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+<title>Chapter 5. Functions </title>
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+<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="chapter" id="ch5func">
+<div class="titlepage"></div>
+<div class="toc">
+<p><b>Table of Contents</b></p>
+<dl>
+<dt><span class="article"><a href="#id2436407">Functional Approach</a></span></dt>
+<dd><dl>
+<dt><span class="section"><a href="#id2487445">1. Function scope</a></span></dt>
+<dt><span class="section"><a href="#id2487503">2. Default Arguments</a></span></dt>
+<dt><span class="section"><a href="#id2487542">3. Keyword Arguments</a></span></dt>
+<dt><span class="section"><a href="#id2487658">4. Parameter Packing and Unpacking</a></span></dt>
+<dt><span class="section"><a href="#id2487758">5. Nested Functions and Scopes</a></span></dt>
+<dt><span class="section"><a href="#id2487811">6. map, reduce and filter functions</a></span></dt>
+<dd><dl><dt><span class="section"><a href="#id2488001">6.1. List Comprehensions</a></span></dt></dl></dd>
+</dl></dd>
+</dl>
+</div>
+<div class="article" title="Functional Approach">
+<div class="titlepage">
+<div><div><h2 class="title">
+<a name="id2436407"></a>Functional Approach</h2></div></div>
+<hr>
+</div>
+<div class="toc">
+<p><b>Table of Contents</b></p>
+<dl>
+<dt><span class="section"><a href="#id2487445">1. Function scope</a></span></dt>
+<dt><span class="section"><a href="#id2487503">2. Default Arguments</a></span></dt>
+<dt><span class="section"><a href="#id2487542">3. Keyword Arguments</a></span></dt>
+<dt><span class="section"><a href="#id2487658">4. Parameter Packing and Unpacking</a></span></dt>
+<dt><span class="section"><a href="#id2487758">5. Nested Functions and Scopes</a></span></dt>
+<dt><span class="section"><a href="#id2487811">6. map, reduce and filter functions</a></span></dt>
+<dd><dl><dt><span class="section"><a href="#id2488001">6.1. List Comprehensions</a></span></dt></dl></dd>
+</dl>
+</div>
+<p id="ch5func_1"></a><span class="emphasis"><em>Functions</em></span> allow us to enclose a set of statements and call the function again
+and again instead of repeating the group of statements everytime. Functions also
+allow us to isolate a piece of code from all the other code and provides the
+convenience of not polluting the global variables.</p>
+<p id="ch5func_2"></a><span class="emphasis"><em>Function</em></span> in python is defined with the keyword <span class="strong"><strong>def</strong></span> followed by the name
+of the function, in turn followed by a pair of parenthesis which encloses the
+list of parameters to the function. The definition line ends with a ':'. The
+definition line is followed by the body of the function intended by one block.
+The <span class="emphasis"><em>Function</em></span> must return a value:</p>
+<pre class="programlisting"> def factorial(n):
+ fact = 1
+ for i in range(2, n):
+ fact *= i
+
+ return fact</pre>
+<p id="ch5func_3"></a>The code snippet above defines a function with the name factorial, takes the
+number for which the factorial must be computed, computes the factorial and
+returns the value.</p>
+<p id="ch5func_4"></a>A <span class="emphasis"><em>Function</em></span> once defined can be used or called anywhere else in the program. We
+call a fucntion with its name followed by a pair of parenthesis which encloses
+the arguments to the function.</p>
+<p id="ch5func_5"></a>The value that function returns can be assigned to a variable. Let's call the
+above function and store the factorial in a variable:</p>
+<pre class="programlisting"> fact5 = factorial(5)</pre>
+<p id="ch5func_6"></a>The value of fact5 will now be 120, which is the factorial of 5. Note that we
+passed 5 as the argument to the function.</p>
+<p id="ch5func_7"></a>It may be necessary to document what the function does, for each of the function
+to help the person who reads our code to understand it better. In order to do
+this Python allows the first line of the function body to be a string. This
+string is called as <span class="emphasis"><em>Documentation String</em></span> or <span class="emphasis"><em>docstring</em></span>. <span class="emphasis"><em>docstrings</em></span> prove
+to be very handy since there are number of tools which can pull out all the
+docstrings from Python functions and generate the documentation automatically
+from it. <span class="emphasis"><em>docstrings</em></span> for functions can be written as follows:</p>
+<pre class="programlisting"> def factorial(n):
+ 'Returns the factorial for the number n.'
