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8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	4	<title>Chapter 5. Functions </title>
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8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	13	<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="chapter" id="ch5func">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	14	<div class="titlepage"></div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	15	<div class="toc">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	16	<p><b>Table of Contents</b></p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	17	<dl>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	18	<dt><span class="article"><a href="#id2436407">Functional Approach</a></span></dt>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	19	<dd><dl>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	20	<dt><span class="section"><a href="#id2487445">1. Function scope</a></span></dt>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	21	<dt><span class="section"><a href="#id2487503">2. Default Arguments</a></span></dt>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	22	<dt><span class="section"><a href="#id2487542">3. Keyword Arguments</a></span></dt>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	23	<dt><span class="section"><a href="#id2487658">4. Parameter Packing and Unpacking</a></span></dt>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	24	<dt><span class="section"><a href="#id2487758">5. Nested Functions and Scopes</a></span></dt>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	25	<dt><span class="section"><a href="#id2487811">6. map, reduce and filter functions</a></span></dt>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	26	<dd><dl><dt><span class="section"><a href="#id2488001">6.1. List Comprehensions</a></span></dt></dl></dd>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	27	</dl></dd>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	28	</dl>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	29	</div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	30	<div class="article" title="Functional Approach">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	31	<div class="titlepage">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	32	<div><div><h2 class="title">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	33	<a name="id2436407"></a>Functional Approach</h2></div></div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	34	<hr>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	35	</div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	36	<div class="toc">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	37	<p><b>Table of Contents</b></p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	38	<dl>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	39	<dt><span class="section"><a href="#id2487445">1. Function scope</a></span></dt>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	40	<dt><span class="section"><a href="#id2487503">2. Default Arguments</a></span></dt>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	41	<dt><span class="section"><a href="#id2487542">3. Keyword Arguments</a></span></dt>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	42	<dt><span class="section"><a href="#id2487658">4. Parameter Packing and Unpacking</a></span></dt>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	43	<dt><span class="section"><a href="#id2487758">5. Nested Functions and Scopes</a></span></dt>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	44	<dt><span class="section"><a href="#id2487811">6. map, reduce and filter functions</a></span></dt>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	45	<dd><dl><dt><span class="section"><a href="#id2488001">6.1. List Comprehensions</a></span></dt></dl></dd>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	46	</dl>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	47	</div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	48	<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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	49	and again instead of repeating the group of statements everytime. Functions also
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	50	allow us to isolate a piece of code from all the other code and provides the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	51	convenience of not polluting the global variables.</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	52	<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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	53	of the function, in turn followed by a pair of parenthesis which encloses the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	54	list of parameters to the function. The definition line ends with a ':'. The
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	55	definition line is followed by the body of the function intended by one block.
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	56	The <span class="emphasis"><em>Function</em></span> must return a value:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	57	<pre class="programlisting"> def factorial(n):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	58	fact = 1
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	59	for i in range(2, n):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	60	fact *= i
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	61
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	62	return fact</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	63	<p id="ch5func_3"></a>The code snippet above defines a function with the name factorial, takes the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	64	number for which the factorial must be computed, computes the factorial and
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	65	returns the value.</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	66	<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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	67	call a fucntion with its name followed by a pair of parenthesis which encloses
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	68	the arguments to the function.</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	69	<p id="ch5func_5"></a>The value that function returns can be assigned to a variable. Let's call the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	70	above function and store the factorial in a variable:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	71	<pre class="programlisting"> fact5 = factorial(5)</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	72	<p id="ch5func_6"></a>The value of fact5 will now be 120, which is the factorial of 5. Note that we
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	73	passed 5 as the argument to the function.</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	74	<p id="ch5func_7"></a>It may be necessary to document what the function does, for each of the function
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	75	to help the person who reads our code to understand it better. In order to do
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	76	this Python allows the first line of the function body to be a string. This
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	77	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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	78	to be very handy since there are number of tools which can pull out all the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	79	docstrings from Python functions and generate the documentation automatically
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	80	from it. <span class="emphasis"><em>docstrings</em></span> for functions can be written as follows:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	81	<pre class="programlisting"> def factorial(n):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	82	'Returns the factorial for the number n.'
