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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
"http://www.w3.org/TR/REC-html40/loose.dtd">
<HTML>
<HEAD><TITLE></TITLE>
<META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<META name="GENERATOR" content="hevea 1.07">
</HEAD>
<BODY >
<!--HEVEA command line is: hevea -O -e latexonly png.hva htmlonly cfbook -->
<!--HTMLHEAD-->
<!--ENDHTML-->
<!--PREFIX <ARG ></ARG>-->
<!--CUT DEF chapter 1 -->
<BR>
<BR>
<FONT SIZE=5>Python for Informatics: Exploring Information</FONT><BR>
<BR>
<FONT SIZE=4>Charles Severance</FONT><BR>
<BR>
Version 0.0.7<BR>
<BR>
<!--TOC chapter Preface-->
<H1><A NAME="htoc1">Chapter 0</A> Preface</H1><!--SEC END -->
<!--TOC section Python for Informatics: Remixing an Open Book-->
<H2>Python for Informatics: Remixing an Open Book</H2><!--SEC END -->
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.<BR>
<BR>
In December of 2009, I was preparing to teach
<B>SI502 - Networked Programming</B> 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 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.<BR>
<BR>
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. <BR>
<BR>
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. <BR>
<BR>
The chapters 2-10 are similar to the <EM>Think Python</EM> book
but there have been some changes. Nearly all number-oriented
exercises have been replaced with data-oriented exercises.
Topics are presented in the order to needed to build increasingly
sophisticated data analysis solutions. Some topics like <TT>try</TT> and
<TT>except</TT> are pulled forward and presented as part of the chapter
on conditionals while other concepts like functions are left until
they are needed to handle program complexity rather introduced
as an early lesson in abstraction. The word ``recursion'' does not
appear in the book at all.<BR>
<BR>
In chapters 1 and 11-15, 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 data, and creating
and using databases using Structured Query Language.<BR>
<BR>
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 applied even if one chooses not to
become a professional programmer.<BR>
<BR>
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 computing in general and computational thinking
that are covered in <EM>Think Python</EM>.
And given that the books have a similar writing style and at times
have identical text and examples, you should be
able to move quickly through <EM>Think Python</EM> with a minimum of effort.<BR>
<BR>
<A NAME="@default0"></A>
<A NAME="@default1"></A>
<A NAME="@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.<BR>
<BR>
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.<BR>
<BR>
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.<BR>
<BR>
Charles Severance<BR>
www.dr-chuck.com<BR>
Ann Arbor, MI, USA<BR>
September 9, 2013<BR>
<BR>
Charles Severance is a
Clinical Associate Professor
at the University of Michigan School of Information.<BR>
<BR>
<!--TOC section Preface for ``Think Python''-->
<H2>Preface for ``Think Python''</H2><!--SEC END -->
<!--TOC subsection The strange history of ``Think Python''-->
<H3>The strange history of ``Think Python''</H3><!--SEC END -->
(Allen B. Downey)<BR>
<BR>
In January 1999 I was preparing to teach an introductory programming
class in Java. I had taught it three times and I was getting
frustrated. The failure rate in the class was too high and, even for
students who succeeded, the overall level of achievement was too low.<BR>
<BR>
One of the problems I saw was the books.
They were too big, with too much unnecessary detail about Java, and
not enough high-level guidance about how to program. And they all
suffered from the trap door effect: they would start out easy,
proceed gradually, and then somewhere around Chapter 5 the bottom would
fall out. The students would get too much new material, too fast,
and I would spend the rest of the semester picking up the pieces.<BR>
<BR>
Two weeks before the first day of classes, I decided to write my
own book.
My goals were:
<UL><LI>Keep it short. It is better for students to read 10 pages
than not read 50 pages.<BR>
<BR>
<LI>Be careful with vocabulary. I tried to minimize the jargon
and define each term at first use.<BR>
<BR>
<LI>Build gradually. To avoid trap doors, I took the most difficult
topics and split them into a series of small steps. <BR>
<BR>
<LI>Focus on programming, not the programming language. I included
the minimum useful subset of Java and left out the rest.</UL>
I needed a title, so on a whim I chose <EM>How to Think Like
a Computer Scientist</EM>.<BR>
<BR>
My first version was rough, but it worked. Students did the reading,
and they understood enough that I could spend class time on the hard
topics, the interesting topics and (most important) letting the
students practice.<BR>
<BR>
I released the book under the GNU Free Documentation License,
which allows users to copy, modify, and distribute the book.<BR>
<BR>
<A NAME="@default3"></A>
<A NAME="@default4"></A><BR>
<BR>
What happened next is the cool part. Jeff Elkner, a high school
teacher in Virginia, adopted my book and translated it into
Python. He sent me a copy of his translation, and I had the
unusual experience of learning Python by reading my own book.<BR>
<BR>
Jeff and I revised the book, incorporated a case study by
Chris Meyers, and in 2001 we released <EM>How to Think Like
a Computer Scientist: Learning with Python</EM>, also under
the GNU Free Documentation License.
