* Functions

*** Outline

***** Functions

******* review of what's been done in solving equations tutorial

********* def

********* name

********* arguments

********* indented block

********* calling a function

******* arguments are local to a function

******* return values

******* doc strings - with example.

******* code reading exercises?

******* default arguments

******* keyword arguments

******* availability library functions

*** Script

    Welcome friends. 

    In this tutorial we shall be looking at Functions. We already have

    looked at the basics of functions in the tutorial on solving

    equations. We shall first review these basics. Next we shall look

    at other details like doc-strings, default arguments and keyword

    arguments. 

    Let's write a simple function that prints a Hello message, after

    accepting a name. 

        def welcome(name):

	    print "Hello", name 

    You would recall that def is a keyword that indicates a function

    definition. 'welcome' is the name of the function and 'name' is

    the lone argument to the function. Note that the function is

    defined within an indented block, similar to any other block. Our

    function welcome just has one line in it's definition.  

    We can call our function by passing, as follows -

        welcome("World")

    (arguments are local to a function)

    In general functions should be accompanied by documentation on how

    to use it. Python provides a convenient way of writing within the

    function itself, using doc strings. They were mentioned in the

    tutorial on strings. Let's look at how to write them here. 

    Let us add a simple doc string to our welcome function. 

        def welcome(name):

	    """ Prints a hello message to a person, given a name. """

	    print "Hello", name 

    Notice that the doc string uses triple quotes. If the doc-string

    exceeded one line, we could have used new line characters in

    it. Also, as expected the doc-string is indented as is required

    for anything within a block. 

    We shall now look at default arguments. 

    [show slide with examples of functions with default arguments]

    The strip function has been used in two different ways in the

    previous tutorials - one for splitting on spaces and the other for

    splitting on commas. 

    The function split is being called with no arguments and one

    argument, respectively. In the first case, white space is being

    used as a default value. Let's now edit our function, welcome, to

    use default values. (For convenience, we have dropped the doc-string)

        def welcome(name="World!"):

	    print "Hello", name 

    Now, we call the function 'welcome' without passing any arguments

    to it. 

        welcome()

    "World!" is used as a default argument, when no name argument is

    passed to 'welcome'. 

    Let's now look at the use of keyword arguments. 

    [show slide with examples of functions with keyword arguments]

    We have already looked at functions and keyword arguments in these

    examples. loc, linewidth, xy, labels are all keywords. 

    Let's now see, how we can call our function 'welcome', using

    keyword arguments. We can call the function in a variety of ways.

        welcome("Hello", "James")

	welcome("Hi", name="Guido")

	welcome(name="Guido", greet="Hello")

    Keyword arguments allow us to call functions by passing arguments

    in any order and removes need to remember the order of arguments

    in the function definition. 

***** return values

***** arguments are local to a function

***** availability library functions

***** code reading exercises?

    We come to the end of this tutorial on functions. In this tutorial

    we have learnt about functions in a greater detail. We looked at

    how to define functions, calling them, default and keyword

    arguments. 

*** Notes