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<HTML><HEAD><TITLE>Python for Newbies</TITLE>
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<H1 align=center>Python for Newbies</H1></A>
<H2 align=center>by <A href="mailto:skinite@home.com">Pupp3tM</A>, a.k.a. David
Borowitz</H2>
<H3>Table of Contents</H3>
<OL>
<LI><A
href="#intro">Introduction</A>
<OL type=a>
<LI><A
href="#whatis">What
is Python?</A>
<LI><A
href="#interp">The
Interpreter</A> </LI></OL>
<LI><A
href="#first">Your
First Program</A>
<OL type=a>
<LI><A
href="#hello">Hello,
World!</A>
<LI><A
href="#explain">Explanation</A>
</LI></OL>
<LI><A
href="#varmath">Variables
and Math</A>
<OL type=a>
<LI><A
href="#var">Variables:
Strings, Numbers, etc.</A>
<LI><A
href="#math">Math
and Operators</A> </LI></OL>
<LI><A
href="#io">Input/Output</A>
<OL type=a>
<LI><A
href="#print">Printing
information</A>
<LI><A
href="#user">Interacting
with the User</A> </LI></OL>
<LI><A
href="#control">Program
Control</A>
<OL type=a>
<LI><A
href="#if">What
If...</A>
<LI><A
href="#for">For
Ever...</A>
<LI><A
href="#while">While
We...and Others</A> </LI></OL>
<LI><A
href="#inter">Intermediate
Python</A>
<OL type=a>
<LI><A
href="#tuples">Tuples!</A>
<LI><A
href="#string">Strings
and Slice Indexing</A>
<LI><A
href="#fileio">File
I/O</A> </LI></OL>
<LI><A
href="#mod">Modules</A>
<OL type=a>
<LI><A
href="#mod1">Overview
and Importing</A>
<LI><A
href="#builtin">Builtin
Modules</A>
<LI><A
href="#yours">User-Defined
Modules and Functions</A> </LI></OL>
<LI><A
href="#close">In
Closing</A>
<OL type=a>
<LI><A
href="#perl">Why
Python is Better than Perl</A>
<LI><A
href="#ref">References</A>
</LI></OL></LI></OL>
<HR>
<H2 align=center><A name=intro>Introduction</A></H2>
<H3 align=center><A name=whatis>What is Python?</A></H3><BR>
<P>Python is, in short, a scripting language. It is similar in function to Perl,
but to my knowledge is not nearly as popular. To fix that is part of the goal of
this tutorial. It is a very high-level language that can perform complex tasks,
but is surprisingly easy to learn. Many different add=ons (modules) are
available to control everything from MP3s to windowing toolkits (Unix). I find
Python to be a just plain <I>fun</I> language to program in, as it is very
intuitive and suited to a variety of uses. </P>
<P>Python can be run on almost any platform, from 'nix to Windows. To get
Python, first go to <A href="http://www.python.org/">Python.org</A>, and then
download away! </P>
<P>This tutorial is geared towards people who have little experience with
programming but know at least something. I make numerous references to languages
like C and Perl, which are good to know, but you won't lose much if you just
skip over them. </P>
<P>What's with the funky name, you ask? Some sort of carnivorous reptile? No,
dear reader. Python's creators were (are) big Monty Python fans. The name comes
from the BBC show "Monty Python's Flying Circus," and in the official docs, it
says "Making references to Monty Python skits in documentation is not only
allowed, it is encouraged." That said, I'm afraid I haven't seen all too many
Monty Python movies so we'll go a little light on the references :) </P>
<P>So sit back, grab a Coke, <A href="http://www.python.org/download/">download
Python</A>, and get ready to learn :) </P>
<P><FONT size=-2><A
href="#top">Back
to top</A></FONT></P>
<HR>
<H3 align=center><A name=interp>The Interpreter</A></H3><BR>
<P>Python is a scripted, i.e. interpreted, language. Unlike C, which has a
compiler, we have in Python the...(drum roll please)...Python interpreter. Once
you have Python correctly installed, in your Unix shell or Dos box type 'python'
(Note: anytime I tell you to type something and put it in quotes, don't type the
quotes. I'm sure you already figured this out, but...). If you've used perl or
some other interpreted language before, you might notice something: there's a
prompt (those three little ">>>" things)! Not waiting for stdin or
saying "Usage: python <script-file>"! That's right, Python has an
<I>interactive mode</I>. In this mode, you can type any Python command and it
will work just like you typed it from a script, with a few differences. Most
importantly, if you type a variable of some sort or something that returns a
value of some sort (except assignments), it will print the result automatically.
Neat for trying stuff out (and learning!). Now type '1 + 1' at the prompt you
should still be at. What do you see? 2! Now ain't that a powerful language: <PRE>>>> 1 + 1
2
</PRE>You can do this with any kind of variable or math expression (see part <A
href="#math">3b</A>)
and get some output. Works with strings too: <PRE>>>> "elp! I'm being oppressed"
"elp! I'm being oppressed"
</PRE>Note the bad Monty Python reference :). Anyway, that's all for the
interpreter. Now we get into the meat of things. If at any point there's a
sample script and you don't feel like creating a new file for it, feel free to
type (or better yet copy & paste) it into the interpreter, it should work
the same. Wait, scratch that :). If I put something in a script, it most likely
means you only get the effect from a script and it won't work in interactive
mode. Sorry.
