An example of declaring/using/calling functions, and passing data to and from them. Written as a reference for students in one of my classes.

example.py

#!/usr/bin/python3.1
#
import random

# single_roll: A function which rolls a single 6 sided die, and returns
#               the result back to the calling function
def single_roll():
    # returns a random value between 1 and 6
    return random.randint(1,6)


# roll_dice: A function which rolls a given number of 6 sided die,
#              and returns the result back to the calling
#              function (main in this case).
def roll_dice(number_of_dice):

    # Loop for a given number of dice.
    #
    # First initialize some of the variables we will need.
    i = 0
    roll_sum = 0
    while i < number_of_dice:
        # increment i
        i += 1

        # Call our helper function `single_roll()` that rolls a single
        # dice for us.  It takes no input arguments (since we defined it
        # as having none.
        new_roll = single_roll()

        # Print out the result of the single roll.
        print("Die roll number %d: %d" % (i, new_roll))

        # Update the running total of the sum of all the rolls
        # so far.
        roll_sum += new_roll

    # return the sum of all the rolls back to main
    return roll_sum


def main():
    # randomize the seed
    random.seed()

    # Get user input: How many dice do you want to roll?
    number = input("How many dice do you want to roll? ").strip()

    print() # an empty print for formatting purposes

    # input() returns a string, so we need to convert to an integer
    try:
        number = int(number)
    except:
        # couldn't convert number to an integer
        print("Couldn't convert `%s` to an integer. Exiting..." % number)
        return

    # Call the function, and receive back the return value
    # into a variable called `my_sum`
    my_sum = roll_dice(number)

    # Print the result
    print() # an empty line
    print("The sum of all the dice rolls is: %d" % my_sum)

    # End of function

# We've defined our functions above (including main() ), but
# we have to call them to use them.  So call main() here.
if __name__ == '__main__':
    main()