EXTREME NOOBIE

[Mike Fletcher]

This is rather rough, but might give you somewhere to start...

A function is a "factory", the factory takes some number (sometimes zero) of
inputs (raw material), performs some number of operations on the inputs, and
either returns a result (a finished product), or raises an exception (blows
up).  We make these factories so that we can more readily understand how our
end product (what we want our program to do) will be created.  That is, we
create an abstraction by saying "this function (factory) has the job of
doing <<this>>, and preferably only <<this>>", that is its role, or, if you
like, it's "contract" with the rest of our programming world.

Once we have the factory built, we don't need to worry about the machinery
inside anymore, as we can look at the "contract" of the factory, and see how
the factory fits into our larger goals (instead of always having to decide
which spout on which machine needs to be connected to which conveyor belt on
which other machine).

A factory can have a number of inputs.  That is, it can require different
types of "materials" to accomplish its job.  For instance, if our factory's
job is to create cars, it requires some wheels, some steel, some seats, some
glass etc.  Depending on how much of each material is received, different
processing might occur within the factory (if we receive only three wheels,
we create a funky three wheeled car or blow up). In programming, we
differentiate these "materials" (which we call "parameters") by giving them
names.  A typical definition of a set of inputs to a function might look
like this:

def factory( steel, glass, tires ):
	# the machinery of the function goes here

In this function definition, we have defined three parameters to the
function named "factory".  These parameters are each accessible within the
"walls" of the function as "variables" (effectively bins in which material
is stored).  To use the function (produce the desired output from the
input), we pass the materials to the function like so:

factory( 3000, 50, 4 )

The order of the parameters determines which parameter is matched to which
value [aside: there are other ways to match up parameters and the input
values, but you will learn those later]. Therefore values available within
the function are steel = 3000, glass = 50, tires = 4.  The function performs
its black box operation on the inputs, deciding what processing needs to be
done given the types of materials passed, and produces some result (or blows
up).  To see what this looks like in code:

def factory( steel, glass, tires ):
	# processing in here
	return myAutomobile

In Python, code uses the "return" statement to return a particular value to
the outside world.  [Geek Note: every function in Python either returns a
value or blows up.  If no value is explicitly returned (i.e. you don't use
the "return something" statement), the None object is returned from your
function.]  Within the "body" of the function, you use normal code to
manipulate the values available to you, including the inputs (parameters) to
the function, and those features which are part of the environment (called
global variables or "globals") around the function.

The best explanation of global variables I can think of would be the analogy
of a nearby stream.  The factory can decide to use the water from the
stream, or dump pollution into the stream, and can thereby make the
processing of inputs faster, more efficient, cheaper etc. The trade-off is
that other users of those global resources are affected by the changes to
the resources (which means you again have to think about the internal
workings of the factory when you're considering your overall plan for your
system). In some cases, the changes are beneficial, but care must be taken
when using them.

Hope this helps,
Mike


-----Original Message-----
From: skeetor [mailto:skeetornospam at bellsouth.net]
Sent: Tuesday, August 29, 2000 1:51 AM
To: python-list at python.org
Subject: EXTREME NOOBIE


Whats this i hear about a function?  I just downloaded  this Python thing,
and i am going through the online tutorials when I get to the "defining of a
function..."  The tutorial is real basic until that part (thats where it
starts assuming you know how to program, Doh!)

Just wondering  if some kind person can clearly and adequately explain this
defining of a function thing...


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