Function

     In this post I want to share some basic concepts about the functions in python. I think this post is very useful for all who want to know the function in python.

     Functions are reusable pieces of programs. They allow you to give name to a block of statements and you can run that block using that name anywhere in your program and any number of times. Functions are defined using the def keyword. This keyword followed by the function name followed by a pair of parentheses which may enclose some names of variables and the lines ends with a colon. Next follows the block of statements that are part of this function.

Example:

def hello():

     print 'Hello World.'

Output

hello() # call the function

Hello World

Parameters

Parameters are specified within the pair of parentheses in the function definition, separated by commas. When we call the function, we supply the values in the same way. The names given in the function definition are called parameters. Whereas the values you supply in the function are called arguments.

Example1

>>> def max(a,b):

...         if a>b:

...             print a,' is maximum'

...         else:

...             print b,' is maximum'

Output

>>> max(7,5) # Directly give the literal values

7 is maximum

Example 2

>>> def max(a,b):

... if a>b:

... print a,' is maximum'

... else:

... print b,' is maximum'

Output

>>> x=5

>>> y=7

>>> max(x,y) # Give variables as arguments

7 is maximum

Local Variable

     Variables defined in the functions are not related to any other variables defined with the same name outside the function. That is the variable names are local to the function.

Example

>>> def fun(x):

...         print 'x is',x

...         x=5

...         print 'changed local x to',x

Output

>>> x=10

>>> fun(x)

x is 10

changed local x to 5

>>> print 'x is still',x

x is still 10

Global statement

    Global statements are defined using the global keyword. It is impossible to assign a value to a variable defined outside a function without the 'global' statement.

Example

>>> def fun():

...         global x

...         print 'x is',x

...         x=5

...         print 'changed global x to',x

Output

>>> x=10

>>> fun()

x is 10

changed local x to 5

>>> print 'Value of x is :',x

Value of x is : 5

Default Argument values

     This type of arguments are used to assign default values to the parameters.

Example

>>> def name(message,num=1):

... print message*num

Output

>>> name('Alice')

Alice

>>> name('Alice',6)

AliceAliceAliceAliceAliceAlice

    An important thing is that, default values are assigned only to the parameters which are at the end of the parameter list. That is name(message,num=5) is valid, but name(message=5,num) is not valid.

Keyword Arguments

     In some situations, functions have many parameters, but we want to only specify the values of some of them. In such situation we assign values to this parameters by naming them, this is called keyword arguments. We use the names instead of position to specify the arguments to functions.

Example

>>> def fun(x,y=2,z=10):

...       print 'x is:',x,' y is:',y,' z is:',z

Output

>>> fun(3,7)

x is: 3 y is: 7 z is: 10

>>> fun(25,z=24)

x is: 25 y is: 2 z is: 24

>>> fun(z=50,x=100)

x is: 100 y is: 2 z is: 50

    Two advantages of using the Keyword arguments are: (1) We dont worry about the order of the arguments. (2) We can give values to only those parameters that we want.

    In this example first, the parameter x gets the value 3, y gets the value 7 and the parameter z uses the default value 10. In second case, parameter x gets the value 25, y uses the default value and z gets the value 24. In third case x gets the value 100, y gets the default value and z gets the value 50. Here order of arguments are not important. We can pass argument values to parameters in any order by using the name of parameters.

Return Statement

Return statement used to return from the function. Optionally we can return value from a function by using the return statement.

Example

>>> def largest(a,b):

...         if a>b:

...             return a

...         else:

...             return b

Output

>>> print largest(67,45)

67

Every function implicitly contain an return statement. If you not return any value, that return statement return a none value.

DocStrings

Python has a feature called Documentation String, which is usually referred to as docstrings. It helps to document the program better and makes it more easy to understand.

Example

>>> def max(x,y):

...         '''Prints the maximum of two numbers.

...         the two values must be integers.'''

...         x=int(x)

...         y=int(y)

...         if(x>y):

...             print x,'is maximum'

...         else:

...             print y,'is maximum'

Output

>>> max(5,8)

8 is maximum

>>> print max.__doc__

Prints the maximum of two numbers.

the two values must be integers.

     docstring is the first logical line of a function. Note that docstring also applied to modules and classes. Here the first line start with a capital letter and ends with a dot. Then the second line is blank followed by any detailed explanation starting from the third line.

     These are the basic concepts of function in python. Detailed description of function are described in the following posts.