+ fact = 1
+ for i in range(2, n):
+ fact *= i
+
+ return fact</pre>
+<p id="ch5func_8"></a>An important point to note at this point is that, a function can return any
+Python value or a Python object, which also includes a <span class="emphasis"><em>Tuple</em></span>. A <span class="emphasis"><em>Tuple</em></span> is
+just a collection of values and those values themselves can be of any other
+valid Python datatypes, including <span class="emphasis"><em>Lists</em></span>, <span class="emphasis"><em>Tuples</em></span>, <span class="emphasis"><em>Dictionaries</em></span> among other
+things. So effectively, if a function can return a tuple, it can return any
+number of values through a tuple</p>
+<p id="ch5func_9"></a>Let us write a small function to swap two values:</p>
+<pre class="programlisting"> def swap(a, b):
+ return b, a
+
+c, d = swap(a, b)</pre>
+<div class="section" title="1. Function scope">
+<div class="titlepage"><div><div><h2 class="title" style="clear: both">
+<a name="id2487445"></a>1. Function scope</h2></div></div></div>
+<p id="ch5func_a"></a>The variables used inside the function are confined to the function's scope
+and doesn't pollute the variables of the same name outside the scope of the
+function. Also the arguments passed to the function are passed by-value if
+it is of basic Python data type:</p>
+<pre class="programlisting"> def cant_change(n):
+ n = 10
+
+n = 5
+cant_change(n)</pre>
+<p id="ch5func_b"></a>Upon running this code, what do you think would have happened to value of n
+which was assigned 5 before the function call? If you have already tried out
+that snippet on the interpreter you already know that the value of n is not
+changed. This is true of any immutable types of Python like <span class="emphasis"><em>Numbers</em></span>, <span class="emphasis"><em>Strings</em></span>
+and <span class="emphasis"><em>Tuples</em></span>. But when you pass mutable objects like <span class="emphasis"><em>Lists</em></span> and <span class="emphasis"><em>Dictionaries</em></span>
+the values are manipulated even outside the function:</p>
+<pre class="programlisting"> >>> def can_change(n):
+... n[1] = James
+...
+
+>>> name = ['Mr.', 'Steve', 'Gosling']
+>>> can_change(name)
+>>> name
+['Mr.', 'James', 'Gosling']</pre>
+<p id="ch5func_c"></a>If nothing is returned by the function explicitly, Python takes care to return
+None when the funnction is called.</p>
+</div>
+<div class="section" title="2. Default Arguments">
+<div class="titlepage"><div><div><h2 class="title" style="clear: both">
+<a name="id2487503"></a>2. Default Arguments</h2></div></div></div>
+<p id="ch5func_d"></a>There may be situations where we need to allow the functions to take the
+arguments optionally. Python allows us to define function this way by providing
+a facility called <span class="emphasis"><em>Default Arguments</em></span>. For example, we need to write a function
+that returns a list of fibonacci numbers. Since our function cannot generate an
+infinite list of fibonacci numbers, we need to specify the number of elements
+that the fibonacci sequence must contain. Suppose, additionally, we want to the
+function to return 10 numbers in the sequence if no option is specified we can
+define the function as follows:</p>
+<pre class="programlisting"> def fib(n=10):
+ fib_list = [0, 1]
+ for i in range(n - 2):
+ next = fib_list[-2] + fib_list[-1]
+ fib_list.append(next)
+ return fib_list</pre>
+<p id="ch5func_e"></a>When we call this function, we can optionally specify the value for the
