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<TITLE>A quick course in Python for programmers</TITLE>
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<H2><FONT FACE="Verdana, Tahoma, Arial, Helvetica, Sans">
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="_Toc534420056"></A><A NAME="Heading5"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H1 ALIGN="LEFT">
A quick course in Python for programmers</H1></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia" SIZE=4>This book assumes you’re an
experienced programmer, and it’s best if you have learned Python through
another book. For everyone else, this chapter gives a fast introduction to the
language.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0001">Add Comment</A></FONT><A NAME="_Toc524504143"></A><A NAME="_Toc534420057"></A><BR></P></DIV>
<A NAME="Heading6"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H2 ALIGN="LEFT">
Python overview</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">This</FONT><FONT FACE="Georgia"> brief
introduction is for the experienced programmer (which is what you should be if
you’re reading this book). You can refer to the full documentation at
<I>www.Python.org</I> (especially the incredibly useful HTML page <I>A Python
Quick Reference</I>),<I> </I>and also numerous books such as <I>Learning
Python</I> by Mark Lutz and David Ascher (O’Reilly, 1999).
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0002">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Python is often referred to as a
scripting language, but scripting languages tend to be limiting, especially in
the scope of the problems that they solve. Python, on the other hand, is a
programming language that also supports scripting. It <I>is</I> marvelous for
scripting, and you may find yourself replacing all your batch files, shell
scripts, and simple programs with Python scripts. But it is far more than a
scripting language.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0003">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Python is designed to be very clean to
write and especially to read. You will find that it’s quite easy to read
your own code long after you’ve written it, and also to read other
people’s code. This is accomplished partially through clean, to-the-point
syntax, but a major factor in code readability is indentation – scoping in
Python is determined by indentation. For example:
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0004">Add Comment</A></FONT><BR></P></DIV>
<BLOCKQUOTE><FONT SIZE = "+1"><PRE>#: c01:<font color=#0000ff>if</font>.py
response = <font color=#004488>"yes"</font>
<font color=#0000ff>if</font> response == <font color=#004488>"yes"</font>:
<font color=#0000ff>print</font> <font color=#004488>"affirmative"</font>
val = 1
<font color=#0000ff>print</font> <font color=#004488>"continuing..."</font>
#:~</PRE></FONT></BLOCKQUOTE><DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The ‘<B>#</B>’
denotes a comment that goes until the end of the line, just like C++ and Java
‘<B>//</B>’ comments.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0005">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">First notice that the basic syntax of
Python is C-ish as you can see in the <B>if</B> statement. But in a C <B>if</B>,
you would be required to use parentheses around the conditional, whereas they
are not necessary in Python (it won’t complain if you use them anyway).
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0006">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The conditional clause ends with a colon,
and this indicates that what follows will be a group of indented statements,
which are the “then” part of the <B>if</B> statement. In this case,
there is a “print” statement which sends the result to standard
output, followed by an assignment to a variable named <B>val</B>. The subsequent
statement is not indented so it is no longer part of the <B>if</B>. Indenting
can nest to any level, just like curly braces in C++ or Java, but unlike those
languages there is no option (and no argument) about where the braces are placed
– the compiler forces everyone’s code to be formatted the same way,
which is one of the main reasons for Python’s consistent readability.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0007">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Python normally has only one statement
per line (you can put more by separating them with semicolons), thus no
terminating semicolon is necessary. Even from the brief example above you can
see that the language is designed to be as simple as possible, and yet still
very readable.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0008">Add Comment</A></FONT><A NAME="_Toc524504144"></A><A NAME="_Toc534420058"></A><BR></P></DIV>
<A NAME="Heading7"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H3 ALIGN="LEFT">
Built-in containers</H3></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">With languages like C++ and Java,
containers are add-on libraries and not integral to the language. In Python, the
essential nature of containers for programming is acknowledged by building them
into the core of the language: both lists and associative arrays (a.k.a. maps,
dictionaries, hash tables) are fundamental data types. This adds much to the
elegance of the language.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0009">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">In addition, the <B>for</B> statement
automatically iterates through lists rather than just counting through a
sequence of numbers. This makes a lot of sense when you think about it, since
you’re almost always using a <B>for</B> loop to step through an array or a
container. Python formalizes this by automatically making <B>for</B> use an
iterator that works through a sequence. Here’s an example:
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0010">Add Comment</A></FONT><BR></P></DIV>
<BLOCKQUOTE><FONT SIZE = "+1"><PRE>#: c01:list.py
list = [ 1, 3, 5, 7, 9, 11 ]
<font color=#0000ff>print</font> list
list.append(13)
<font color=#0000ff>for</font> x <font color=#0000ff>in</font> list:
<font color=#0000ff>print</font> x
#:~</PRE></FONT></BLOCKQUOTE><DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The first line creates a list.