As Green Tea Press, I published the book and started selling
hard copies through Amazon.com and college book stores.
Other books from Green Tea Press are available at
<TT>greenteapress.com</TT>.<BR>
<BR>
In 2003 I started teaching at Olin College and I got to teach
Python for the first time. The contrast with Java was striking.
Students struggled less, learned more, worked on more interesting
projects, and generally had a lot more fun.<BR>
<BR>
Over the last five years I have continued to develop the book,
correcting errors, improving some of the examples and
adding material, especially exercises. In 2008 I started work
on a major revision---at the same time, I was
contacted by an editor at Cambridge University Press who
was interested in publishing the next edition. Good timing!<BR>
<BR>
I hope you enjoy working with this book, and that it helps
you learn to program and think, at least a little bit, like
a computer scientist.<BR>
<BR>
<!--TOC subsection Acknowledgements for ``Think Python''-->
<H3>Acknowledgements for ``Think Python''</H3><!--SEC END -->
(Allen B. Downey)<BR>
<BR>
First and most importantly, I thank Jeff Elkner, who
translated my Java book into Python, which got this project
started and introduced me to what has turned out to be my
favorite language.<BR>
<BR>
I also thank Chris Meyers, who contributed several sections
to <EM>How to Think Like a Computer Scientist</EM>.<BR>
<BR>
And I thank the Free Software Foundation for developing
the GNU Free Documentation License, which helped make
my collaboration with Jeff and Chris possible.<BR>
<BR>
<A NAME="@default5"></A>
<A NAME="@default6"></A><BR>
<BR>
I also thank the editors at Lulu who worked on
<EM>How to Think Like a Computer Scientist</EM>.<BR>
<BR>
I thank all the students who worked with earlier
versions of this book and all the contributors (listed
in an Appendix) who sent in corrections and suggestions.<BR>
<BR>
And I thank my wife, Lisa, for her work on this book, and Green
Tea Press, and everything else, too.<BR>
<BR>
Allen B. Downey <BR>
Needham MA<BR>
Allen Downey is an Associate Professor of Computer Science at
the Franklin W. Olin College of Engineering.<BR>
<BR>
<!--TOC chapter Why should you learn to write programs?-->
<H1><A NAME="htoc2">Chapter 1</A> Why should you learn to write programs?</H1><!--SEC END -->
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. <BR>
<BR>
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?''.<BR>
<DIV ALIGN=center><IMG SRC="cfbook001.png"></DIV>
<BR>
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.<BR>
<BR>
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.<BR>
<BR>
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:
<PRE><FONT SIZE=4 COLOR=blue>
python words.py
Enter file:words.txt
to 16
</FONT></PRE><FONT COLOR=black>Our ``personal information analysis assistant'' quickly
told us that the word ``to'' was used sixteen times in the
first three paragraphs of this chapter.<BR>
<BR>
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. </FONT><BR>
<BR>
<!--TOC section <FONT COLOR=black><FONT SIZE=3>Creativity and motivation</FONT></FONT>-->
<H2><FONT COLOR=black><A NAME="htoc3">1.1</A> Creativity and motivation</FONT></H2><!--SEC END -->
<FONT COLOR=black>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.<BR>
<BR>
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:<BR>
</FONT><DIV ALIGN=center><FONT COLOR=black><IMG SRC="cfbook002.png"></FONT></DIV><FONT COLOR=black>
<BR>
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.</FONT><BR>
<BR>
<!--TOC section <FONT COLOR=black><FONT SIZE=3>Computer hardware architecture</FONT></FONT>-->
<H2><FONT COLOR=black><A NAME="htoc4">1.2</A> Computer hardware architecture</FONT></H2><!--SEC END -->
<A NAME="@default7"></A>
<A NAME="@default8"></A>
<FONT COLOR=black>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:<BR>
</FONT><DIV ALIGN=center><FONT COLOR=black><IMG SRC="cfbook003.png"></FONT></DIV><FONT COLOR=black>
<BR>
The high-level definitions of these parts are as follows:
</FONT><UL><LI><FONT COLOR=black>The <B>Central Processing Unit</B> (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.</FONT><BR>
<BR>
<LI><FONT COLOR=black>The <B>Main Memory</B> 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.</FONT><BR>
<BR>
<LI><FONT COLOR=black>The <B>Secondary Memory</B> 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).</FONT><BR>
<BR>
<LI><FONT COLOR=black>The <B>Input and Output Devices</B> are simply our
screen, keyboard, mouse, microphone, speaker, touchpad, etc.
