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Hello World Code Running It Walkthrough Conclusion

Hello World Code

hello.py

#!/usr/bin/env python

def main():
	print "Hello World!"
	name = raw_input("What is your name?\n> ")
	print "Hello", name + "."

if __name__ == "__main__":
	main()

Running It

Running the above code should output something like this:

$ ./hello.py
Hello World!
What is your Name?
> David
Hello David.
$
Note: If in a linux environment you must make sure the program is executable ("chmod a+x hello.py"), or run it through the interpreter directly ("python hello.py").

Walkthrough

Let us walk through this program line by line.

1) The Interpreter Line

#!/usr/bin/env python
The "/usr/bin/env" program will search for and run the specified program. The reason for doing this as opposed to "/usr/bin/python" is simply that someone may have installed their copy of python elsewhere, "/usr/local/bin/python" for example.

This opening statement is an attempt at cross-platform coding. In a linux environment, it will tell the shell you are using to run python as the interpreter. Windows will simply execute whatever program you have associated with the ".py" extension

2) Make a Function

def main():

This line begins a new function called "main" with no parameters. As python structures its syntax around indentation, all lines following this one which are indented will be a part of this function.

3) Output Some Text

	print "Hello World!"

The "print" command in python will output whatever follows it to the console, appending a newline at the end.

4) Collect Some Input

	name = raw_input("What is your name?\n> ")
The variable "name" will be created on this line based on the type that is stored in it. In this case it will be a string.

The "raw_input" function will print a prompt to the console, then await input until a newline is received (enter is pressed). The results will then be stored in a variable called "name" for use later in our program.

5) More Output

	print "Hello", name + "."

You may provide "print" with a comma-separated list of strings. The function will print these strings to the same line on the console, separated by a single space. In the case of the period at the end, we do not wish for there to be a space, so we simply concatenate it with the name variable.

6) Getting the "main" function to run

if __name__ == "__main__":
	main()
Please note that there are two underscores in front and two more in back of both '__name__' and '__main__'.

These two lines are designed to make your python code more re-usable. Suffice to say that they will cause your 'main' function to be called when your file is run directly, without causing problems when it is loaded as a module. We will learn about modules in a later tutorial, and hopefully the purpose of these lines will become more clear.

Conclusion

This example illustrates a basic form which you can apply to most of the python files that you write which are intended to be run directly.

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About the author.
Copyright © 2009 David Braden V. See license for details.