<document><title>Functional Approach</title><paragraph>Functions 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.</paragraph><paragraph>Function in python is defined with the keyword def 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 Function must return a value:</paragraph><literal_block>def factorial(n): + fact = 1 + for i in range(2, n): + fact *= i + + return fact</literal_block><paragraph>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.</paragraph><paragraph>A Function 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.</paragraph><paragraph>The value that function returns can be assigned to a variable. Let's call the +above function and store the factorial in a variable:</paragraph><literal_block>fact5 = factorial(5)</literal_block><paragraph>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.</paragraph><paragraph>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 Documentation String or docstring. docstrings 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. docstrings for functions can be written as follows:</paragraph><literal_block>def factorial(n): + 'Returns the factorial for the number n.' + fact = 1 + for i in range(2, n): + fact *= i + + return fact</literal_block><paragraph>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 Tuple. A Tuple is +just a collection of values and those values themselves can be of any other +valid Python datatypes, including Lists, Tuples, Dictionaries among other +things. So effectively, if a function can return a tuple, it can return any +number of values through a tuple</paragraph><paragraph>Let us write a small function to swap two values:</paragraph><literal_block>def swap(a, b): + return b, a + +c, d = swap(a, b)</literal_block>

+1. Function scope

+<paragraph>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:</paragraph><literal_block>def cant_change(n): + n = 10 + +n = 5 +cant_change(n)</literal_block><paragraph>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 Numbers, Strings +and Tuples. But when you pass mutable objects like Lists and Dictionaries +the values are manipulated even outside the function:</paragraph><literal_block>>>> def can_change(n): +... n[1] = James +... + +>>> name = ['Mr.', 'Steve', 'Gosling'] +>>> can_change(name) +>>> name +['Mr.', 'James', 'Gosling']</literal_block><paragraph>If nothing is returned by the function explicitly, Python takes care to return +None when the funnction is called.</paragraph> +

+2. Default Arguments

+<paragraph>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 Default Arguments. 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:</paragraph><literal_block>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</literal_block><paragraph>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:</paragraph><literal_block>fib() +[0, 1, 1, 2, 3, 5, 8, 13, 21, 34] +fib(5) +[0, 1, 1, 2, 3]</literal_block> +

+3. Keyword Arguments

+<paragraph>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 Keyword Arguments.</paragraph><paragraph>In a function call, Keyword arguments can be used for each argument, in the +following fashion:</paragraph><literal_block>argument_name=argument_value +Also denoted as: keyword=argument + +def wish(name='World', greetings='Hello'): + print "%s, %s!" % (greetings, name)</literal_block><paragraph>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 Keyword arguments +can be specified. Also note, that we have combined Keyword arguments with +Default arguments in this example, however it is not necessary:</paragraph><literal_block>wish(name='Guido', greetings='Hey') +wish(greetings='Hey', name='Guido')</literal_block><paragraph>Calling functions by specifying arguments in the order of parameters specified +in the function definition is called as Positional arguments, as opposed to +Keyword arguments. It is possible to use both Positional arguments and +Keyword arguments 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 Positional arguments which is in turn followed by Keyword +arguments:</paragraph><literal_block>def my_func(x, y, z, u, v, w): + # initialize variables. + ... + # do some stuff + ... + # return the value</literal_block><paragraph>It is valid to call the above functions in the following ways:</paragraph><literal_block>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)</literal_block><paragraph>Following lists some of the invalid calls:</paragraph><literal_block>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)</literal_block> +

+4. Parameter Packing and Unpacking

+<paragraph>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 *, for collecting positional +parameters, in turn followed by the dictionary collecting parameter, whose name +is preceded by a **</paragraph><literal_block>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</literal_block><paragraph>The above function can be called as. Note, the order of keyword parameters can +be interchanged:</paragraph><literal_block>>>> 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</literal_block><paragraph>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 Positional arguments or Keyword arguments respectively +using * or **</paragraph><literal_block>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</literal_block> +

+5. Nested Functions and Scopes

+<paragraph>Python allows nesting one function inside another. This style of programming +turns out to be extremely flexible and powerful features when we use Python +decorators. We will not talk about decorators is beyond the scope of this +course. If you are interested in knowing more about decorator programming in +Python you are suggested to read:</paragraph><line_block><line>

http://avinashv.net/2008/04/python-decorators-syntactic-sugar/

</line><line>

http://personalpages.tds.net/~kent37/kk/00001.html

</line></line_block><paragraph>However, the following is an example for nested functions in Python:</paragraph><literal_block>def outer(): + print "Outer..." + def inner(): + print "Inner..." + print "Outer..." + inner() + +>>> outer()</literal_block> +

+6. map, reduce and filter functions

+<paragraph>Python provides several built-in functions for convenience. The map(), +reduce() and filter() functions prove to be very useful with sequences like +Lists.</paragraph><paragraph>The map (function, sequence) function takes two arguments: function +and a sequence argument. The function argument must be the name of the +function which in turn takes a single argument, the individual element of the +sequence. The map function calls function(item), for each item in the +sequence and returns a list of values, where each value is the value returned +by each call to function(item). map() function allows to pass more than +one sequence. In this case, the first argument, function 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 None if one of the +sequence is exhausted:</paragraph><literal_block>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])</literal_block><paragraph>The filter (function, sequence) function takes two arguments, similar to +the map() function. The filter function calls function(item), for each +item in the sequence and returns all the elements in the sequence for which +function(item) returned True:</paragraph><literal_block>def even(x): + if x % 2: + return True + else: + return False + +>>> filter(even, range(1, 10)) +[1, 3, 5, 7, 9]</literal_block><paragraph>The reduce (function, sequence) function takes two arguments, similar to +map function, however multiple sequences are not allowed. The reduce +function calls function with first two consecutive elements in the sequence, +obtains the result, calls function with the result and the subsequent element +in the sequence and so on until the end of the list and returns the final result:</paragraph><literal_block>def mul(x, y): + return x*y + +>>> reduce(mul, [1, 2, 3, 4]) +24</literal_block>

+6.1. List Comprehensions

+<paragraph>List Comprehension is a convenvience utility provided by Python. It is a +syntatic sugar to create Lists. Using List Comprehensions one can create +Lists from other type of sequential data structures or other Lists itself. +The syntax of List Comprehensions consists of a square brackets to indicate +the result is a List within which we include at least one for clause and +multiple if clauses. It will be more clear with an example:</paragraph><literal_block>>>> 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]</literal_block><paragraph>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."</paragraph> +

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