* 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 in Python. We already

    have looked at the basics of functions in the tutorial on solving

    equations. We shall first review these basics. Then we shall move on to

    other details such as doc-strings, default arguments and keyword

    arguments. 

    First let's start IPython by typing ipython in the terminal.

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

    accepting a name. 

        def welcome(name):

	    print "Hello", name 

    You would recall that def is a keyword that indicates the 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, just like to any other block. Our

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

    We can call our function, as follows -

        welcome("World")

    (all arguments are local to a function)

    In general functions should be accompanied by documentation on how

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

    function itself, using what are called 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 given a name, 

	        passed as argument. """

	    print "Hello", name 

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

    exceeds one line, we can use new line characters in it. 

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

    for anything within a block. Now that we have written the

    documentation, how do we access it? IPython provides the question

    mark feature that we have seen in the previous tutorials. welcome?

    will display the docstring that we have just written.

    We shall now look at default arguments. 

    [show slide with examples of functions with default arguments]

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

    given example - 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 sake, 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()

    As you can see the output is "Hello World!". Here "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 edit our function so that it displays a custom 

    greeting message as well. 

    def welcome( greet = 'Hello', name = 'World!'):

        print greet, name

    Let's now see, how we can call our updated 'welcome' function, 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 the need to remember the order of arguments

    in the function definition. 

    Let's now write a new function 

    def per_square(n):

        i = 1

	while ( i*i < n ):

	    i += 1

	return i*i == n, i

    What does this function do? It checks if the given number is a perfect square.

    If it is, then the function returns True along with the square root of

    the given number. If the number is not a perfect square it returns

    False and the square root of the next perfect square.

    Please observe that this function returns 2 values.

    In Python there is no restriction on the number of values returned by

    a function. Whenever a function has to return more than one value, the multiple

    values are packed into one single tuple and that single tuple is returned.

    With this 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