They are all of the ways we interact with the computer.</FONT><BR>
<BR>
<LI><FONT COLOR=black>These days, most computers also have a
<B>Network Connection</B> 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
<B>Secondary Memory</B></FONT></UL>
<FONT COLOR=black>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.<BR>
<BR>
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.<BR>
</FONT><DIV ALIGN=center><FONT COLOR=black><IMG SRC="cfbook004.png"></FONT></DIV><FONT COLOR=black>
<BR>
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 <B>program</B> and the act
of writing these instructions down and getting the instructions to
be correct <B>programming</B>.</FONT><BR>
<BR>
<!--TOC section <FONT COLOR=black><FONT SIZE=3>Understanding programming</FONT></FONT>-->
<H2><FONT COLOR=black><A NAME="htoc5">1.3</A> Understanding programming</FONT></H2><!--SEC END -->
<FONT COLOR=black>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
<B>programmer</B> --- 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.<BR>
<BR>
<A NAME="@default9"></A><BR>
<BR>
In a sense, you need two skills to be a programmer:
</FONT><UL><LI><FONT COLOR=black>First you need to know the programming language (Python) -
you need to know the vocabulary and the grammar. You need to be able
spell the words in this new language properly and how to construct
well-formed ``sentences'' in this new languages.</FONT><BR>
<BR>
<LI><FONT COLOR=black>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''.</FONT></UL>
<FONT COLOR=black>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.<BR>
<BR>
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. <BR>
<BR>
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. </FONT><BR>
<BR>
<!--TOC section <FONT COLOR=black><FONT SIZE=3>Words and sentences</FONT></FONT>-->
<H2><FONT COLOR=black><A NAME="htoc6">1.4</A> Words and sentences</FONT></H2><!--SEC END -->
<A NAME="@default10"></A>
<A NAME="@default11"></A>
<FONT COLOR=black>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 <B>variables</B>. 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.<BR>
<BR>
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 <B>walk</B> 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<SUP><A NAME="text1" HREF="#note1">1</A></SUP>.<BR>
<BR>
The reserved words in the language where humans talk to
Python incudes the following:
</FONT><PRE><FONT SIZE=4 COLOR=blue>
and del for is raise
assert elif from lambda return
break else global not try
class except if or while
continue exec import pass yield
def ï¬nally in print
</FONT></PRE><FONT COLOR=black>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.<BR>
<BR>
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:
</FONT><PRE><FONT SIZE=4 COLOR=blue>
print 'Hello world!'
</FONT></PRE>
<FONT COLOR=black>And we have even written our first syntactically correct Python sentence.
Our sentence starts with the reserved word <B>print</B> followed
by a string of text of our choosing enclosed in single quotes.</FONT><BR>
<BR>
<!--TOC section <FONT COLOR=black><FONT SIZE=3>Conversing with Python</FONT></FONT>-->
<H2><FONT COLOR=black><A NAME="htoc7">1.5</A> Conversing with Python</FONT></H2><!--SEC END -->
<FONT COLOR=black>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.<BR>
<BR>
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 <TT>www.pythonlearn.com</TT> 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 <B>python</B> and
the Python interpreter will start executing in interactive mode:
and appear somewhat as follows:
</FONT><A NAME="@default12"></A>
<PRE><FONT SIZE=4 COLOR=blue>
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.
>>>
</FONT></PRE><FONT COLOR=black>The <TT>>>></TT> 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.<BR>
<BR>
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:
</FONT><PRE><FONT SIZE=4 COLOR=blue>
>>> 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
>>>
</FONT></PRE><FONT COLOR=black>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.<BR>
<BR>
Luckily you brought a copy of this book on your travels and you thumb to
this very page and try again:
</FONT><PRE><FONT SIZE=4 COLOR=blue>
>>> print 'Hello world!'
Hello world!
