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- Welcome to Lesson Four, Functions.

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Up to this point we've used
a few built-in functions

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from the Python Standard Library,

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and here we're going to start talking

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about all the different syntax issues

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for Creating your own
custom functions in Python,

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which of course is a key aspect

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of Procedural Programming,

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and then separately when
we eventually do get

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to Object Oriented
Programming in Part Three,

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we'll create functions in
the context of classes,

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which we would then called Methods,

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and the same syntax issues
apply there as well.

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Now another key aspect of this lesson

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is that we're going to start

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to Import capabilities from other aspects

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of the Python Standard Library.

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So we'll learn the Import Statement

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for the first time,

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and that's going to be similar

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to what you do in other C-based languages.

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So for example, in C and C++,

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we use the Pound Include Directive,

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in Java we use the Import Keyword,

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and in C sharp we use Using

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in the same context.

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We'll also demonstrate the concepts

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of Passing data between
functions in Python,

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and because everything
is an object in Python,

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all you ever pass into a function

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is a copy of a reference.

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So in effect,

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every object in Python is passed

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into a function by reference.

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And we'll talk about that and demonstrate

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that to you as well.

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We'll also step into some
Simulation techniques

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as part of this lesson,

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introducing Random number generation

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for the first time.

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We'll talk as part of that discussion

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about Tuples for the first time.

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Tuples are another key
data structure in Python

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for aggregating data,

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and one of the key uses of a tuple

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is to be able to return

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more than one value at a time

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from a function.

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So by definition,

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functions in Python return
one value at a time,

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but that one value could be an object

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and that object could
contain multiple things,

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and that's what tuples
are commonly used for.

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Amongst other things in Python.

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We'll talk about how to create tuples,

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which is called Packing a tuple.

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We'll also talk about
how to extract the data

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in a tuple,

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which is called Unpacking the tuple.

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And we'll take a look at Scope

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in the context of Python functions,

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which is actually somewhat different

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from how Scope works

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in some of the other programming languages

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that you're probably familiar with.

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Some of the other topics

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we'll look at in this lesson,

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we'll show you how to do
Default parameter values,

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which are frequently used in Python.

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We'll also take a look at what are known

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as Keyword arguments,

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which enable you to call a
Python function or method

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with the parameters named,

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and that also means that
you can change the order

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of the parameters

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and the function will
still get called correctly

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if you have named each of the arguments.

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We'll show you how to define

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a Variable-length argument list,

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and we'll finish up

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with a discussion of Methods

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by demonstrating how to call functions

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on objects of the string class.

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So as I've mentioned previously

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in the introduction,

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it's very commonly the case in Python

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that you will create
objects of existing types

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and simply interact with those objects,

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and the primary way

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that you'll do that is
by calling their methods.