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	83	fact = 1
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	84	for i in range(2, n):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	85	fact *= i
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	86
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	87	return fact</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	88	<p id="ch5func_8"></a>An important point to note at this point is that, a function can return any
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	89	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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	90	just a collection of values and those values themselves can be of any other
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	91	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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	92	things. So effectively, if a function can return a tuple, it can return any
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	93	number of values through a tuple</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	94	<p id="ch5func_9"></a>Let us write a small function to swap two values:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	95	<pre class="programlisting"> def swap(a, b):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	96	return b, a
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	97
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	98	c, d = swap(a, b)</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	99	<div class="section" title="1. Function scope">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	100	<div class="titlepage"><div><div><h2 class="title" style="clear: both">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	101	<a name="id2487445"></a>1. Function scope</h2></div></div></div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	102	<p id="ch5func_a"></a>The variables used inside the function are confined to the function's scope
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	103	and doesn't pollute the variables of the same name outside the scope of the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	104	function. Also the arguments passed to the function are passed by-value if
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	105	it is of basic Python data type:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	106	<pre class="programlisting"> def cant_change(n):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	107	n = 10
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	108
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	109	n = 5
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	110	cant_change(n)</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	111	<p id="ch5func_b"></a>Upon running this code, what do you think would have happened to value of n
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	112	which was assigned 5 before the function call? If you have already tried out
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	113	that snippet on the interpreter you already know that the value of n is not
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	114	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>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	115	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>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	116	the values are manipulated even outside the function:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	117	<pre class="programlisting"> >>> def can_change(n):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	118	... n[1] = James
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	119	...
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	120
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	121	>>> name = ['Mr.', 'Steve', 'Gosling']
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	122	>>> can_change(name)
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	123	>>> name
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	124	['Mr.', 'James', 'Gosling']</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	125	<p id="ch5func_c"></a>If nothing is returned by the function explicitly, Python takes care to return
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	126	None when the funnction is called.</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	127	</div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	128	<div class="section" title="2. Default Arguments">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	129	<div class="titlepage"><div><div><h2 class="title" style="clear: both">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	130	<a name="id2487503"></a>2. Default Arguments</h2></div></div></div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	131	<p id="ch5func_d"></a>There may be situations where we need to allow the functions to take the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	132	arguments optionally. Python allows us to define function this way by providing
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	133	a facility called <span class="emphasis"><em>Default Arguments</em></span>. For example, we need to write a function
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	134	that returns a list of fibonacci numbers. Since our function cannot generate an
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	135	infinite list of fibonacci numbers, we need to specify the number of elements
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	136	that the fibonacci sequence must contain. Suppose, additionally, we want to the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	137	function to return 10 numbers in the sequence if no option is specified we can
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	138	define the function as follows:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	139	<pre class="programlisting"> def fib(n=10):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	140	fib_list = [0, 1]
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	141	for i in range(n - 2):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	142	next = fib_list[-2] + fib_list[-1]
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	143	fib_list.append(next)
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	144	return fib_list</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	145	<p id="ch5func_e"></a>When we call this function, we can optionally specify the value for the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	146	parameter n, during the call as an argument. Calling with no argument and
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	147	argument with n=5 returns the following fibonacci sequences:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	148	<pre class="programlisting"> fib()
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	149	[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	150	fib(5)
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	151	[0, 1, 1, 2, 3]</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	152	</div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	153	<div class="section" title="3. Keyword Arguments">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	154	<div class="titlepage"><div><div><h2 class="title" style="clear: both">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	155	<a name="id2487542"></a>3. Keyword Arguments</h2></div></div></div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	156	<p id="ch5func_f"></a>When a function takes a large number of arguments, it may be difficult to
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	157	remember the order of the parameters in the function definition or it may
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	158	be necessary to pass values to only certain parameters since others take
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	159	the default value. In either of these cases, Python provides the facility
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	160	of passing arguments by specifying the name of the parameter as defined in
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	161	the function definition. This is known as <span class="emphasis"><em>Keyword Arguments</em></span>.</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	162	<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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	163	following fashion:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	164	<pre class="programlisting"> argument_name=argument_value
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	165	Also denoted as: keyword=argument
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	166
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	167	def wish(name='World', greetings='Hello'):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	168	print "%s, %s!" % (greetings, name)</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	169	<p id="ch5func_11"></a>This function can be called in one of the following ways. It is important to
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	170	note that no restriction is imposed in the order in which <span class="emphasis"><em>Keyword arguments</em></span>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	171	can be specified. Also note, that we have combined <span class="emphasis"><em>Keyword arguments</em></span> with
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	172	<span class="emphasis"><em>Default arguments</em></span> in this example, however it is not necessary:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	173	<pre class="programlisting"> wish(name='Guido', greetings='Hey')
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	174	wish(greetings='Hey', name='Guido')</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	175	<p id="ch5func_12"></a>Calling functions by specifying arguments in the order of parameters specified
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	176	in the function definition is called as <span class="emphasis"><em>Positional arguments</em></span>, as opposed to
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	177	<span class="emphasis"><em>Keyword arguments</em></span>. It is possible to use both <span class="emphasis"><em>Positional arguments</em></span> and
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	178	<span class="emphasis"><em>Keyword arguments</em></span> in a single function call. But Python doesn't allow us to
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	179	bungle up both of them. The arguments to the function, in the call, must always
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	180	start with <span class="emphasis"><em>Positional arguments</em></span> which is in turn followed by <span class="emphasis"><em>Keyword
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	181	arguments</em></span>:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	182	<pre class="programlisting"> def my_func(x, y, z, u, v, w):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	183	# initialize variables.
