1 * Functions

     2 *** Outline

     3 ***** Functions

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

     5 ********* def

     6 ********* name

     7 ********* arguments

     8 ********* indented block

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

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

    11 ******* return values

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

    13 ******* code reading exercises?

    14 ******* default arguments

    15 ******* keyword arguments

    16 ******* availability library functions

    17 *** Script

    18     Welcome friends. 

    19 

    20     In this tutorial we shall be looking at Functions in Python. We already

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

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

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

    24     arguments. 

    25     

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

    27 

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

    29     accepting a name. 

    30 

    31         def welcome(name):

    32 	    print "Hello", name 

    33 

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

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

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

    37     defined within an indented block, just like to any other block. Our

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

    39     

    40     We can call our function, as follows -

    41         welcome("World")

    42 

    43     (all arguments are local to a function)

    44 

    45     In general functions should be accompanied by documentation on how

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

    47     function itself, using what are called doc strings. They were mentioned in the

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

    49 

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

    51 

    52         def welcome(name):

    53 	    """ Prints a hello message given a name, 

    54 	        passed as argument. """

    55 	    print "Hello", name 

    56     

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

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

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

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

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

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

    63     will display the docstring that we have just written.

    64 

    65     We shall now look at default arguments. 

    66     [show slide with examples of functions with default arguments]

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

    68     given example - one for splitting on spaces and the other for

    69     splitting on commas.

    70 

    71     The function split is being called with no arguments and one

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

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

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

    75 

    76         def welcome(name="World!"):

    77 	    print "Hello", name 

    78     

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

    80     to it. 

    81         welcome()

    82 

    83     As you can see the output is "Hello World!". Here "World!" is used as a

    84     default argument, when no name argument is passed to 'welcome'. 

    85 

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

    87     [show slide with examples of functions with keyword arguments]

    88     We have already looked at functions and keyword arguments in these

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

    90 

    91     Let's now edit our function so that it displays a custom 

    92     greeting message as well. 

    93 

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

    95         print greet, name

    96 

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

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

    99         welcome("Hello", "James")

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

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

   102 

   103     Keyword arguments allow us to call functions by passing arguments

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

   105     in the function definition. 

   106 

   107     Let's now write a new function 

   108 

   109     def per_square(n):

   110         i = 1

   111 	while ( i*i < n ):

   112 	    i += 1

   113 	return i*i == n, i

   114 

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

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

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

   118     False and the square root of the next perfect square.

   119 

   120     Please observe that this function returns 2 values.

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

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

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

   124 

   125     With this we come to the end of this tutorial on functions. In this tutorial

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

   127     how to define functions, calling them, default and keyword

   128     arguments. 

   129 

   130 *** Notes