreter uses keywords to recognize the structure of the program,
and they cannot be used as variable names.</span></p><p><a id="hevea_default52"></a></p><p><span class="c006">Python reserves 31 keywords</span><sup><a id="text3" href="#note3"><span class="c006">1</span></a></sup><span class="c006"> for its use:</span></p><pre class="verbatim"><span class="c004">and del from not while
as elif global or with
assert else if pass yield
break except import print
class exec in raise
continue finally is return
def for lambda try
</span></pre><p><span class="c006">You might want to keep this list handy. If the interpreter complains
about one of your variable names and you don’t know why, see if it
is on this list.</span></p><span class="c005">
</span><!--TOC section id="sec21" <span class="c000"><span class="c005">Statements</span></span>-->
<h2 class="section" id="sec21"><span class="c006">2.4  Statements</span></h2><!--SEC END --><p><span class="c006">A <span class="c009">statement</span> is a unit of code that the Python interpreter can
execute. We have seen two kinds of statements: print
and assignment.</span></p><p><a id="hevea_default53"></a><span class="c005">
</span><a id="hevea_default54"></a><span class="c005">
</span><a id="hevea_default55"></a></p><p><span class="c006">When you type a statement in interactive mode, the interpreter
executes it and displays the result, if there is one.</span></p><p><span class="c006">A script usually contains a sequence of statements. If there
is more than one statement, the results appear one at a time
as the statements execute.</span></p><p><span class="c006">For example, the script</span></p><pre class="verbatim"><span class="c004">print 1
x = 2
print x
</span></pre><p><span class="c006">produces the output</span></p><pre class="verbatim"><span class="c004">1
2
</span></pre><p><span class="c006">The assignment statement produces no output.</span></p><span class="c005">
</span><!--TOC section id="sec22" <span class="c000"><span class="c005">Operators and operands</span></span>-->
<h2 class="section" id="sec22"><span class="c006">2.5  Operators and operands</span></h2><!--SEC END --><p><span class="c005">
</span><a id="hevea_default56"></a><span class="c005">
</span><a id="hevea_default57"></a><span class="c005">
</span><a id="hevea_default58"></a><span class="c005">
</span><a id="hevea_default59"></a></p><p><span class="c006"><span class="c009">Operators</span> are special symbols that represent computations like
addition and multiplication. The values the operator is applied to
are called <span class="c009">operands</span>.</span></p><p><span class="c006">The operators <span class="c001">+</span>, <span class="c001">-</span>, <span class="c001">*</span>, <span class="c001">/</span> and <span class="c001">**</span>
perform addition, subtraction, multiplication, division and
exponentiation, as in the following examples:</span></p><pre class="verbatim"><span class="c004">20+32 hour-1 hour*60+minute minute/60 5**2 (5+9)*(15-7)
</span></pre><p><span class="c006">The division operator might not do what you expect:</span></p><pre class="verbatim"><span class="c004">>>> minute = 59
>>> minute/60
0
</span></pre><p><span class="c006">The value of <span class="c001">minute</span> is 59, and in conventional arithmetic 59
divided by 60 is 0.98333, not 0. The reason for the discrepancy is
that Python is performing <span class="c009">floor division</span></span><sup><a id="text4" href="#note4"><span class="c006">2</span></a></sup><span class="c006">.</span></p><p><a id="hevea_default60"></a><span class="c005">
</span><a id="hevea_default61"></a><span class="c005">
</span><a id="hevea_default62"></a><span class="c005">
</span><a id="hevea_default63"></a><span class="c005">
</span><a id="hevea_default64"></a></p><p><span class="c006">When both of the operands are integers, the result is also an
integer; floor division chops off the fraction
part, so in this example it rounds down to zero.</span></p><p><span class="c006">If either of the operands is a floating-point number, Python performs
floating-point division, and the result is a <span class="c001">float</span>:</span></p><pre class="verbatim"><span class="c004">>>> minute/60.0
0.98333333333333328
</span></pre><span class="c005">
</span><!--TOC section id="sec23" <span class="c000"><span class="c005">Expressions</span></span>-->
<h2 class="section" id="sec23"><span class="c006">2.6  Expressions</span></h2><!--SEC END --><p><span class="c006">An <span class="c009">expression</span> is a combination of values, variables, and operators.