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	184	...
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	185	# do some stuff
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	186	...
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	187	# return the value</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	188	<p id="ch5func_13"></a>It is valid to call the above functions in the following ways:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	189	<pre class="programlisting"> my_func(10, 20, 30, u=1.0, v=2.0, w=3.0)
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	190	my_func(10, 20, 30, 1.0, 2.0, w=3.0)
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	191	my_func(10, 20, z=30, u=1.0, v=2.0, w=3.0)
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	192	my_func(x=10, y=20, z=30, u=1.0, v=2.0, w=3.0)</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	193	<p id="ch5func_14"></a>Following lists some of the invalid calls:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	194	<pre class="programlisting"> my_func(10, 20, z=30, 1.0, 2.0, 3.0)
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	195	my_func(x=10, 20, z=30, 1.0, 2.0, 3.0)
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	196	my_func(x=10, y=20, z=30, u=1.0, v=2.0, 3.0)</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	197	</div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	198	<div class="section" title="4. Parameter Packing and Unpacking">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	199	<div class="titlepage"><div><div><h2 class="title" style="clear: both">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	200	<a name="id2487658"></a>4. Parameter Packing and Unpacking</h2></div></div></div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	201	<p id="ch5func_15"></a>The positional arguments passed to a function can be collected in a tuple
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	202	parameter and keyword arguments can be collected in a dictionary. Since keyword
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	203	arguments must always be the last set of arguments passed to a function, the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	204	keyword dictionary parameter must be the last parameter. The function definition
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	205	must include a list explicit parameters, followed by tuple paramter collecting
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	206	parameter, whose name is preceded by a <span class="strong"><strong>*</strong></span>, for collecting positional
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	207	parameters, in turn followed by the dictionary collecting parameter, whose name
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	208	is preceded by a <span class="strong"><strong>**</strong></span></p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	209	<pre class="programlisting"> def print_report(title, args, *name):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	210	"""Structure of args
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	211	(age, email-id)
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	212	Structure of name
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	213	{
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	214	'first': First Name
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	215	'middle': Middle Name
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	216	'last': Last Name
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	217	}
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	218	"""
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	219
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	220	print "Title: %s" % (title)
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	221	print "Full name: %(first)s %(middle)s %(last)s" % name
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	222	print "Age: %d\nEmail-ID: %s" % args</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	223	<p id="ch5func_16"></a>The above function can be called as. Note, the order of keyword parameters can
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	224	be interchanged:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	225	<pre class="programlisting"> >>> print_report('Employee Report', 29, 'johny@example.com', first='Johny',
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	226	last='Charles', middle='Douglas')
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	227	Title: Employee Report
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	228	Full name: Johny Douglas Charles
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	229	Age: 29
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	230	Email-ID: johny@example.com</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	231	<p id="ch5func_17"></a>The reverse of this can also be achieved by using a very identical syntax while
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	232	calling the function. A tuple or a dictionary can be passed as arguments in
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	233	place of a list of <span class="emphasis"><em>Positional arguments</em></span> or <span class="emphasis"><em>Keyword arguments</em></span> respectively
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	234	using <span class="strong"><strong></strong></span> or <span class="strong"><strong>*</strong></span></p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	235	<pre class="programlisting"> def print_report(title, age, email, first, middle, last):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	236	print "Title: %s" % (title)
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	237	print "Full name: %s %s %s" % (first, middle, last)
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	238	print "Age: %d\nEmail-ID: %s" % (age, email)
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	239
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	240	>>> args = (29, 'johny@example.com')
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	241	>>> name = {
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	242	'first': 'Johny',
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	243	'middle': 'Charles',
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	244	'last': 'Douglas'
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	245	}
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	246	>>> print_report('Employee Report', args, *name)
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	247	Title: Employee Report
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	248	Full name: Johny Charles Douglas
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	249	Age: 29
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	250	Email-ID: johny@example.com</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	251	</div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	252	<div class="section" title="5. Nested Functions and Scopes">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	253	<div class="titlepage"><div><div><h2 class="title" style="clear: both">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	254	<a name="id2487758"></a>5. Nested Functions and Scopes</h2></div></div></div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	255	<p id="ch5func_18"></a>Python allows nesting one function inside another. This style of programming
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	256	turns out to be extremely flexible and powerful features when we use <span class="emphasis"><em>Python
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	257	decorators</em></span>. We will not talk about decorators is beyond the scope of this
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	258	course. If you are interested in knowing more about <span class="emphasis"><em>decorator programming</em></span> in
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	259	Python you are suggested to read:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	260	<span style="color: black"><line_block><span style="color: black"><line><div class="reference">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	261	<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">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	262	<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>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	263	<pre class="programlisting"> def outer():
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	264	print "Outer..."
