We use def keyword to define a function. general syntax is like

def functionname(params):
    statement1
    statement2

Let us write a function which will take two integers as input and then return the sum.

>>> def sum(a, b):
...     return a + b

In the second line with the return keyword, we are sending back the value of a + b to the caller. You must call it like

>>> res = sum(234234, 34453546464)
>>> res
34453780698L

Remember the palindrome program we wrote in the last chapter. Let us write a function which will check if a given string is palindrome or not, then return True or False.

#!/usr/bin/env python
def palindrome(s):
    z = s
    z = [x for x in z]
    z.reverse()
    if s == "".join(z):
        return True
    else:
        return False
s = raw_input("Enter a string: ")
if palindrome(s):
    print "Yay a palindrome"
else:
    print "Oh no, not a palindrome"

Now run the code :)