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What is Function?

Think of a situation where you do a particualar job over and over again. Now what if you could teach someone else to do the job and whenever you need that particular task done you can call him. Taking this situation into programming we get the concept of functions. A function is a block of reusable code that can be used to perform a particular function.

Basics of Functions

Defining a Function

Defining a function in python requires the use of the keyword def followed by the function name and arguments.

def function_name(argument1,argument2):
    # Code for the function   
    return <expression>
You can call the function by:
where arg1,arg2 are the values you are passing to the function.

Sample Function

Let's define a function to print the sum of two numbers.

def sum(a,b):
    print "Sum is",c
Now we can call the function:
The output for the following function call will be
Sum is 15
Sum is 12
Here on the first function call a takes the value 5 and b takes 10. So the output is Sum is 15. And in the second function call a takes 4 and b takes 8 and output is Sum is 12.

Return Statement

The return statement can be used exit a function.It can also be used to send a value back from the function.
Syntax is :-

return <expression>
Let's take a look at a function to calculate the sum of first n natural numbers :-
def sum(n):
    s = 0
    while(n >= 0):
        s += n
        n -= 1
    return s
And we can call this function :-
su = sum(10)
print "sum is",su
And the output will be:-
sum is 55
Here the sum is calculated by the function and stored in the variable s. The value in s is returned by the function and stored in a variable su and thus we get the output mentioned above.

More on functions

Global and local variables

The variables declared within a function is a local variable of that particular function. And the variables outside a function is called global variable. The value of the variable depends on the scope of the variable.
Consider the following code:-

def func(a,b):
    out = a*b
    print "Output is",out
out = 50
print "Output is",out
The output of the following code will be:-
Output is 42
Output is 50
Here the global out is given the value 50. When the func() is called, the local out will have a value 42 since preference is given to local variables and we get Output is 42. After the function is called, the global out comes into scope. So for the second print out will have the value 50 and thus we get Output is 50.

Default arguments

You can set a default value for a particular argument of a function. So when you dont pass a certain argument to the function the function will assume the default value for that argument.
Consider the following code:-

def greet(name,grt = "Hi"):
    print grt,name
The output for the code will be:-
Hi Robert
Hello Joe
On the first call of greet(), only one argument is given so grt assumes its default value Hi and we get the output Hi Robert. On the second call two arguments are passed to the function and so the Hello will overwrite the default value of grt and thus we will get the output Hello Joe.

Recursive function

Recursion is a tricky concept. It can be used when a certain problem can be divided into smaller sections of the same problem.Just consider calculating the factorial of a number. 6! is 6*5*4*3*2*1 which can be written as 6x5!, similarly 5! can be written as 5x4! and so on. So as you can see calculating factorial of 5 is divided into smaller and similar task.
Now with this idea lets write a recursive function which returns the factorial of a number:-

def factorial(n):
        return 1
    return n*factorial(n-1)
So the above function will return the factorial of a number. Now we can call the function:-
f = factorial(6)
print "6! is",f
The output is predictable, but still :-
6! is 720
In the factorial() the n==0 is called ext condition. We need the recursive function call to stop at when this condition is satisfied. And we returned 1 in that case because 0! = 1. The statement n*factorial(n-1) will execute the recursion process.
In the above mentioned case, for 6! we will get 6*factorial(5), and for 5! we will get 5*factorial(4) and so on till n is 0, In that case that particular call will return 1 to its previous call, and that that function call will return whatever value it will get to its previous function call and after all the returning back to the previous function call it will return the desired value.

Imported Functions

Till now we spend time defining functions to do a particular task. Now we can use pre-defined functions to do a particular task. All we have to do is import the library which contains that particular function.
The syntax to import a function from a module is :-

from <module> import <function>
For example, to find the square root of a number you can use sqrt() from the math library
from math import sqrt
You can call this function like any other function:-
print sqrt(9)
print sqrt(5)
And output will be :-