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	265	def inner():
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	266	print "Inner..."
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	267	print "Outer..."
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	268	inner()
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	269
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	270	>>> outer()</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	271	</div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	272	<div class="section" title="6. map, reduce and filter functions">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	273	<div class="titlepage"><div><div><h2 class="title" style="clear: both">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	274	<a name="id2487811"></a>6. map, reduce and filter functions</h2></div></div></div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	275	<p id="ch5func_1a"></a>Python provides several built-in functions for convenience. The <span class="strong"><strong>map()</strong></span>,
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	276	<span class="strong"><strong>reduce()</strong></span> and <span class="strong"><strong>filter()</strong></span> functions prove to be very useful with sequences like
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	277	<span class="emphasis"><em>Lists</em></span>.</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	278	<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>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	279	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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	280	function which in turn takes a single argument, the individual element of the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	281	<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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	282	sequence and returns a list of values, where each value is the value returned
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	283	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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	284	one sequence. In this case, the first argument, <span class="emphasis"><em>function</em></span> must take as many
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	285	arguments as the number of sequences passed. This function is called with each
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	286	corresponding element in the each of the sequences, or <span class="strong"><strong>None</strong></span> if one of the
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	287	sequence is exhausted:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	288	<pre class="programlisting"> def square(x):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	289	return x*x
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	290
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	291	>>> map(square, [1, 2, 3, 4])
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	292	[1, 4, 9, 16]
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	293
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	294	def mul(x, y):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	295	return x*y
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	296
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	297	>>> map(mul, [1, 2, 3, 4], [6, 7, 8, 9])</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	298	<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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	299	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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	300	item in the sequence and returns all the elements in the sequence for which
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	301	<span class="emphasis"><em>function(item)</em></span> returned True:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	302	<pre class="programlisting"> def even(x):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	303	if x % 2:
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	304	return True
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	305	else:
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	306	return False
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	307
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	308	>>> filter(even, range(1, 10))
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	309	[1, 3, 5, 7, 9]</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	310	<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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	311	<span class="strong"><strong>map</strong></span> function, however multiple sequences are not allowed. The <span class="strong"><strong>reduce</strong></span>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	312	function calls <span class="emphasis"><em>function</em></span> with first two consecutive elements in the sequence,
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	313	obtains the result, calls <span class="emphasis"><em>function</em></span> with the result and the subsequent element
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	314	in the sequence and so on until the end of the list and returns the final result:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	315	<pre class="programlisting"> def mul(x, y):
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	316	return x*y
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	317
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	318	>>> reduce(mul, [1, 2, 3, 4])
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	319	24</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	320	<div class="section" title="6.1. List Comprehensions">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	321	<div class="titlepage"><div><div><h3 class="title">
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	322	<a name="id2488001"></a>6.1. List Comprehensions</h3></div></div></div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	323	<p id="ch5func_1e"></a>List Comprehension is a convenvience utility provided by Python. It is a
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	324	syntatic sugar to create <span class="emphasis"><em>Lists</em></span>. Using <span class="emphasis"><em>List Comprehensions</em></span> one can create
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	325	<span class="emphasis"><em>Lists</em></span> from other type of sequential data structures or other <span class="emphasis"><em>Lists</em></span> itself.
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	326	The syntax of <span class="emphasis"><em>List Comprehensions</em></span> consists of a square brackets to indicate
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	327	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
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	328	multiple <span class="strong"><strong>if</strong></span> clauses. It will be more clear with an example:</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	329	<pre class="programlisting"> >>> num = [1, 2, 3]
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	330	>>> sq = [x*x for x in num]
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	331	>>> sq
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	332	[1, 4, 9]
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	333	>>> all_num = [1, 2, 3, 4, 5, 6, 7, 8, 9]
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	334	>>> even = [x for x in all_num if x%2 == 0]</pre>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	335	<p id="ch5func_1f"></a>The syntax used here is very clear from the way it is written. It can be
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	336	translated into english as, "for each element x in the list all_num,
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	337	if remainder of x divided by 2 is 0, add x to the list."</p>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	338	</div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	339	</div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	340	</div>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	341	</div></body>
8083d21c0020 The first commit of all the required files for the review app amit@thunder parents: diff changeset	342	</html>

author	amit@thunder
	Mon, 25 Jan 2010 18:56:45 +0530
changeset 0	8083d21c0020
child 1	672eaaab9204
permissions	-rw-r--r--