Now, let us 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 but it teaches you a lot on how to use the tools of that language. As Simon Cozens [1] says, it is the 'traditional incantation to the programming gods to help you learn the language better' :-).
In DrPython, click on -> . If you prefer the keyboard, press Ctrl-N. Enter the program as shown below.
#!/usr/bin/env python # Filename: helloworld.py print 'Hello World'
Click on Ctrl-S. Enter the name
of the file as helloworld.py
and store it in a
folder of your choice.
To run the program, click on Ctrl-E.
-> . If you prefer the keyboard, pressIf the Ctrl-D to stop the interpreter.
option is not enabled, then it probably means you have an interpreter already running. Click on -> or pressIf you get an error, check whether you have typed the program exactly as shown above and try again. Be careful about use of spaces and tabs in your file.
You can also run the same program in the shell using the python command:
$ python helloworld.py Hello World
Note that, in the shell, you will have to switch to the
directory containing helloworld.py
for the
above command to work.
Let us consider the first two lines of the program. You
will notice the #
symbol at the start of the
lines. In Python, any text to the right of the
#
symbol are considered to be
comments. Comments are optional but are
useful as notes for the readers of the program.
However, the first line is a special case:
#!/usr/bin/env python
The first two characters in this line, and in this file,
are #
and !
respectively.
This is called the she-bang line. This line
allows you to tell your Linux/BSD system to run the given source
file with the interpreter that we specify in that line. This is
explained in detail in the
next section.
Although the usage of the she-bang line is specific for Linux/BSD
systems, it is considered good practice to include this line even
if you are using only Windows for your programs.
We use the /usr/bin/env command (env is
short for environment) to find the
python command for us and then our script is run with
that command. We could have directly specify the location of the
python command as
#!/usr/bin/python
, but some systems have the Python
command located at /usr/local/bin/python.
Using env allows us to avoid such problems and
make our programs more portable (portable means allowing a
program to run on different kinds of systems without requiring
changes).
You can always specify the command first and then the
source file by running python
helloworld.py
. This method
works across platforms.
Writing good comments is important because it helps other readers of your program to understand the program easily. Good comments should concentrate on what the program is doing rather than how the program works (which is explained by the code itself).
Remember that six months after you have written your programs, your comments will help you immensely to recollect how the program is working!
Following the two comment lines, the third line is a Python
statement. The print
statement is supplied the text 'Hello
World'
which is promptly printed to the
screen.