A value all by itself is considered an expression, and so is
a variable, so the following are all legal expressions
(assuming that the variable <span class="c001">x</span> has been assigned a value):</span></p><p><a id="hevea_default65"></a><span class="c005">
</span><a id="hevea_default66"></a></p><pre class="verbatim"><span class="c004">17
x
x + 17
</span></pre><p><span class="c006">If you type an expression in interactive mode, the interpreter
<span class="c009">evaluates</span> it and displays the result:</span></p><pre class="verbatim"><span class="c004">>>> 1 + 1
2
</span></pre><p><span class="c006">But in a script, an expression all by itself doesn’t
do anything! This is a common
source of confusion for beginners.</span></p><div class="theorem"><span class="c006"><span class="c009">Exercise 1</span>  <em>
Type the following statements in the Python interpreter to see
what they do:</em></span><pre class="verbatim"><span class="c004"><em>5
x = 5
x + 1
</em></span></pre></div><span class="c005">
</span><!--TOC section id="sec24" <span class="c000"><span class="c005">Order of operations</span></span>-->
<h2 class="section" id="sec24"><span class="c006">2.7  Order of operations</span></h2><!--SEC END --><p><span class="c005">
</span><a id="hevea_default67"></a><span class="c005">
</span><a id="hevea_default68"></a><span class="c005">
</span><a id="hevea_default69"></a></p><p><span class="c006">When more than one operator appears in an expression, the order of
evaluation depends on the <span class="c009">rules of precedence</span>. For
mathematical operators, Python follows mathematical convention.
The acronym <span class="c009">PEMDAS</span> is a useful way to
remember the rules:</span></p><p><a id="hevea_default70"></a></p><ul class="itemize"><li class="li-itemize"><span class="c006"><span class="c009">P</span>arentheses have the highest precedence and can be used
to force an expression to evaluate in the order you want. Since
expressions in parentheses are evaluated first, <span class="c001">2 * (3-1)</span> is 4,
and <span class="c001">(1+1)**(5-2)</span> is 8. You can also use parentheses to make an
expression easier to read, as in <span class="c001">(minute * 100) / 60</span>, even
if it doesn’t change the result.</span></li><li class="li-itemize"><span class="c006"><span class="c009">E</span>xponentiation has the next highest precedence, so
<span class="c001">2**1+1</span> is 3, not 4, and <span class="c001">3*1**3</span> is 3, not 27.</span></li><li class="li-itemize"><span class="c006"><span class="c009">M</span>ultiplication and <span class="c009">D</span>ivision have the same precedence,
which is higher than <span class="c009">A</span>ddition and <span class="c009">S</span>ubtraction, which also
have the same precedence. So <span class="c001">2*3-1</span> is 5, not 4, and
<span class="c001">6+4/2</span> is 8, not 5.</span></li><li class="li-itemize"><span class="c006">Operators with the same precedence are evaluated from left to
right. So in the expression <span class="c001">5-3-1</span> is 1, not 3 because the
<span class="c001">5-3</span> happens first and then <span class="c001">1</span> is subtracted from <span class="c001">2</span>.</span></li></ul><p><span class="c006">When in doubt always put parentheses in your expressions to make sure
the computations are performed in the order you intend.</span></p><span class="c005">
</span><!--TOC section id="sec25" <span class="c000"><span class="c005">Modulus operator</span></span>-->
<h2 class="section" id="sec25"><span class="c006">2.8  Modulus operator</span></h2><!--SEC END --><p><a id="hevea_default71"></a><span class="c005">
</span><a id="hevea_default72"></a></p><p><span class="c006">The <span class="c009">modulus operator</span> works on integers and yields the remainder
when the first operand is divided by the second. In Python, the
modulus operator is a percent sign (<code>%</code>). The syntax is the same
as for other operators:</span></p><pre class="verbatim"><span class="c004">>>> quotient = 7 / 3
>>> print quotient
2
>>> remainder = 7 % 3
>>> print remainder
1
</span></pre><p><span class="c006">So 7 divided by 3 is 2 with 1 left over.</span></p><p><span class="c006">The modulus operator turns out to be surprisingly useful. For
example, you can check whether one number is divisible by another—if
<span class="c001">x % y</span> is zero, then <span class="c001">x</span> is divisible by <span class="c001">y</span>.</span></p><p><a id="hevea_default73"></a></p><p><span class="c006">Also, you can extract the right-most digit