</FONT></PRE><FONT COLOR=black>This is looking much better so you try to communicate some
more:
</FONT><PRE><FONT SIZE=4 COLOR=blue>
>>> 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
>>>
</FONT></PRE><FONT COLOR=black>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.<BR>
<BR>
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.<BR>
<BR>
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.<BR>
<BR>
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:
</FONT><PRE><FONT SIZE=4 COLOR=blue>
>>> 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()
</FONT></PRE><FONT COLOR=black>You will notice that the error is different for the first two
incorrect attempts. The second error is different because
<B>if</B> 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.<BR>
<BR>
The proper way to say ``good-bye'' to Python is to enter
<B>quit()</B> at the interactive chevron <TT>>>></TT> 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.</FONT><BR>
<BR>
<!--TOC section <FONT COLOR=black><FONT SIZE=3>Terminology: interpreter and compiler</FONT></FONT>-->
<H2><FONT COLOR=black><A NAME="htoc8">1.6</A> Terminology: interpreter and compiler</FONT></H2><!--SEC END -->
<FONT COLOR=black>Python is a <B>high-level</B> 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.<BR>
<BR>
The CPU understands a language we call <B>machine-language</B>. Machine
language is very simple and frankly very tiresome to write because it
is represented all in zeros and ones:
</FONT><PRE><FONT SIZE=4 COLOR=blue>
01010001110100100101010000001111
11100110000011101010010101101101
...
</FONT></PRE><FONT COLOR=black>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.<BR>
<BR>
Since machine language is tied to the computer hardware, machine language
is not <B>portable</B> 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.<BR>
<BR>
These programming language translators fall into two general categories:
(1) interpreters and (2) compilers.<BR>
<BR>
An <B>interpreter</B> 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. <BR>
<BR>
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 <B>variable</B> to refer to the labels we use to refer to this stored data.
</FONT><PRE><FONT SIZE=4 COLOR=blue>
>>> x = 6
>>> print x
6
>>> y = x * 7
>>> print y
42
>>>
</FONT></PRE><FONT COLOR=black>In this example, we ask Python to remember the value six and use the label <B>x</B>
so we can retrieve the value later. We verify that Python has actually remembered
the value using <B>print</B>. Then we ask Python to retrieve <B>x</B> and multiply
it by seven and put the newly-computed value in <B>y</B>. Then we ask Python to print out
the value currently in <B>y</B>.<BR>
<BR>
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.<BR>
<BR>
It is the nature of an <B>interpreter</B> to be able to have an interactive conversation
as shown above. A <B>compiler</B> 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.<BR>
<BR>
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.<BR>
<BR>
If you were to open an executable file in a text editor, it would look
completely crazy and be unreadable:
</FONT><PRE><FONT SIZE=4 COLOR=blue>
^?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
....
</FONT></PRE><FONT COLOR=black>It is not easy to read or write machine language so it is nice that we have
<B>interpreters</B> and <B>compilers</B> that allow us to write in a high-level
language like Python or C.<BR>
<BR>
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?<BR>
<BR>
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 <TT>www.python.org</TT> 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:
</FONT><PRE><FONT SIZE=4 COLOR=blue>
C:\Python27\python.exe
</FONT></PRE><FONT COLOR=black>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.</FONT><BR>
<BR>
<!--TOC section <FONT COLOR=black><FONT SIZE=3>Writing a program</FONT></FONT>-->
<H2><FONT COLOR=black><A NAME="htoc9">1.7</A> Writing a program</FONT></H2><!--SEC END -->
<FONT COLOR=black>Typing commands into the Python interpreter is a great way to experiment
with Pythonâs features, but it is not recommended for solving more complex problems.<BR>
<BR>
When we want to write a program,
we use a text editor to write the Python instructions into a file,
which is called a <B>script</B>. By
convention, Python scripts have names that end with <TT>.py</TT>.<BR>
<BR>
<A NAME="@default13"></A><BR>
<BR>
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 <TT>python hello.py</TT> as follows:
</FONT><PRE><FONT SIZE=4 COLOR=blue>
csev$ cat hello.py
print 'Hello world!'
csev$ python hello.py
Hello world!
csev$
</FONT></PRE><FONT COLOR=black>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.<BR>
<BR>
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.<BR>
<BR>
You will notice that there was no need to have <B>quit()</B> at the end of
the Python program in the file. When Python is reading your source code
form a file, it knows to stop when it reaches the end of the file.</FONT><BR>
<BR>
<!--TOC section <FONT COLOR=black><FONT SIZE=3>What is a program?</FONT></FONT>-->
<H2><FONT COLOR=black><A NAME="htoc10">1.8</A> What is a program?</FONT></H2><!--SEC END -->
<FONT COLOR=black>The definition of a <B>program</B> at its most basic is a sequence
of Python statements that have been crafted to do something.
