Beginning Python - From Novice to Professional

136 CHAPTER 6 ■ ABSTRACTION
In the original example, reduce takes care of the sum and the looping, while the lambda
represents the expression sum + number. Let’s take a peek at what’s happening. The following
defines a function for adding numbers that also prints out its arguments:
def peek_sum(x, y):
print 'Adding', x, 'and', y
return x + y
Let’s use this with reduce:
>>> reduce(peek_sum, [1, 2, 3, 4, 5])
Adding 1 and 2
Adding 3 and 3
Adding 6 and 4
Adding 10 and 5
15
What happens is that reduce first adds 1 and 2, then adds the result with 3, and so on until
all the elements have been added. Finally, after printing out all the operations it goes through,
the sum (15) is returned.
■Note There is a built-in function called sum, which returns the sum of a sequence. (It cannot be used to
join a sequence of strings.)
As another example, let’s imagine that you could only use max with two arguments (in fact, it
works with entire sequences) and you wanted to use it on a sequence. Then you could use reduce:
>>> reduce(max, numbers)
119
The max function is used here to return the maximum of two numbers, and instead of
keeping track of a sum, reduce keeps track of the maximum so far. Let’s take another peek
under the hood:
def peek_max(x, y):
print 'Finding max of', x, 'and', y
return max(x, y)
Just like peek_sum, peek_max prints out its arguments when it is executed. Let’s use it
with reduce:
>>> reduce(peek_max, [3, 5, 2, 6, 9, 2])
Finding max of 3 and 5
Finding max of 5 and 2
Finding max of 5 and 6
Finding max of 6 and 9
Finding max of 9 and 2
9