This does nothing but print the message "My while now". The expression and command sequence are ignored because Tcl treats them as parameters to the while function and the while function expects them but ignores them! So you can see how we define procedures in Tcl and how we can abuse that to create very confusing programs - don't do it unless you have a very good reason!

Creating our own modules

So now we can create our own functions and call these from other parts of our program. That's good because it can save us a lot of typing and, more importantly, makes our programs easier to understand because we can forget about some of the details after we create the function that hides them. (This principle of wrapping up the complex bits of a program inside functions is called information hiding for fairly obvious reasons.) But how can we use these functions in other programs? We create a module.

Python Modules

A module in Python is nothing special. It's just a plain text file full of Python program statements. Usually these statements are function definitions. Thus when we type:

from sys import *

we effectively copy the contents of sys.py into our program, almost like a cut n' paste operation. (its not really like that but the concept is OK). In fact in some programming languages (noteably C++) the translator literally does simply copy module files into the current program as required.

So to recap, we create a module by creating a Python file containing the functions we want to reuse in other programs. Then we just import our module exactly like we do the standard modules. Easy eh? Let's do it.

Copy the function below into a file by itself and save the file with the name timestab.py

def print_table(multiplier):

print "--- Printing the %d times table ---" % multiplier for n in range(1,13):

print "%d x %d = %d" % (n, multiplier, n*multiplier)

Now at the Python prompt type:

>>>import timestab

>>>timestab.print_table(12)

Heh presto! You've created a module and used it.

Important Note:If you didn't start Python from the same directory that you stored the timestab.py file then Python might not have been able to find the file and reported an error. If so then you can create an environment variable called PYTHONPATH that holds a list of valid directories to search for modules (in addition to the standard modules supplied with Python).

Creating environment variables is a platform specific operation which I assume you either know how to do or can find out!

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Modules in BASIC and Tcl

What about BASIC? That's more complex.... In QBASIC and other older varieties there is no real module concept. You have to manually cut n' paste from previous projects into your current one using your text editor. However in Visual Basic there is a module concept and you can load a module via the Integrated Development Environment (IDE) File|Open Module... menu. There are a few restrictions as to what kind of things you can do inside a BASIC module but since we're not using Visual Basic on this course I won't go into that any further. (Note: there is a cut down version of Visual Basic known as the COM Controls Edition, CCE, available for free download on Microsoft's website if you feel like experimenting. Also Windows 98, 2000 and IE5 all install a cut down version of VB called VBScript which you can use in files ending .vbs)

Finally Tcl, as ever(!), takes a somewhat eclectic, but nonetheless interesting, path with regard to reusing modules (or as it prefers to call them libraries).

At the simplest level you can just create a file of Tcl functions as we do in Python and then, in your program, source the file. This literally causes the interpreter to read your file and those programs become available for use. But there is a more interesting option:

You can create your files as above, put them all in a directory/folder and then run a mk_index command. This builds an index of all the functions and files in the folder. Then in your program you simply call the required function and the Tcl interpreter will realize the function is not available and automatically look in the index file. It will then source the relevant source file from the library and execute the function.

Once sourced the function stays available so there is little performance overhead involved. The only snag is that the programmer must avoid having more than one function with the same name. This feature of Tcl is known as autoloading.

Next we'll take a look at files and text handling and then as promised revisit the business of counting words in a file. In fact we're eventually going to create a module of text handling functions for our convenience.

Things to remember

•Functions are a form of module

•Functions return values, procedures don't

•Python modules normally consist of function definitions in a file

•Create new functions with the def keyword in Python

•Use SUB or FUN in BASIC and proc in Tcl

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