+parameter n, during the call as an argument. Calling with no argument and
+argument with n=5 returns the following fibonacci sequences:</p>
+<pre class="programlisting"> fib()
+[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
+fib(5)
+[0, 1, 1, 2, 3]</pre>
+</div>
+<div class="section" title="3. Keyword Arguments">
+<div class="titlepage"><div><div><h2 class="title" style="clear: both">
+<a name="id2487542"></a>3. Keyword Arguments</h2></div></div></div>
+<p id="ch5func_f"></a>When a function takes a large number of arguments, it may be difficult to
+remember the order of the parameters in the function definition or it may
+be necessary to pass values to only certain parameters since others take
+the default value. In either of these cases, Python provides the facility
+of passing arguments by specifying the name of the parameter as defined in
+the function definition. This is known as <span class="emphasis"><em>Keyword Arguments</em></span>.</p>
+<p id="ch5func_10"></a>In a function call, <span class="emphasis"><em>Keyword arguments</em></span> can be used for each argument, in the
+following fashion:</p>
+<pre class="programlisting"> argument_name=argument_value
+Also denoted as: keyword=argument
+
+def wish(name='World', greetings='Hello'):
+ print "%s, %s!" % (greetings, name)</pre>
+<p id="ch5func_11"></a>This function can be called in one of the following ways. It is important to
+note that no restriction is imposed in the order in which <span class="emphasis"><em>Keyword arguments</em></span>
+can be specified. Also note, that we have combined <span class="emphasis"><em>Keyword arguments</em></span> with
+<span class="emphasis"><em>Default arguments</em></span> in this example, however it is not necessary:</p>
+<pre class="programlisting"> wish(name='Guido', greetings='Hey')
+wish(greetings='Hey', name='Guido')</pre>
+<p id="ch5func_12"></a>Calling functions by specifying arguments in the order of parameters specified
+in the function definition is called as <span class="emphasis"><em>Positional arguments</em></span>, as opposed to
+<span class="emphasis"><em>Keyword arguments</em></span>. It is possible to use both <span class="emphasis"><em>Positional arguments</em></span> and
+<span class="emphasis"><em>Keyword arguments</em></span> in a single function call. But Python doesn't allow us to
+bungle up both of them. The arguments to the function, in the call, must always
+start with <span class="emphasis"><em>Positional arguments</em></span> which is in turn followed by <span class="emphasis"><em>Keyword
+arguments</em></span>:</p>
+<pre class="programlisting"> def my_func(x, y, z, u, v, w):
+ # initialize variables.
+ ...
+ # do some stuff
+ ...
+ # return the value</pre>
+<p id="ch5func_13"></a>It is valid to call the above functions in the following ways:</p>
+<pre class="programlisting"> my_func(10, 20, 30, u=1.0, v=2.0, w=3.0)
+my_func(10, 20, 30, 1.0, 2.0, w=3.0)
+my_func(10, 20, z=30, u=1.0, v=2.0, w=3.0)
+my_func(x=10, y=20, z=30, u=1.0, v=2.0, w=3.0)</pre>
+<p id="ch5func_14"></a>Following lists some of the invalid calls:</p>
+<pre class="programlisting"> my_func(10, 20, z=30, 1.0, 2.0, 3.0)
+my_func(x=10, 20, z=30, 1.0, 2.0, 3.0)
+my_func(x=10, y=20, z=30, u=1.0, v=2.0, 3.0)</pre>
+</div>
+<div class="section" title="4. Parameter Packing and Unpacking">
+<div class="titlepage"><div><div><h2 class="title" style="clear: both">
+<a name="id2487658"></a>4. Parameter Packing and Unpacking</h2></div></div></div>
+<p id="ch5func_15"></a>The positional arguments passed to a function can be collected in a tuple