Even our simple <B>hello.py</B> 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.<BR>
<BR>
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.<BR>
<BR>
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.<BR>
<BR>
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.
</FONT><PRE><FONT SIZE=4 COLOR=blue>
the clown ran after the car and the car ran into the tent
and the tent fell down on the clown and the car
</FONT></PRE><FONT COLOR=black>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.<BR>
<BR>
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.
</FONT><PRE><FONT SIZE=4 COLOR=blue>
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
</FONT></PRE><FONT COLOR=black>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 <B>words.py</B> and run it or you download the source
code from <TT>http://www.pythonlearn.com/code/</TT> and run it.<BR>
<BR>
<A NAME="@default14"></A>
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.</FONT><BR>
<BR>
<!--TOC section <FONT COLOR=black><FONT SIZE=3>The building blocks of programs</FONT></FONT>-->
<H2><FONT COLOR=black><A NAME="htoc11">1.9</A> The building blocks of programs</FONT></H2><!--SEC END -->
<FONT COLOR=black>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.<BR>
<BR>
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.
</FONT><DL COMPACT=compact><DT><FONT COLOR=black><B>input:</B></FONT><DD><FONT COLOR=black> Get data from the 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.</FONT><BR>
<BR>
<DT><FONT COLOR=black><B>output:</B></FONT><DD><FONT COLOR=black> 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.</FONT><BR>
<BR>
<DT><FONT COLOR=black><B>sequential execution:</B></FONT><DD><FONT COLOR=black> Perform statements one after
another in the order they are encountered in the script.</FONT><BR>
<BR>
<DT><FONT COLOR=black><B>conditional execution:</B></FONT><DD><FONT COLOR=black> Check for certain conditions and
execute or skip a sequence of statements.</FONT><BR>
<BR>
<DT><FONT COLOR=black><B>repeated execution:</B></FONT><DD><FONT COLOR=black> Perform some set of statements
repeatedly, usually with
some variation.</FONT><BR>
<BR>
<DT><FONT COLOR=black><B>reuse:</B></FONT><DD><FONT COLOR=black> Write a set of instructions once and give them a name
and then reuse those instructions as needed throughout your program.</FONT></DL>
<FONT COLOR=black>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.<BR>
<BR>
The word counting program above directly uses all of
these patterns except for one.</FONT><BR>
<BR>
<!--TOC section <FONT COLOR=black><FONT SIZE=3>What could possibly go wrong?</FONT></FONT>-->
<H2><FONT COLOR=black><A NAME="htoc12">1.10</A> What could possibly go wrong?</FONT></H2><!--SEC END -->
<FONT COLOR=black>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.<BR>
<BR>
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.
</FONT><PRE><FONT SIZE=4 COLOR=blue>
>>> 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
>>>
</FONT></PRE><FONT COLOR=black>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.<BR>
<BR>
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 (<TT>>>></TT>) 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 (<TT>>>></TT>).''<BR>
<BR>
As your programs become increasingly sophisticated, you will encounter three
general types of errors:
</FONT><DL COMPACT=compact><DT><FONT COLOR=black><B>Syntax errors:</B></FONT><DD><FONT COLOR=black> 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.</FONT><BR>
<BR>
<DT><FONT COLOR=black><B>Logic errors:</B></FONT><DD><FONT COLOR=black> 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.''</FONT><BR>
<BR>
<DT><FONT COLOR=black><B>Semantic errors:</B></FONT><DD><FONT COLOR=black> 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.
The you say ``did you turn left or right 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.''. </FONT></DL>
<FONT COLOR=black>Again in all three types of errors, Python is merely trying its hardest to
do exactly what you have asked.</FONT><BR>
<BR>
<!--TOC section <FONT COLOR=black><FONT SIZE=3>The learning journey</FONT></FONT>-->
<H2><FONT COLOR=black><A NAME="htoc13">1.11</A> The learning journey</FONT></H2><!--SEC END -->
<FONT COLOR=black>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 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.<BR>
<BR>
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.<BR>
<BR>
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.<BR>
<BR>
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 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.</FONT><BR>
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<H2><FONT COLOR=black><A NAME="htoc14">1.12</A> Glossary</FONT></H2><!--SEC END -->
<DL COMPACT=compact><DT><FONT COLOR=black><B>bug:</B></FONT><DD><FONT COLOR=black> An error in a program.
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<DT><FONT COLOR=black><B>central processing unit:</B></FONT><DD><FONT COLOR=black> The heart of any computer. It is what
runs the software that we write; also called ``CPU'' or ``the processor''.
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