or digits from a number. For example, <span class="c001">x % 10</span> yields the
right-most digit of <span class="c001">x</span> (in base 10). Similarly <span class="c001">x % 100</span>
yields the last two digits.</span></p><span class="c005">
</span><!--TOC section id="sec26" <span class="c000"><span class="c005">String operations</span></span>-->
<h2 class="section" id="sec26"><span class="c006">2.9  String operations</span></h2><!--SEC END --><p><span class="c005">
</span><a id="hevea_default74"></a><span class="c005">
</span><a id="hevea_default75"></a></p><p><span class="c006">The <span class="c001">+</span> operator works with strings, but it
is not addition in the mathematical sense. Instead it performs
<span class="c009">concatenation</span>, which means joining the strings by
linking them end-to-end. For example:</span></p><p><a id="hevea_default76"></a></p><pre class="verbatim"><span class="c004">>>> first = 10
>>> second = 15
>>> print first+second
25
>>> first = '100'
>>> second = '150'
>>> print first + second
100150
</span></pre><p><span class="c006">The output of this program is <span class="c001">100150</span>.</span></p><span class="c005">
</span><!--TOC section id="sec27" <span class="c000"><span class="c005">Asking the user for input</span></span>-->
<h2 class="section" id="sec27"><span class="c006">2.10  Asking the user for input</span></h2><!--SEC END --><p><span class="c005">
</span><a id="hevea_default77"></a></p><p><span class="c006">Sometimes we would like to take the value for a variable from the user
via their keyboard.
Python provides a built-in function called <code>raw_input</code> that gets
input from the keyboard</span><sup><a id="text5" href="#note5"><span class="c006">3</span></a></sup><span class="c006">. When this function is called, the program stops and
waits for the user to type something. When the user presses <span class="c003">Return</span> or <span class="c003">Enter</span>, the program resumes and <code>raw_input</code>
returns what the user typed as a string.</span></p><p><a id="hevea_default78"></a><span class="c005">
</span><a id="hevea_default79"></a><span class="c005">
</span><a id="hevea_default80"></a></p><pre class="verbatim"><span class="c004">>>> input = raw_input()
Some silly stuff
>>> print input
Some silly stuff
</span></pre><p><span class="c006">Before getting input from the user, it is a good idea to print a
prompt telling the user what to input. You can pass a string
to <code>raw_input</code> to be displayed to the user before pausing
for input:</span></p><p><a id="hevea_default81"></a></p><pre class="verbatim"><span class="c004">>>> name = raw_input('What is your name?\n')
What is your name?
Chuck
>>> print name
Chuck
</span></pre><p><span class="c006">The sequence <code>\n</code> at the end of the prompt represents a <span class="c009">newline</span>,
which is a special character that causes a line break.
That’s why the user’s input appears below the prompt.</span></p><p><a id="hevea_default82"></a></p><p><span class="c006">If you expect the user to type an integer, you can try to convert
the return value to <span class="c001">int</span> using the <span class="c001">int()</span> function:</span></p><pre class="verbatim"><span class="c004">>>> prompt = 'What...is the airspeed velocity of an unladen swallow?\n'
>>> speed = raw_input(prompt)
What...is the airspeed velocity of an unladen swallow?
17
>>> int(speed)
17
>>> int(speed) + 5
22
</span></pre><p><span class="c006">But if the user types something other than a string of digits,
you get an error:</span></p><pre class="verbatim"><span class="c004">>>> speed = raw_input(prompt)
What...is the airspeed velocity of an unladen swallow?
What do you mean, an African or a European swallow?
>>> int(speed)
ValueError: invalid literal for int()
</span></pre><p><span class="c006">We will see how to handle this kind of error later.</span></p><p><a id="hevea_default83"></a><span class="c005">
</span><a id="hevea_default84"></a></p><span class="c005">
</span><!--TOC section id="sec28" <span class="c000"><span class="c005">Comments</span></span>-->
<h2 class="section" id="sec28"><span class="c006">2.11  Comments</span></h2><!--SEC END --><p><span class="c005">
</span><a id="hevea_default85"></a></p><p><span class="c006">As programs get bigger and more complicated, they get more difficult
to read. Formal languages are dense, and it is often difficult to
look at a piece of code and figure out what it is doing, or why.</span></p><p><span class="c006">For this reason, it is a good idea to add notes to your programs to explain