o_18ufhmfmq19t513t3lgmn5l1qa8a.pdf

CHAPTER 6
■ ■ ■
Abstraction
In this chapter, you learn how to group statements into functions, which enables you to tell
the computer how to do something, and to tell it only once. You won’t have to give it the same
detailed instructions over and over. The chapter provides a thorough introduction to parameters
and scoping; you learn what recursion is and what it can do for your programs, and you see how
functions themselves can be used as parameters, just like numbers, strings, and other objects.
Laziness Is a Virtue
The programs we’ve written so far have been pretty small, but if you want to make something
bigger, you’ll soon run into trouble. Consider what happens if you have written some code in
one place and need to use it in another place as well. For example, let’s say you wrote a snippet
of code that computed some Fibonacci numbers (a series of numbers in which each number is
the sum of the two previous ones):
fibs = [0, 1]
for i in range(8):
fibs.append(fibs[-2] + fibs[-1])
After running this, fibs contains the first ten Fibonacci numbers:
>>> fibs
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
This is all right if what you want is to calculate the first ten Fibonacci numbers once. You
could even change the for loop to work with a dynamic range, with the length of the resulting
sequence supplied by the user:
fibs = [0, 1]
num = input('How many Fibonacci numbers do you want? ')
for i in range(num-2):
fibs.append(fibs[-2] + fibs[-1])
print fibs
■Note Remember that you can use raw_input if you want to read in a plain string. In this case, you would
then have had to convert it to an integer by using the int function.
109