.. _tut-intro:

**********************

Whetting Your Appetite

**********************

If you do much work on computers, eventually you find that there's some task

you'd like to automate.  For example, you may wish to perform a

search-and-replace over a large number of text files, or rename and rearrange a

bunch of photo files in a complicated way. Perhaps you'd like to write a small

custom database, or a specialized GUI application, or a simple game.

If you're a professional software developer, you may have to work with several

C/C++/Java libraries but find the usual write/compile/test/re-compile cycle is

too slow.  Perhaps you're writing a test suite for such a library and find

writing the testing code a tedious task.  Or maybe you've written a program that

could use an extension language, and you don't want to design and implement a

whole new language for your application.

Python is just the language for you.

You could write a Unix shell script or Windows batch files for some of these

tasks, but shell scripts are best at moving around files and changing text data,

not well-suited for GUI applications or games. You could write a C/C++/Java

program, but it can take a lot of development time to get even a first-draft

program.  Python is simpler to use, available on Windows, Mac OS X, and Unix

operating systems, and will help you get the job done more quickly.

Python is simple to use, but it is a real programming language, offering much

more structure and support for large programs than shell scripts or batch files

can offer.  On the other hand, Python also offers much more error checking than

C, and, being a *very-high-level language*, it has high-level data types built

in, such as flexible arrays and dictionaries.  Because of its more general data

types Python is applicable to a much larger problem domain than Awk or even

Perl, yet many things are at least as easy in Python as in those languages.

Python allows you to split your program into modules that can be reused in other

Python programs.  It comes with a large collection of standard modules that you

can use as the basis of your programs --- or as examples to start learning to

program in Python.  Some of these modules provide things like file I/O, system

calls, sockets, and even interfaces to graphical user interface toolkits like

Tk.

Python is an interpreted language, which can save you considerable time during

program development because no compilation and linking is necessary.  The

interpreter can be used interactively, which makes it easy to experiment with

features of the language, to write throw-away programs, or to test functions

during bottom-up program development. It is also a handy desk calculator.

Python enables programs to be written compactly and readably.  Programs written

in Python are typically much shorter than equivalent C,  C++, or Java programs,

for several reasons:

* the high-level data types allow you to express complex operations in a single

  statement;

* statement grouping is done by indentation instead of beginning and ending

  brackets;

* no variable or argument declarations are necessary.

Python is *extensible*: if you know how to program in C it is easy to add a new

built-in function or module to the interpreter, either to perform critical

operations at maximum speed, or to link Python programs to libraries that may

only be available in binary form (such as a vendor-specific graphics library).

Once you are really hooked, you can link the Python interpreter into an

application written in C and use it as an extension or command language for that

application.

By the way, the language is named after the BBC show "Monty Python's Flying

Circus" and has nothing to do with reptiles.  Making references to Monty

Python skits in documentation is not only allowed, it is encouraged!

Now that you are all excited about Python, you'll want to examine it in some

more detail.  Since the best way to learn a language is to use it, the tutorial

invites you to play with the Python interpreter as you read.

In the next chapter, the mechanics of using the interpreter are explained.  This

is rather mundane information, but essential for trying out the examples shown

later.

The rest of the tutorial introduces various features of the Python language and

system through examples, beginning with simple expressions, statements and data

types, through functions and modules, and finally touching upon advanced

concepts like exceptions and user-defined classes.