[Tutor] Runing a Python program

[Alan Gauld]

Hi Henry,

> As a new python programmer, I created a directory in 
> 'C:\python24\myPythonFiles', and added a simple python 
> under the myPythonFiles directory; but when I tried 
> running it on the Python Shell, I got the following error.
>
>>> import myPythonFiles.readOut
>  ImportError: No module named myPythonFiles.readOut

Ok, the first thing to say is that you are not running the 
program but importing it, there is a difference.

To run a Python program in Windows either double click it 
in Windows Explorer (it should have a snake icon to show 
that the association is set up correctly) or at a DOS 
command prompt (or the Start->Run dialog) type

C:\> python C:\python24\myPythonFiles\readOut.py

In either case you may find the program runs so fast you 
can't see the output. In that case add a final line like:

raw_input("Hit ENTER to quit")

When you import a module it does indeed execute
the code in that module and so might appear to run 
the program, but there is one important distinction
(which you may not have come across yet!) You can 
put a clause into the module like:

if __name__ == "__main__":
     # some code here

and the code under the if will only be executed when the 
file is run as a program, it will NOT be executed when 
the file is imported as a module. (This is a powerful feature 
that greatly eases the task of writing reusable code 
in Python.)

Now the second thing to note is that your import syntax 
is wrong.

To make the files in your directory visible toi the import 
command you need to modify the value in sys.path.
There are a number of ways to do this, the one that I 
use is to create a PYTHONPATH environment variable
(MyComputer->Properties->Advanced->Environment Variables)
which points to all the folders where I keep Python code, 
in the same way that the DOS PATH variable points to 
my executable files.

HTH,

Alan Gauld
Author of the Learn to Program web site
http://www.freenetpages.co.uk/hp/alan.gauld