You can print the list and it will look exactly as you put it in (in contrast,
remember that I had to create a special <B>Arrays2</B> class in <I>Thinking in
Java, 2<SUP>nd</SUP> Edition </I>in order to print arrays in Java). Lists are
like Java containers – you can add new elements to them (here,
<B>append( ) </B>is used) and they will automatically resize themselves.
The <B>for</B> statement creates an iterator <B>x</B> which takes on each value
in the list.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0011">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You can create a list of numbers with the
<B>range( )</B> function, so if you really need to imitate C’s
<B>for</B>, you can.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0012">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Notice that there aren’t any type
declarations – the object names simply appear, and Python infers their
type by the way that you use them. It’s as if Python is designed so that
you only need to press the keys that absolutely must. You’ll find after
you’ve worked with Python for a short while that you’ve been using
up a lot of brain cycles parsing semicolons, curly braces, and all sorts of
other extra verbiage that was demanded by your non-Python programming language
but didn’t actually describe what your program was supposed to do.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0013">Add Comment</A></FONT><A NAME="_Toc524504145"></A><A NAME="_Toc534420059"></A><BR></P></DIV>
<A NAME="Heading8"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H3 ALIGN="LEFT">
Functions</H3></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">To create a function in Python, you use
the <B>def</B> keyword, followed by the function name and argument list, and a
colon to begin the function body. Here is the first example turned into a
function:
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0014">Add Comment</A></FONT><BR></P></DIV>
<BLOCKQUOTE><FONT SIZE = "+1"><PRE>#: c01:myFunction.py
<font color=#0000ff>def</font> myFunction(response):
val = 0
<font color=#0000ff>if</font> response == <font color=#004488>"yes"</font>:
<font color=#0000ff>print</font> <font color=#004488>"affirmative"</font>
val = 1
<font color=#0000ff>print</font> <font color=#004488>"continuing..."</font>
<font color=#0000ff>return</font> val
<font color=#0000ff>print</font> myFunction(<font color=#004488>"no"</font>)
<font color=#0000ff>print</font> myFunction(<font color=#004488>"yes"</font>)
#:~</PRE></FONT></BLOCKQUOTE><DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Notice there is no type
information in the function signature – all it specifies is the name of
the function and the argument identifiers, but no argument types or return
types. Python is a <I>weakly-typed</I> language, which means it puts the minimum
possible requirements on typing. For example, you could pass and return
different types from the same function:
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0015">Add Comment</A></FONT><BR></P></DIV>
<BLOCKQUOTE><FONT SIZE = "+1"><PRE>#: c01:differentReturns.py
<font color=#0000ff>def</font> differentReturns(arg):
<font color=#0000ff>if</font> arg == 1:
<font color=#0000ff>return</font> <font color=#004488>"one"</font>
<font color=#0000ff>if</font> arg == <font color=#004488>"one"</font>:
<font color=#0000ff>return</font> 1
<font color=#0000ff>print</font> differentReturns(1)
<font color=#0000ff>print</font> differentReturns(<font color=#004488>"one"</font>)
#:~</PRE></FONT></BLOCKQUOTE><DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The only constraints on an
object that is passed into the function are that the function can apply its
operations to that object, but other than that, it doesn’t care. Here, the
same function applies the ‘<B>+</B>’ operator to integers and
strings:
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0016">Add Comment</A></FONT><BR></P></DIV>
<BLOCKQUOTE><FONT SIZE = "+1"><PRE>#: c01:sum.py
<font color=#0000ff>def</font> sum(arg1, arg2):
<font color=#0000ff>return</font> arg1 + arg2
<font color=#0000ff>print</font> sum(42, 47)
<font color=#0000ff>print</font> sum('spam ', <font color=#004488>"eggs"</font>)
#:~</PRE></FONT></BLOCKQUOTE><DIV ALIGN="LEFT"><P><FONT FACE="Georgia">When the operator
‘<B>+</B>’ is used with strings, it means concatenation (yes, Python
supports operator overloading, and it does a nice job of it).
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A2_0017">Add Comment</A></FONT><A NAME="_Toc524504146"></A><A NAME="_Toc534420060"></A><BR></P></DIV>
<A NAME="Heading9"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H3 ALIGN="LEFT">
Strings</H3></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The above example also shows a little bit
about Python string handling, which is the best of any language I’ve
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