<P></P>
<P>Wait! One note before we leave. To quit the interpreter, you can't do
anything fancy like type 'quit'. For some reason (beats me), that doesn't work.
You have to type Ctrl+D to quit (on Unix at least, haven't tested it on
Windows). Anyway if you type 'quit' it'll say something like 'Use Ctrl-D (i.e.
EOF) to exit.' so listen to that. </P>
<P><FONT size=-2><A
href="#top">Back
to top</A></FONT></P>
<HR>
<H2 align=center><A name=first>Your First Program</A></H2>
<H3 align=center><A name=hello>Hello, World!</A></H3><BR>Alright, if you've ever
read a programming tutorial before you know they all have some sort of program
that just prints "Hello, World!" or something on the screen. Unfortunately, this
tutorial is no different. Alright, here goes: <PRE>#!/usr/bin/python
#You only need the above line for Unix; replace that with your path to python
hello = "Hello, World!"
print hello
</PRE>That's it! Put that in a text file, name it hello.py, run it, and we're
good. On Unix you may have to 'chmod +x hello.py' to run it, or just type
'python hello.py'. Windows, I'm pretty sure .py files are associated with the
Python interpreter automatically.
<P></P>
<P><FONT size=-2><A
href="#top">Back
to top</A></FONT></P>
<HR>
<H3 align=center><A name=explain>Explanation</A></H3><BR>"Wow," you may be
saying, "that was easy." Indeed it was. Now, though, you probably only have some
remote clue of what's going on. We'll take it line by line. <PRE>#!/usr/bin/python
</PRE>This line, like you should've read, is only needed for Unix users. It
tells your shell where the python executable is so it can run the script. <PRE>#You only need the above line for Unix; replace that with your path to python
</PRE>A pretty useless line, you may think, but it illustrates an important
point: comments! Any good programmer knows that you need comments in your code,
so just put # and anything after that on that line is a comment and will be
ignored by the interpreter. <PRE>hello = "Hello, World!"
</PRE>I didn't really need this line, but I thought a Hello, World! script with
only one line is kinda lame, so I added it anyway. hello is a string variable,
which we'll talk about later, and the = operator assigns "Hello, World!" as its
value. I'm sure if you've programmed before you've seen all this. <PRE>print hello
</PRE>This last line, as you may well guess, prints the contents of hello. If
you're a perl guy (or girl :), you need to know that if you're printing a
variable you can't put the name of the variable in quotes. But that's not too
inconvenient, as we should see later.
<P></P>
<P>Alright! Our first program! Now just a note or two before we move on. First,
you may notice that there's nothing on the end of any lines, no ; like in perl
or C. That's one of the convenient features of Python. I think it's more
intuitive because blocks (later dammit!) are determined by indenting and every
statement has its own line...you may disagree I guess but you'll just have to
get used to it. </P>
<P><FONT size=-2><A
href="#top">Back
to top</A></FONT></P>
<HR>
<H2 align=center><A name=varmath>Variables and Math</A></H2>
<H3 align=center><A name=var>Variables: Strings, Numbers, etc.</A></H3><BR>In
Python, just like any other language worth its salt, there are a number of data
types, Mainly, there are numbers (of arbitrary precision), strings, lists, and
tuples. Tuples are more of an "advanced" variable type, kind of like lists, so I
won't go into them here, but I'll touch on them a bit in the <A
href="#inter">Intermediate
Python</A> section.
<P></P>
<P>Numbers. What can I say? Numbers are numbers. To assign them, just like you
did with the string in Hello World!, just put the name of your variable, the =
operator, and what you want it to be: <PRE>>>> a = 5
>>>
</PRE>A word on variable names: I'm not quite sure exactly how long Python
allows them to be (should I know?), but as a general rule you shouldn't have
names more than 20 or so characters. Like other languages, variable names must
be all alphanumeric characters and underscores (_), but you <I>can</I> start
names with underscores, unlike some languages.
<P></P>
<P>Anyway, on to strings. Strings store, obviously, strings of characters.
Nothing you haven't heard before. For strings, you can use either type of quotes
("" or ''), but be sure you know which kind. If you want to use, say, an
apostrophe inside a string, you need to make sure of some things. If the string
has "" quotes, don't worry about it, just put your apostraph. If you were in ''
quotes, though, you would need to escape the apostrophe. This is so the
interpreter doesn't think the apostrophe, which is just like a quote, is the end
of your string. To escape the apostrophe, just put a \ (backslash) in front of
the quote. Try it! It's great: <PRE>>>> "I'm Dave"
"I'm Dave"
>>> 'Python\'s my favorite language'
"Python's my favorite language"
>>> "\"I can't think of anything else to say!\" he said"
'"I can\'t think of anything else to say!" he said'
>>> 'That's too bad'
File "<stdin>", line 1
'That's too bad'
^
SyntaxError: invalid syntax
</PRE>Note the last two examples. The first one illustrates escaping of the ""
quotes and also this: If you have both types of quotes in a string, Python will
default to one and escape the other. Don't worry; if you're printing this to the
screen it won't show the \ character in your output. In the last example, that's
what happens when you don't escape quotes. You may notice Python has a nice way
of handling errors, but we won't go into that.
<P></P>
<P>Speaking of escaping, here are a few more characters you can escape (if it
says 'n' then you use '\n' to escape it):
<UL>
<LI>n - Prints a newline, i.e. starts a new line.
<LI>t - Horizontal tab
<LI>b - Backslash. Deletes the last character typed
<LI>a - System beep
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