Chapter 3. First Steps¶

Start the intepreter on the command line by entering python at the shell prompt. Now enter print ‘Hello World’ followed by the Enter key. You should see the words Hello World as output.

For Windows users, you can run the interpreter in the command line if you have set the PATH variable appropriately. Alternatively, you can use the IDLE program. IDLE is short for Integrated DeveLopment Environment. Click on Start -> Programs -> Python 2.3 -> IDLE (Python GUI). Linux users can use IDLE too.

Note that the <<< signs are the prompt for entering Python statements.

$ python
Python 2.3.4 (#1, Oct 26 2004, 16:42:40)
[GCC 3.4.2 20041017 (Red Hat 3.4.2-6.fc3)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> print 'hello world'
hello world
>>>

To exit the prompt, press Ctrl-d if you are using IDLE or are using a Linux/BSD shell. In case of the Windows command prompt, press Ctrl-z followed by Enter.

Before we move on to writing Python programs in source files, we need an editor to write the source files. The choice of an editor is crucial indeed. You have to choose an editor as you would choose a car you would buy. A good editor will help you write Python programs easily, making your journey more comfortable and helps you reach your destination (achieve your goal) in a much faster and safer way.

One of the very basic requirements is syntax highlighting where all the different parts of your Python program are colorized so that you can see your program and visualize its running.

If you are using Windows, then I suggest that you use IDLE. IDLE does syntax highlighting and a lot more such as allowing you to run your programs within IDLE among other things. A special note: don’t use Notepad - it is a bad choice because it does not do syntax highlighting and also importantly it does not support indentation of the text which is very important in our case as we will see later. Good editors such as IDLE (and also VIM) will automatically help you do this.

If you are using Linux/FreeBSD, then you have a lot of choices for an editor. If you are an experienced programmer, then you must be already using VIM or Emacs. Needless to say, these are two of the most powerful editors and you will be benefitted by using them to write your Python programs. I personally use VIM for most of my programs. If you are a beginner programmer, then you can use Kate which is one of my favorites. In case you are willing to take the time to learn VIM or Emacs, then I highly recommend that you do learn to use either of them as it will be very useful for you in the long run.

If you still want to explore other choices of an editor, see the comprehensive list of Python editors and make your choice. You can also choose an IDE (Integrated Development Environment) for Python. See the comprehensive list of IDEs that support Python for more details. Once you start writing large Python programs, IDEs can be very useful indeed.

I repeat once again, please choose a proper editor - it can make writing Python programs more fun and easy.

Now let’s get back to programming. There is a tradition that whenever you learn a new programming language, the first program that you write and run is the ‘Hello World’ program - all it does is just say ‘Hello World’ when you run it. As Simon Cozens ^[1] puts it, it is the ‘traditional incantation to the programming gods to help you learn the language better’ :) .

Start your choice of editor, enter the following program and save it as helloworld.py

#!/usr/bin/python
# Filename : helloworld.py
print 'Hello World'

(Source file: code/helloworld.py)

$ python helloworld.py
Hello World

Use comments sensibly in your program to explain some important details of your program - this is useful for readers of your program so that they can easily understand what the program is doing. Remember, that person can be yourself after six months!

The comments are followed by a Python statement - this just prints the text ‘Hello World’. The print is actually an operator and ‘Hello World’ is referred to as a string - don’t worry, we will explore these terminologies in detail later.

This applies only to Linux/Unix users but Windows users might be curious as well about the first line of the program. First, we have to give the program executable permission using the chmod command then run the source program.

$ chmod a+x helloworld.py
$ ./helloworld.py
Hello World

The chmod command is used here to change the mode of the file by giving execute permission to all users of the system. Then, we execute the program directly by specifying the location of the source file. We use the ./ to indicate that the program is located in the current directory.

To make things more fun, you can rename the file to just helloworld and run it as ./helloworld and it will still work since the system knows that it has to run the program using the interpreter whose location is specified in the first line in the source file.

You are now able to run the program as long as you know the exact path of the program - but what if you wanted to be able to run the program from anywhere? You can do this by storing the program in one of the directories listed in the PATH environment variable. Whenever you run any program, the system looks for that program in each of the directories listed in the PATH environment variable and then runs that program. We can make this program available everywhere by simply copying this source file to one of the directories listed in PATH.

$ echo $PATH
/opt/mono/bin:/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/home/swaroop
/bin
$ cp helloworld.py /home/swaroop/bin/helloworld
$ helloworld
Hello World

We can display the PATH variable using the echo command and prefixing the variable name by $ to indicate to the shell that we need the value of this variable. We see that /home/swaroop/bin is one of the directories in the PATH variable where swaroop is the username I am using in my system. There will usually be a similar directory for your username on your system. Alternatively, you can add a directory of your choice to the PATH variable - this can be done by running PATH=$PATH:/home/swaroop/mydir where ‘/home/swaroop/mydir’ is the directory I want to add to the PATH variable.

This method is very useful if you want to write useful scripts that you want to run the program anytime, anywhere. It is like creating your own commands just like cd or any other commands that you use in the Linux terminal or DOS prompt.

If you need quick information about any function or statement in Python, then you can use the built-in help functionality. This is very useful especially when using the interpreter prompt. For example, run help(str) - this displays the help for the str class which is used to store all text (strings) that you use in your program. Classes will be explained in detail in the chapter on object-oriented programming.

$ env PYTHONDOCS=/usr/share/doc/python-docs-2.3.4/html/ python
Python 2.3.4 (#1, Oct 26 2004, 16:42:40)
[GCC 3.4.2 20041017 (Red Hat 3.4.2-6.fc3)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> help('print')

You should now be able to write, save and run Python programs at ease. Now that you are a Python user, let’s learn some more Python concepts. _________________________________________________________

^[1] one of the leading Perl6/Parrot hackers and the author of the amazing ‘Beginning Perl’ book