Programming Python, Part I

Python is a programming language that is highly regarded for its simplicity and ease of use. It often is recommended to programming newcomers as a good starting point. Python also is a program that interprets programs written in Python. There are other implementations of Python, such as Jython (in Java), CLPython (Common Lisp), IronPython (.NET) and possibly more. Here, we use only Python.

Installing Python and getting it running is the first step. These days, it should be very easy. If you are running Gentoo GNU/Linux, you already have Python 2.4 installed. The packaging system for Gentoo, Portage, is written in Python. If you don't have it, your installation is broken.

If you are running Debian GNU/Linux, Ubuntu, Kubuntu or MEPIS, simply run the following (or log in as root and leave out sudo):

sudo apt-get install python

One catch is that Debian's stable Python is 2.3, while for the rest of the distributions, you are likely to find 2.4. They are not very different, and most code will run on both versions. The main differences I have encountered are in the API of some library classes, new features added to 2.4 and some internals, which shouldn't concern us here.

If you are running some other distribution, it is very likely that Python is prepackaged for it. Use the usual resources and tools you use for other packages to find the Python package.

If all that fails, you need to do a manual installation. It is not difficult, but be aware that it is easy to break your system unless you follow this simple guideline: install Python into a well-isolated place, I like /opt/python/2.4.3, or whatever version it is.

To perform the installation, download Python, unpack it, and run the following commands:

./configure --prefix=/opt/python2.4/
make
make install

This task is well documented on Python's README, which is included in the downloaded tarball; take a look at it for further details. The only missing task here is adding Python to your path. Alternatively, you can run it directly by calling it with its path, which I recommend for initial exploration.

Now that we have Python running, let's jump right in to programming and examine the language as we go along. To start, let's build a blog engine. By engine, I mean that it won't have any kind of interface, such as a Web interface, but it's a good exercise anyway.

Python comes with an REPL—a nice invention courtesy of the Lisp community. REPL stands for Read Eval Print Loop, and it means there's a program that can read expressions and statements, evaluate them, print the result and wait for more. Let's run the REPL (adjust your path according to where you installed Python in the previous section):

$ python
Python 2.4.3 (#1, Sep  1 2006, 18:35:05)
[GCC 4.1.1 (Gentoo 4.1.1)] on linux2
Type "help", "copyright", "credits" or "license" for
more information.
>>>

Those three greater-than signs (>>>) are the Python prompt where you write statements and expressions. To quit Python, press Ctrl-D.

Let's type some simple expressions:

>>> 5
5

The value of 5 is, well, 5.

>>> 10 + 4
14

That's more interesting, isn't it?

There are other kinds of expressions, such as a string:

>>> "Hello"
'Hello'

Quotes are used to create strings. Single or double quotes are treated essentially the same. In fact, you can see that I used double quotes, and Python showed the strings in single quotes.

Another kind of expression is a list:

>>> [1,3,2]
[1, 3, 2]

Square brackets are used to create lists in which items are separated by commas. And, as we can add numbers, we can add—actually concatenate—lists:

>>> [1,3,2] + [11,3,2]
[1, 3, 2, 11, 3, 2]

By now, you might be getting bored. Let's switch to something more exciting—a blog. A blog is a sequence of posts, and a Python list is a good way to represent a blog, with posts as strings. In the REPL, we can build a simple blog like this:

>>> ["My first post", "Python is cool"]
['My first post', 'Python is cool']
>>>

That's a list of strings. You can make lists of whatever you want, including a list of lists. So far, all our expressions are evaluated, shown and lost. We have no way to recall our blog to add more items or to show them in a browser. Assignment comes to the rescue:

>>> blog = ["My first post", "Python is cool"]
>>> 

Now blog, a so-called variable, contains the list. Unlike in the previous example, nothing was printed this time, because it is an assignment. Assignments are statements, and statements don't have a return value. Simply evaluating the variable shows us the content:

>>> blog
['My first post', 'Python is cool']

Accessing our blog is easy. We simply identify each post by number:

>>> blog[0]
'My first post'
>>> blog[1]
'Python is cool'