diff -r 401a46c880b0 -r 5076574b7b83 basic_python/intro.rst --- a/basic_python/intro.rst Thu Sep 17 14:33:08 2009 +0530 +++ b/basic_python/intro.rst Fri Sep 18 03:04:02 2009 +0530 @@ -714,267 +714,3 @@ 'Guido Van Rossum' >>> numvar = int(stringvar) - -Functions in Python: **def** -============================ - -*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. - -*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:: - - def factorial(n): - fact = 1 - for i in range(2, n): - fact *= i - - return fact - -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. - -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. - -The value that function returns can be assigned to a variable. Let's call the -above function and store the factorial in a variable:: - - fact5 = factorial(5) - -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. - -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:: - - def factorial(n): - 'Returns the factorial for the number n.' - fact = 1 - for i in range(2, n): - fact *= i - - return fact - -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 - -Let us write a small function to swap two values:: - - def swap(a, b): - return b, a - - c, d = swap(a, b) - -Function scope ---------------- -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:: - - def cant_change(n): - n = 10 - - n = 5 - cant_change(n) - -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:: - - >>> def can_change(n): - ... n[1] = James - ... - - >>> name = ['Mr.', 'Steve', 'Gosling'] - >>> can_change(name) - >>> name - ['Mr.', 'James', 'Gosling'] -If nothing is returned by the function explicitly, Python takes care to return -None when the funnction is called. - -Default Arguments ------------------ - -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:: - - 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 - -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:: - - fib() - [0, 1, 1, 2, 3, 5, 8, 13, 21, 34] - fib(5) - [0, 1, 1, 2, 3] - -Keyword Arguments ------------------ - -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*. - -In a function call, *Keyword arguments* can be used for each argument, in the -following fashion:: - - argument_name=argument_value - Also denoted as: keyword=argument - - def wish(name='World', greetings='Hello'): - print "%s, %s!" % (greetings, name) - -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:: - - wish(name='Guido', greetings='Hey') - wish(greetings='Hey', name='Guido') - -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*:: - - def my_func(x, y, z, u, v, w): - # initialize variables. - ... - # do some stuff - ... - # return the value - -It is valid to call the above functions in the following ways:: - - 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) - -Following lists some of the invalid calls:: - - 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) - -Parameter Packing and Unpacking -------------------------------- - -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 ****** :: - - 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 - -The above function can be called as. Note, the order of keyword parameters can -be interchanged:: - - >>> 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 - -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 ****** :: - - 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 - -Nested Functions and Scopes ---------------------------- - -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: - -| http://avinashv.net/2008/04/python-decorators-syntactic-sugar/ -| http://personalpages.tds.net/~kent37/kk/00001.html - -However, the following is an example for nested functions in Python:: - - def outer(): - print "Outer..." - def inner(): - print "Inner..." - print "Outer..." - inner() - - >>> outer() -