+parameter and keyword arguments can be collected in a dictionary. Since keyword
+arguments must always be the last set of arguments passed to a function, the
+keyword dictionary parameter must be the last parameter. The function definition
+must include a list explicit parameters, followed by tuple paramter collecting
+parameter, whose name is preceded by a <span class="strong"><strong>*</strong></span>, for collecting positional
+parameters, in turn followed by the dictionary collecting parameter, whose name
+is preceded by a <span class="strong"><strong>**</strong></span></p>
+<pre class="programlisting"> def print_report(title, *args, **name):
+ """Structure of *args*
+ (age, email-id)
+ Structure of *name*
+ {
+ 'first': First Name
+ 'middle': Middle Name
+ 'last': Last Name
+ }
+ """
+
+ print "Title: %s" % (title)
+ print "Full name: %(first)s %(middle)s %(last)s" % name
+ print "Age: %d\nEmail-ID: %s" % args</pre>
+<p id="ch5func_16"></a>The above function can be called as. Note, the order of keyword parameters can
+be interchanged:</p>
+<pre class="programlisting"> >>> print_report('Employee Report', 29, 'johny@example.com', first='Johny',
+ last='Charles', middle='Douglas')
+Title: Employee Report
+Full name: Johny Douglas Charles
+Age: 29
+Email-ID: johny@example.com</pre>
+<p id="ch5func_17"></a>The reverse of this can also be achieved by using a very identical syntax while
+calling the function. A tuple or a dictionary can be passed as arguments in
+place of a list of <span class="emphasis"><em>Positional arguments</em></span> or <span class="emphasis"><em>Keyword arguments</em></span> respectively
+using <span class="strong"><strong>*</strong></span> or <span class="strong"><strong>**</strong></span></p>
+<pre class="programlisting"> def print_report(title, age, email, first, middle, last):
+ print "Title: %s" % (title)
+ print "Full name: %s %s %s" % (first, middle, last)
+ print "Age: %d\nEmail-ID: %s" % (age, email)
+
+>>> args = (29, 'johny@example.com')
+>>> name = {
+ 'first': 'Johny',
+ 'middle': 'Charles',
+ 'last': 'Douglas'
+ }
+>>> print_report('Employee Report', *args, **name)
+Title: Employee Report
+Full name: Johny Charles Douglas
+Age: 29
+Email-ID: johny@example.com</pre>
+</div>
+<div class="section" title="5. Nested Functions and Scopes">
+<div class="titlepage"><div><div><h2 class="title" style="clear: both">
+<a name="id2487758"></a>5. Nested Functions and Scopes</h2></div></div></div>
+<p id="ch5func_18"></a>Python allows nesting one function inside another. This style of programming
+turns out to be extremely flexible and powerful features when we use <span class="emphasis"><em>Python
+decorators</em></span>. We will not talk about decorators is beyond the scope of this
+course. If you are interested in knowing more about <span class="emphasis"><em>decorator programming</em></span> in
+Python you are suggested to read:</p>
+<span style="color: black"><line_block><span style="color: black"><line><div class="reference">
+<div class="titlepage"><hr></div>http://avinashv.net/2008/04/python-decorators-syntactic-sugar/</div></line></span><span style="color: black"><line><div class="reference">
+<div class="titlepage"><hr></div>http://personalpages.tds.net/~kent37/kk/00001.html</div></line></span></line_block></span><p id="ch5func_19"></a>However, the following is an example for nested functions in Python:</p>
+<pre class="programlisting"> def outer():
+ print "Outer..."
+ def inner():
+ print "Inner..."
+ print "Outer..."
