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Thinking in Python<br>
<small>Revision 0.1.2 (12/31/01) -- Incomplete and Unfinished</small></FONT></H2>
<H3><FONT FACE="Verdana, Tahoma, Arial, Helvetica, Sans">
by Bruce Eckel ©2002 MindView, Inc.</FONT></H3>
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</P></DIV><A NAME="_Toc534420053"></A><A NAME="Heading2"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H1 ALIGN="LEFT">
Introduction</H1></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia" SIZE=4>This is a book about design that I
have been working on for years, basically ever since I first started trying to
read <I>Design Patterns</I> (Gamma, Helm, Johnson & Vlissides,
Addison-Wesley, 1995), commonly referred to as the <I>Gang of
Four</I></FONT><A NAME="fnB1" HREF="#fn1">[1]</A><FONT FACE="Georgia" SIZE=4> or
just GoF).
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_6">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">There is a chapter on design patterns in
the first edition of <I>Thinking in C++</I>, which has evolved in Volume 2 of
the second edition of <I>Thinking in C++</I>, and you’ll also find a
chapter on patterns in the first edition of <I>Thinking in Java</I>. I took that
chapter out of the second edition of <I>Thinking in Java</I> because that book
was getting too big, and also because I had decided to write <I>Thinking in
Patterns</I>. That book, still to be finished, has become this one. The ease of
expressing these more complex ideas in Python will, I think, finally allow me to
get it all out.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_7">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">This is not an introductory book. I am
assuming that you have worked your way through at least <I>Learning Python</I>
(by Mark Lutz & David Ascher; OReilly, 1999) or an equivalent text before
coming to this book.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_8">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">In addition, I assume you have more than
just a grasp of the syntax of Python. You should have a good understanding of
objects and what they’re about, including polymorphism.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_9">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">On the other hand, by going through this
book you’re going to learn a <I>lot</I> about object-oriented programming
by seeing objects used in many different situations. If your knowledge of
objects is rudimentary, it will get much stronger in the process of
understanding the designs in this book.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_10">Add Comment</A></FONT><A NAME="_Toc534420054"></A><BR></P></DIV>
<A NAME="Heading3"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H2 ALIGN="LEFT">
The Y2K syndrome</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">In a book that has “problem-solving
techniques” in its subtitle, it’s worth mentioning one of the
biggest pitfalls in programming: premature optimization. Every time I bring this
concept forward, virtually everyone agrees to it. Also, everyone seems to
reserve in their own mind a special case “except for this thing that I
happen to know is a particular problem.”
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_11">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The reason I call this the Y2K syndrome
has to do with that special knowledge. Computers are a mystery to most people,
so when someone announced that those silly computer programmers had forgotten to
put in enough digits to hold dates past the year 1999, then suddenly everyone
became a computer expert – “these things aren’t so difficult
after all, if I can see such an obvious problem.” For example, my
background was originally in computer engineering, and I started out by
programming embedded systems. As a result, I know that many embedded systems
have no idea what the date or time is, and even if they do that data often
isn’t used in any important calculations. And yet I was told in no
uncertain terms that all the embedded systems were going to crash on January 1,
2000</FONT><A NAME="fnB2" HREF="#fn2">[2]</A><FONT FACE="Georgia">. As far as I
can tell the only memory that was lost on that particular date was that of the
people who were predicting doom – it’s as if they had never said any
of that stuff.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_12">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The point is that it’s very easy to
fall into a habit of thinking that the particular algorithm or piece of code
that you happen to partly or thoroughly understand is naturally going to be the
bottleneck in your system, simply because you can imagine what’s going on
in that piece of code and so you think that it must somehow be much less
efficient than all the other pieces of code that you don’t know about. But
unless you’ve run actual tests, typically with a profiler, you can’t
really know what’s going on. And even if you are right, that a piece of
code is very inefficient, remember that most programs spend something like 90%
of their time in less than 10% of the code in the program, so unless the piece
of code you’re thinking about happens to fall into that 10% it isn’t
going to be important.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_13">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">“Premature optimization is the root
of all evil.” is sometimes referred to as “Knuth’s law”
(from Donald E. Knuth).
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_14">Add Comment</A></FONT><A NAME="_Toc534420055"></A><BR></P></DIV>
<A NAME="Heading4"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H2 ALIGN="LEFT">
Context and composition</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">One of the terms you will see used over
and over in design patterns literature is <I>context</I>. In fact, one common
definition of a design pattern is “a solution to a problem in a
context.” The GoF patterns often have a “context object” that
the client programmer interacts with. At one point it occurred to me that such
objects seemed to dominate the landscape of many patterns, and so I began asking
what they were about.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_15">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The context object often acts as a little
façade to hide the complexity of the rest of the pattern, and in addition
it will often be the controller that manages the operation of the pattern.
Initially, it seemed to me that these were not really essential to the
implementation, use and understanding of the pattern. However, I remembered one
of the more dramatic statements made in the GoF: “prefer composition to
inheritance.” The context object allows you to use the pattern in a
composition, and that may be its primary value.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A1_15">Add Comment</A></FONT><BR></P></DIV>
<HR><DIV ALIGN="LEFT"><P><A NAME="fn1" HREF="#fnB1">[1]</A><FONT FACE="Georgia"> This
is a tongue-in-cheek reference to an event in China after the death of Mao-Tze
Tung, when four persons including Mao’s widow made a power play, and were
demonized by the Chinese Communist Party under that name.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><A NAME="fn2" HREF="#fnB2">[2]</A><FONT FACE="Georgia"> These
same people were also convinced that all the computers were going to crash then,
too. But since virtually everyone had the experience of their Windows machine
crashing all the time without particularly dire results, this didn’t seem
to carry the same drama of impending doom.</FONT><BR></P></DIV>
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