+ inner()
+
+>>> outer()</pre>
+</div>
+<div class="section" title="6. map, reduce and filter functions">
+<div class="titlepage"><div><div><h2 class="title" style="clear: both">
+<a name="id2487811"></a>6. map, reduce and filter functions</h2></div></div></div>
+<p id="ch5func_1a"></a>Python provides several built-in functions for convenience. The <span class="strong"><strong>map()</strong></span>,
+<span class="strong"><strong>reduce()</strong></span> and <span class="strong"><strong>filter()</strong></span> functions prove to be very useful with sequences like
+<span class="emphasis"><em>Lists</em></span>.</p>
+<p id="ch5func_1b"></a>The <span class="strong"><strong>map</strong></span> (<span class="emphasis"><em>function</em></span>, <span class="emphasis"><em>sequence</em></span>) function takes two arguments: <span class="emphasis"><em>function</em></span>
+and a <span class="emphasis"><em>sequence</em></span> argument. The <span class="emphasis"><em>function</em></span> argument must be the name of the
+function which in turn takes a single argument, the individual element of the
+<span class="emphasis"><em>sequence</em></span>. The <span class="strong"><strong>map</strong></span> function calls <span class="emphasis"><em>function(item)</em></span>, for each item in the
+sequence and returns a list of values, where each value is the value returned
+by each call to <span class="emphasis"><em>function(item)</em></span>. <span class="strong"><strong>map()</strong></span> function allows to pass more than
+one sequence. In this case, the first argument, <span class="emphasis"><em>function</em></span> must take as many
+arguments as the number of sequences passed. This function is called with each
+corresponding element in the each of the sequences, or <span class="strong"><strong>None</strong></span> if one of the
+sequence is exhausted:</p>
+<pre class="programlisting"> def square(x):
+ return x*x
+
+>>> map(square, [1, 2, 3, 4])
+[1, 4, 9, 16]
+
+def mul(x, y):
+ return x*y
+
+>>> map(mul, [1, 2, 3, 4], [6, 7, 8, 9])</pre>
+<p id="ch5func_1c"></a>The <span class="strong"><strong>filter</strong></span> (<span class="emphasis"><em>function</em></span>, <span class="emphasis"><em>sequence</em></span>) function takes two arguments, similar to
+the <span class="strong"><strong>map()</strong></span> function. The <span class="strong"><strong>filter</strong></span> function calls <span class="emphasis"><em>function(item)</em></span>, for each
+item in the sequence and returns all the elements in the sequence for which
+<span class="emphasis"><em>function(item)</em></span> returned True:</p>
+<pre class="programlisting"> def even(x):
+ if x % 2:
+ return True
+ else:
+ return False
+
+>>> filter(even, range(1, 10))
+[1, 3, 5, 7, 9]</pre>
+<p id="ch5func_1d"></a>The <span class="strong"><strong>reduce</strong></span> (<span class="emphasis"><em>function</em></span>, <span class="emphasis"><em>sequence</em></span>) function takes two arguments, similar to
+<span class="strong"><strong>map</strong></span> function, however multiple sequences are not allowed. The <span class="strong"><strong>reduce</strong></span>
+function calls <span class="emphasis"><em>function</em></span> with first two consecutive elements in the sequence,
+obtains the result, calls <span class="emphasis"><em>function</em></span> with the result and the subsequent element
+in the sequence and so on until the end of the list and returns the final result:</p>
+<pre class="programlisting"> def mul(x, y):
+ return x*y
+
+>>> reduce(mul, [1, 2, 3, 4])
+24</pre>
+<div class="section" title="6.1. List Comprehensions">
+<div class="titlepage"><div><div><h3 class="title">
+<a name="id2488001"></a>6.1. List Comprehensions</h3></div></div></div>
+<p id="ch5func_1e"></a>List Comprehension is a convenvience utility provided by Python. It is a
+syntatic sugar to create <span class="emphasis"><em>Lists</em></span>. Using <span class="emphasis"><em>List Comprehensions</em></span> one can create
+<span class="emphasis"><em>Lists</em></span> from other type of sequential data structures or other <span class="emphasis"><em>Lists</em></span> itself.
+The syntax of <span class="emphasis"><em>List Comprehensions</em></span> consists of a square brackets to indicate
+the result is a <span class="emphasis"><em>List</em></span> within which we include at least one <span class="strong"><strong>for</strong></span> clause and
+multiple <span class="strong"><strong>if</strong></span> clauses. It will be more clear with an example:</p>
+<pre class="programlisting"> >>> num = [1, 2, 3]
+>>> sq = [x*x for x in num]
+>>> sq
+[1, 4, 9]
+>>> all_num = [1, 2, 3, 4, 5, 6, 7, 8, 9]
+>>> even = [x for x in all_num if x%2 == 0]</pre>
+<p id="ch5func_1f"></a>The syntax used here is very clear from the way it is written. It can be
+translated into english as, "for each element x in the list all_num,
+if remainder of x divided by 2 is 0, add x to the list."</p>
+</div>
+</div>
+</div>
+</div></body>
+</html>